A chemical element is a chemical substance whose atoms all have the same number of protons. The number of protons is called the atomic number of that element. For example, oxygen has an atomic number of 8, meaning each oxygen atom has 8 protons in its nucleus. Atoms of the same element can have different numbers of neutrons in their nuclei, known as isotopes of the element. Two or more atoms can combine to form molecules. Some elements are formed from molecules of identical atoms, e. g. atoms of hydrogen (H) form diatomic molecules (H₂). Chemical compounds are substances made of atoms of different elements; they can have molecular or non-molecular structure. Mixtures are materials containing different chemical substances; that means (in case of molecular substances) that they contain different types of molecules. Atoms of one element can be transformed into atoms of a different element in nuclear reactions, which change an atom's atomic number.

Historically, the term "chemical element" meant a substance that cannot be broken down into constituent substances by chemical reactions, and for most practical purposes this definition still has validity. There was some controversy in the 1920s over whether isotopes deserved to be recognized as separate elements if they could be separated by chemical means.

The term "(chemical) element" is used in two different but closely related meanings: it can mean a chemical substance consisting of a single kind of atoms, or it can mean that kind of atoms as a component of various chemical substances. For example, molecules of water (H₂O) contain atoms of hydrogen (H) and oxygen (O), so water can be said as a compound consisting of the elements hydrogen (H) and oxygen (O) even though it does not contain the chemical substances (di)hydrogen (H₂) and (di)oxygen (O₂), as H₂O molecules are different from H₂ and O₂ molecules. For the meaning "chemical substance consisting of a single kind of atoms", the terms "elementary substance" and "simple substance" have been suggested, but they have not gained much acceptance in English chemical literature, whereas in some other languages their equivalent is widely used. For example, the French chemical terminology distinguishes élément chimique (kind of atoms) and corps simple (chemical substance consisting of a single kind of atoms); the Russian chemical terminology distinguishes химический элемент and простое вещество.

Almost all baryonic matter in the universe is composed of elements (among rare exceptions are neutron stars). When different elements undergo chemical reactions, atoms are rearranged into new compounds held together by chemical bonds. Only a few elements, such as silver and gold, are found uncombined as relatively pure native element minerals. Nearly all other naturally occurring elements occur in the Earth as compounds or mixtures. Air is mostly a mixture of molecular nitrogen and oxygen, though it does contain compounds including carbon dioxide and water, as well as atomic argon, a noble gas which is chemically inert and therefore does not undergo chemical reactions.

The history of the discovery and use of elements began with early human societies that discovered native minerals like carbon, sulfur, copper and gold (though the modern concept of an element was not yet understood). Attempts to classify materials such as these resulted in the concepts of classical elements, alchemy, and similar theories throughout history. Much of the modern understanding of elements developed from the work of Dmitri Mendeleev, a Russian chemist who published the first recognizable periodic table in 1869. This table organizes the elements by increasing atomic number into rows ("periods") in which the columns ("groups") share recurring ("periodic") physical and chemical properties. The periodic table summarizes various properties of the elements, allowing chemists to derive relationships between them and to make predictions about elements not yet discovered, and potential new compounds.

By November 2016, the International Union of Pure and Applied Chemistry (IUPAC) had recognized a total of 118 elements. The first 94 occur naturally on Earth, and the remaining 24 are synthetic elements produced in nuclear reactions. Save for unstable radioactive elements (radioelements) which decay quickly, nearly all elements are available industrially in varying amounts. The discovery and synthesis of further new elements is an ongoing area of scientific study.

The lightest elements are hydrogen and helium, both created by Big Bang nucleosynthesis in the first 20 minutes of the universe in a ratio of around 3:1 by mass (or 12:1 by number of atoms), along with tiny traces of the next two elements, lithium and beryllium. Almost all other elements found in nature were made by various natural methods of nucleosynthesis. On Earth, small amounts of new atoms are naturally produced in nucleogenic reactions, or in cosmogenic processes, such as cosmic ray spallation. New atoms are also naturally produced on Earth as radiogenic daughter isotopes of ongoing radioactive decay processes such as alpha decay, beta decay, spontaneous fission, cluster decay, and other rarer modes of decay.

Of the 94 naturally occurring elements, those with atomic numbers 1 through 82 each have at least one stable isotope (except for technetium, element 43 and promethium, element 61, which have no stable isotopes). Isotopes considered stable are those for which no radioactive decay has yet been observed. Elements with atomic numbers 83 through 94 are unstable to the point that radioactive decay of all isotopes can be detected. Some of these elements, notably bismuth (atomic number 83), thorium (atomic number 90), and uranium (atomic number 92), have one or more isotopes with half-lives long enough to survive as remnants of the explosive stellar nucleosynthesis that produced the heavy metals before the formation of our Solar System. At over 1.9×10 years, over a billion times longer than the estimated age of the universe, bismuth-209 has the longest known alpha decay half-life of any isotope, and is almost always considered on par with the 80 stable elements. The heaviest elements (those beyond plutonium, element 94) undergo radioactive decay with half-lives so short that they are not found in nature and must be synthesized.

There are now 118 known elements. In this context, "known" means observed well enough, even from just a few decay products, to have been differentiated from other elements. Most recently, the synthesis of element 118 (since named oganesson) was reported in October 2006, and the synthesis of element 117 (tennessine) was reported in April 2010. Of these 118 elements, 94 occur naturally on Earth. Six of these occur in extreme trace quantities: technetium, atomic number 43; promethium, number 61; astatine, number 85; francium, number 87; neptunium, number 93; and plutonium, number 94. These 94 elements have been detected in the universe at large, in the spectra of stars and also supernovae, where short-lived radioactive elements are newly being made. The first 94 elements have been detected directly on Earth as primordial nuclides present from the formation of the Solar System, or as naturally occurring fission or transmutation products of uranium and thorium.

The remaining 24 heavier elements, not found today either on Earth or in astronomical spectra, have been produced artificially: all are radioactive, with short half-lives; if any of these elements were present at the formation of Earth, they are certain to have completely decayed, and if present in novae, are in quantities too small to have been noted. Technetium was the first purportedly non-naturally occurring element synthesized, in 1937, though trace amounts of technetium have since been found in nature (and also the element may have been discovered naturally in 1925). This pattern of artificial production and later natural discovery has been repeated with several other radioactive naturally occurring rare elements.

List of the elements are available by name, atomic number, density, melting point, boiling point and chemical symbol, as well as ionization energy. The nuclides of stable and radioactive elements are also available as a list of nuclides, sorted by length of half-life for those that are unstable. One of the most convenient, and certainly the most traditional presentation of the elements, is in the form of the periodic table, which groups together elements with similar chemical properties (and usually also similar electronic structures).

The atomic number of an element is equal to the number of protons in each atom, and defines the element. For example, all carbon atoms contain 6 protons in their atomic nucleus; so the atomic number of carbon is 6. Carbon atoms may have different numbers of neutrons; atoms of the same element having different numbers of neutrons are known as isotopes of the element.

The number of protons in the nucleus also determines its electric charge, which in turn determines the number of electrons of the atom in its non-ionized state. The electrons are placed into atomic orbitals that determine the atom's chemical properties. The number of neutrons in a nucleus usually has very little effect on an element's chemical properties; except for hydrogen (for which the kinetic isotope effect is significant). Thus, all carbon isotopes have nearly identical chemical properties because they all have six electrons, even though they may have 6 to 8 neutrons. That is why atomic number, rather than mass number or atomic weight, is considered the identifying characteristic of an element.

The symbol for atomic number is Z.

Isotopes are atoms of the same element (that is, with the same number of protons in their nucleus), but having different numbers of neutrons. Thus, for example, there are three main isotopes of carbon. All carbon atoms have 6 protons, but they can have either 6, 7, or 8 neutrons. Since the mass numbers of these are 12, 13 and 14 respectively, said three isotopes are known as carbon-12, carbon-13, and carbon-14 (C, C, and C). Natural carbon is a mixture of C (about 98.9%), C (about 1.1%) and about 1 atom per trillion of C.

Most (54 of 94) naturally occurring elements have more than one stable isotope. Except for the isotopes of hydrogen (which differ greatly from each other in relative mass—enough to cause chemical effects), the isotopes of a given element are chemically nearly indistinguishable.

All elements have radioactive isotopes (radioisotopes); most of these radioisotopes do not occur naturally. Radioisotopes typically decay into other elements via alpha decay, beta decay, or inverse beta decay; some isotopes of the heaviest elements also undergo spontaneous fission. Isotopes that are not radioactive, are termed "stable" isotopes. All known stable isotopes occur naturally (see primordial nuclide). The many radioisotopes that are not found in nature have been characterized after being artificially produced. Certain elements have no stable isotopes and are composed only of radioisotopes: specifically the elements without any stable isotopes are technetium (atomic number 43), promethium (atomic number 61), and all observed elements with atomic number greater than 82.

Of the 80 elements with at least one stable isotope, 26 have only one stable isotope. The mean number of stable isotopes for the 80 stable elements is 3.1 stable isotopes per element. The largest number of stable isotopes for a single element is 10 (for tin, element 50).

The mass number of an element, A, is the number of nucleons (protons and neutrons) in the atomic nucleus. Different isotopes of a given element are distinguished by their mass number, which is written as a superscript on the left hand side of the chemical symbol (e.g., U). The mass number is always an integer and has units of "nucleons". Thus, magnesium-24 (24 is the mass number) is an atom with 24 nucleons (12 protons and 12 neutrons).

Whereas the mass number simply counts the total number of neutrons and protons and is thus an integer, the atomic mass of a particular isotope (or "nuclide") of the element is the mass of a single atom of that isotope, and is typically expressed in daltons (symbol: Da), or universal atomic mass units (symbol: u). Its relative atomic mass is a dimensionless number equal to the atomic mass divided by the atomic mass constant, which equals 1 Da. In general, the mass number of a given nuclide differs in value slightly from its relative atomic mass, since the mass of each proton and neutron is not exactly 1 Da; since the electrons contribute a lesser share to the atomic mass as neutron number exceeds proton number; and because of the nuclear binding energy and electron binding energy. For example, the atomic mass of chlorine-35 to five significant digits is 34.969 Da and that of chlorine-37 is 36.966 Da. However, the relative atomic mass of each isotope is quite close to its mass number (always within 1%). The only isotope whose atomic mass is exactly a natural number is C, which has a mass of 12 Da; because the dalton is defined as 1/12 of the mass of a free neutral carbon-12 atom in the ground state.

The standard atomic weight (commonly called "atomic weight") of an element is the average of the atomic masses of all the chemical element's isotopes as found in a particular environment, weighted by isotopic abundance, relative to the atomic mass unit. This number may be a fraction that is not close to a whole number. For example, the relative atomic mass of chlorine is 35.453 u, which differs greatly from a whole number as it is an average of about 76% chlorine-35 and 24% chlorine-37. Whenever a relative atomic mass value differs by more than ~1% from a whole number, it is due to this averaging effect, as significant amounts of more than one isotope are naturally present in a sample of that element.

Chemists and nuclear scientists have different definitions of a pure element. In chemistry, a pure element means a substance whose atoms all (or in practice almost all) have the same atomic number, or number of protons. Nuclear scientists, however, define a pure element as one that consists of only one isotope.

For example, a copper wire is 99.99% chemically pure if 99.99% of its atoms are copper, with 29 protons each. However it is not isotopically pure since ordinary copper consists of two stable isotopes, 69% Cu and 31% Cu, with different numbers of neutrons. However, pure gold would be both chemically and isotopically pure, since ordinary gold consists only of one isotope, Au.

Atoms of chemically pure elements may bond to each other chemically in more than one way, allowing the pure element to exist in multiple chemical structures (spatial arrangements of atoms), known as allotropes, which differ in their properties. For example, carbon can be found as diamond, which has a tetrahedral structure around each carbon atom; graphite, which has layers of carbon atoms with a hexagonal structure stacked on top of each other; graphene, which is a single layer of graphite that is very strong; fullerenes, which have nearly spherical shapes; and carbon nanotubes, which are tubes with a hexagonal structure (even these may differ from each other in electrical properties). The ability of an element to exist in one of many structural forms is known as 'allotropy'.

The reference state of an element is defined by convention, usually as the thermodynamically most stable allotrope and physical state at a pressure of 1 bar and a given temperature (typically at 298.15K). However, for phosphorus, the reference state is white phosphorus even though it is not the most stable allotrope, and the reference state for carbon is graphite, because the structure of graphite is more stable than that of the other allotropes. In thermochemistry, an element is defined to have an enthalpy of formation of zero in its reference state.

Several kinds of descriptive categorizations can be applied broadly to the elements, including consideration of their general physical and chemical properties, their states of matter under familiar conditions, their melting and boiling points, their densities, their crystal structures as solids, and their origins.

Several terms are commonly used to characterize the general physical and chemical properties of the chemical elements. A first distinction is between metals, which readily conduct electricity, nonmetals, which do not, and a small group, (the metalloids), having intermediate properties and often behaving as semiconductors.

A more refined classification is often shown in colored presentations of the periodic table. This system restricts the terms "metal" and "nonmetal" to only certain of the more broadly defined metals and nonmetals, adding additional terms for certain sets of the more broadly viewed metals and nonmetals. The version of this classification used in the periodic tables presented here includes: actinides, alkali metals, alkaline earth metals, halogens, lanthanides, transition metals, post-transition metals, metalloids, reactive nonmetals, and noble gases. In this system, the alkali metals, alkaline earth metals, and transition metals, as well as the lanthanides and the actinides, are special groups of the metals viewed in a broader sense. Similarly, the reactive nonmetals and the noble gases are nonmetals viewed in the broader sense. In some presentations, the halogens are not distinguished, with astatine identified as a metalloid and the others identified as nonmetals.

Another commonly used basic distinction among the elements is their state of matter (phase), whether solid, liquid, or gas, at standard temperature and pressure (STP). Most elements are solids at STP, while several are gases. Only bromine and mercury are liquid at 0 degrees Celsius (32 degrees Fahrenheit) and 1 atmosphere pressure; caesium and gallium are solid at that temperature, but melt at 28.4°C (83.2°F) and 29.8°C (85.6°F), respectively.

Melting and boiling points, typically expressed in degrees Celsius at a pressure of one atmosphere, are commonly used in characterizing the various elements. While known for most elements, either or both of these measurements is still undetermined for some of the radioactive elements available in only tiny quantities. Since helium remains a liquid even at absolute zero at atmospheric pressure, it has only a boiling point, and not a melting point, in conventional presentations.

The density at selected standard temperature and pressure (STP) is often used in characterizing the elements. Density is often expressed in grams per cubic centimetre (g/cm). Since several elements are gases at commonly encountered temperatures, their densities are usually stated for their gaseous forms; when liquefied or solidified, the gaseous elements have densities similar to those of the other elements.

When an element has allotropes with different densities, one representative allotrope is typically selected in summary presentations, while densities for each allotrope can be stated where more detail is provided. For example, the three familiar allotropes of carbon (amorphous carbon, graphite, and diamond) have densities of 1.8–2.1, 2.267, and 3.515 g/cm, respectively.

The elements studied to date as solid samples have eight kinds of crystal structures: cubic, body-centered cubic, face-centered cubic, hexagonal, monoclinic, orthorhombic, rhombohedral, and tetragonal. For some of the synthetically produced transuranic elements, available samples have been too small to determine crystal structures.

Chemical elements may also be categorized by their origin on Earth, with the first 94 considered naturally occurring, while those with atomic numbers beyond 94 have only been produced artificially via human-made nuclear reactions.

Of the 94 naturally occurring elements, 83 are considered primordial and either stable or weakly radioactive. The longest-lived isotopes of the remaining 11 elements have half lives too short for them to have been present at the beginning of the Solar System, and are therefore considered transient elements. Of these 11 transient elements, five (polonium, radon, radium, actinium, and protactinium) are relatively common decay products of thorium and uranium. The remaining six transient elements (technetium, promethium, astatine, francium, neptunium, and plutonium) occur only rarely, as products of rare decay modes or nuclear reaction processes involving uranium or other heavy elements.

Elements with atomic numbers 1 through 82, except 43 (technetium) and 61 (promethium), each have at least one isotope for which no radioactive decay has been observed. Observationally stable isotopes of some elements (such as tungsten and lead), however, are predicted to be slightly radioactive with very long half-lives: for example, the half-lives predicted for the observationally stable lead isotopes range from 10 to 10 years. Elements with atomic numbers 43, 61, and 83 through 94 are unstable enough that their radioactive decay can be detected. Three of these elements, bismuth (element 83), thorium (90), and uranium (92) have one or more isotopes with half-lives long enough to survive as remnants of the explosive stellar nucleosynthesis that produced the heavy elements before the formation of the Solar System. For example, at over 1.9×10 years, over a billion times longer than the estimated age of the universe, bismuth-209 has the longest known alpha decay half-life of any isotope. The last 24 elements (those beyond plutonium, element 94) undergo radioactive decay with short half-lives and cannot be produced as daughters of longer-lived elements, and thus are not known to occur in nature at all.

The properties of the elements are often summarized using the periodic table, which powerfully and elegantly organizes the elements by increasing atomic number into rows ("periods") in which the columns ("groups") share recurring ("periodic") physical and chemical properties. The table contains 118 confirmed elements as of 2021.

Although earlier precursors to this presentation exist, its invention is generally credited to Russian chemist Dmitri Mendeleev in 1869, who intended the table to illustrate recurring trends in the properties of the elements. The layout of the table has been refined and extended over time as new elements have been discovered and new theoretical models have been developed to explain chemical behavior.

Use of the periodic table is now ubiquitous in chemistry, providing an extremely useful framework to classify, systematize and compare all the many different forms of chemical behavior. The table has also found wide application in physics, geology, biology, materials science, engineering, agriculture, medicine, nutrition, environmental health, and astronomy. Its principles are especially important in chemical engineering.

The various chemical elements are formally identified by their unique atomic numbers, their accepted names, and their chemical symbols.

The known elements have atomic numbers from 1 to 118, conventionally presented as Arabic numerals. Since the elements can be uniquely sequenced by atomic number, conventionally from lowest to highest (as in a periodic table), sets of elements are sometimes specified by such notation as "through", "beyond", or "from ... through", as in "through iron", "beyond uranium", or "from lanthanum through lutetium". The terms "light" and "heavy" are sometimes also used informally to indicate relative atomic numbers (not densities), as in "lighter than carbon" or "heavier than lead", though the atomic masses of the elements (their atomic weights or atomic masses) do not always increase monotonically with their atomic numbers.

The naming of various substances now known as elements precedes the atomic theory of matter, as names were given locally by various cultures to various minerals, metals, compounds, alloys, mixtures, and other materials, though at the time it was not known which chemicals were elements and which compounds. As they were identified as elements, the existing names for anciently known elements (e.g., gold, mercury, iron) were kept in most countries. National differences emerged over the element names either for convenience, linguistic niceties, or nationalism. For example, German speakers use "Wasserstoff" (water substance) for "hydrogen", "Sauerstoff" (acid substance) for "oxygen" and "Stickstoff" (smothering substance) for "nitrogen"; English and some other languages use "sodium" for "natrium", and "potassium" for "kalium"; and the French, Italians, Greeks, Portuguese and Poles prefer "azote/azot/azoto" (from roots meaning "no life") for "nitrogen".

For purposes of international communication and trade, the official names of the chemical elements both ancient and more recently recognized are decided by the International Union of Pure and Applied Chemistry (IUPAC), which has decided on a sort of international English language, drawing on traditional English names even when an element's chemical symbol is based on a Latin or other traditional word, for example adopting "gold" rather than "aurum" as the name for the 79th element (Au). IUPAC prefers the British spellings "aluminium" and "caesium" over the U.S. spellings "aluminum" and "cesium", and the U.S. "sulfur" over British "sulphur". However, elements that are practical to sell in bulk in many countries often still have locally used national names, and countries whose national language does not use the Latin alphabet are likely to use the IUPAC element names.

According to IUPAC, element names are not proper nouns; therefore, the full name of an element is not capitalized in English, even if derived from a proper noun, as in californium and einsteinium. Isotope names are also uncapitalized if written out, e.g., carbon-12 or uranium-235. Chemical element symbols (such as Cf for californium and Es for einsteinium), are always capitalized (see below).

In the second half of the 20th century, physics laboratories became able to produce elements with half-lives too short for an appreciable amount of them to exist at any time. These are also named by IUPAC, which generally adopts the name chosen by the discoverer. This practice can lead to the controversial question of which research group actually discovered an element, a question that delayed the naming of elements with atomic number of 104 and higher for a considerable amount of time. (See element naming controversy).

Precursors of such controversies involved the nationalistic namings of elements in the late 19th century. For example, lutetium was named in reference to Paris, France. The Germans were reluctant to relinquish naming rights to the French, often calling it cassiopeium. Similarly, the British discoverer of niobium originally named it columbium, in reference to the New World. It was used extensively as such by American publications before the international standardization (in 1950).

Before chemistry became a science, alchemists designed arcane symbols for both metals and common compounds. These were however used as abbreviations in diagrams or procedures; there was no concept of atoms combining to form molecules. With his advances in the atomic theory of matter, John Dalton devised his own simpler symbols, based on circles, to depict molecules.

The current system of chemical notation was invented by Jöns Jacob Berzelius in 1814. In this system, chemical symbols are not mere abbreviations—though each consists of letters of the Latin alphabet. They are intended as universal symbols for people of all languages and alphabets.

Since Latin was the common language of science at Berzelius' time, his symbols were abbreviations based on the Latin names of elements (they may be Classical Latin names of elements known since antiquity or Neo-Latin coinages for later elements). The symbols are not followed by a period (full stop) as with abbreviations. In most cases, Latin names of elements as used by Berzelius have the same roots as the modern English name. For example, hydrogen has the symbol "H" from Neo-Latin hydrogenium, which has the same Greek roots as English hydrogen. However, in eleven cases Latin (as used by Berzelius) and English names of elements have different roots. Eight of them are the seven metals of antiquity and a metalloid also known since antiquity: "Fe" (Latin ferrum) for iron, "Hg" (Latin hydrargyrum) for mercury, "Sn" (Latin stannum) for tin, "Au" (Latin aurum) for gold, "Ag" (Latin argentum) for silver, "Pb" (Latin plumbum) for lead, "Cu" (Latin cuprum) for copper, and "Sb" (Latin stibium) for antimony. The three other mismatches between Neo-Latin (as used by Berzelius) and English names are "Na" (Neo-Latin natrium) for sodium, "K" (Neo-Latin kalium) for potassium, and "W" (Neo-Latin wolframium) for tungsten. These mismatches came from different suggestings of naming the elements in the Modern era. Initially Berzelius had suggested "So" and "Po" for sodium and potassium, but he changed the symbols to "Na" and "K" later in the same year.

Elements discovered after 1814 were also assigned unique chemical symbols, based on the name of the element. The use of Latin as the universal language of science was fading, but chemical names of newly discovered elements came to be borrowed from language to language with little or no modifications. Symbols of elements discovered after 1814 match their names in English, French (ignoring the acute accent on ⟨é⟩), and German (though German often allows alternate spellings with ⟨k⟩ or ⟨z⟩ instead of ⟨c⟩: e.g., the name of calcium may be spelled Calcium or Kalzium in German, but its symbol is always "Ca"). Other languages sometimes modify element name spellings: Spanish iterbio (ytterbium), Italian afnio (hafnium), Swedish moskovium (moscovium); but those modifications do not affect chemical symbols: Yb, Hf, Mc.

Chemical symbols are understood internationally when element names might require translation. There have been some differences in the past. For example, Germans in the past have used "J" (for the name Jod) for iodine, but now use "I" and Iod.

The first letter of a chemical symbol is always capitalized, as in the preceding examples, and the subsequent letters, if any, are always lower case. Thus, the symbols for californium and einsteinium are Cf and Es.

There are also symbols in chemical equations for groups of elements, for example in comparative formulas. These are often a single capital letter, and the letters are reserved and not used for names of specific elements. For example, "X" indicates a variable group (usually a halogen) in a class of compounds, while "R" is a radical, meaning a compound structure such as a hydrocarbon chain. The letter "Q" is reserved for "heat" in a chemical reaction. "Y" is also often used as a general chemical symbol, though it is also the symbol of yttrium. "Z" is also often used as a general variable group. "E" is used in organic chemistry to denote an electron-withdrawing group or an electrophile; similarly "Nu" denotes a nucleophile. "L" is used to represent a general ligand in inorganic and organometallic chemistry. "M" is also often used in place of a general metal.

At least two other, two-letter generic chemical symbols are also in informal use, "Ln" for any lanthanide and "An" for any actinide. "Rg" was formerly used for any rare gas element, but the group of rare gases has now been renamed noble gases and "Rg" now refers to roentgenium.

Isotopes of an element are distinguished by mass number (total protons and neutrons), with this number combined with the element's symbol. IUPAC prefers that isotope symbols be written in superscript notation when practical, for example C and U. However, other notations, such as carbon-12 and uranium-235, or C-12 and U-235, are also used.

As a special case, the three naturally occurring isotopes of hydrogen are often specified as H for H (protium), D for H (deuterium), and T for H (tritium). This convention is easier to use in chemical equations, replacing the need to write out the mass number each time. Thus, the formula for heavy water may be written D₂O instead of H₂O.

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Only about 4% of the total mass of the universe is made of atoms or ions, and thus represented by elements. This fraction is about 15% of the total matter, with the remainder of the matter (85%) being dark matter. The nature of dark matter is unknown, but it is not composed of atoms of elements because it contains no protons, neutrons, or electrons. (The remaining non-matter part of the mass of the universe is composed of the even less well understood dark energy).

The 94 naturally occurring elements were produced by at least four classes of astrophysical process. Most of the hydrogen, helium and a very small quantity of lithium were produced in the first few minutes of the Big Bang. This Big Bang nucleosynthesis happened only once; the other processes are ongoing. Nuclear fusion inside stars produces elements through stellar nucleosynthesis, including all elements from carbon to iron in atomic number. Elements higher in atomic number than iron, including heavy elements like uranium and plutonium, are produced by various forms of explosive nucleosynthesis in supernovae and neutron star mergers. The light elements lithium, beryllium and boron are produced mostly through cosmic ray spallation (fragmentation induced by cosmic rays) of carbon, nitrogen, and oxygen.

In the early phases of the Big Bang, nucleosynthesis of hydrogen resulted in the production of hydrogen-1 (protium, H) and helium-4 (He), as well as a smaller amount of deuterium (H) and tiny amounts (on the order of 10) of lithium and beryllium. Even smaller amounts of boron may have been produced in the Big Bang, since it has been observed in some very old stars, while carbon has not. No elements heavier than boron were produced in the Big Bang. As a result, the primordial abundance of atoms (or ions) consisted of ~75% H, 25% He, and 0.01% deuterium, with only tiny traces of lithium, beryllium, and perhaps boron. Subsequent enrichment of galactic halos occurred due to stellar nucleosynthesis and supernova nucleosynthesis. However, the element abundance in intergalactic space can still closely resemble primordial conditions, unless it has been enriched by some means.

On Earth (and elsewhere), trace amounts of various elements continue to be produced from other elements as products of nuclear transmutation processes. These include some produced by cosmic rays or other nuclear reactions (see cosmogenic and nucleogenic nuclides), and others produced as decay products of long-lived primordial nuclides. For example, trace (but detectable) amounts of carbon-14 (C) are continually produced in the air by cosmic rays impacting nitrogen atoms, and argon-40 (Ar) is continually produced by the decay of primordially occurring but unstable potassium-40 (K). Also, three primordially occurring but radioactive actinides, thorium, uranium, and plutonium, decay through a series of recurrently produced but unstable elements such as radium and radon, which are transiently present in any sample of containing these metals. Three other radioactive elements, technetium, promethium, and neptunium, occur only incidentally in natural materials, produced as individual atoms by nuclear fission of the nuclei of various heavy elements or in other rare nuclear processes.

Besides the 94 naturally occurring elements, several artificial elements have been produced by nuclear physics technology. By 2016, these experiments had produced all elements up to atomic number 118.

The following graph (note log scale) shows the abundance of elements in our Solar System. The table shows the 12 most common elements in our galaxy (estimated spectroscopically), as measured in parts per million by mass. Nearby galaxies that have evolved along similar lines have a corresponding enrichment of elements heavier than hydrogen and helium. The more distant galaxies are being viewed as they appeared in the past, so their abundances of elements appear closer to the primordial mixture. As physical laws and processes appear common throughout the visible universe, however, scientists expect that these galaxies evolved elements in similar abundance.

The abundance of elements in the Solar System is in keeping with their origin from nucleosynthesis in the Big Bang and a number of progenitor supernova stars. Very abundant hydrogen and helium are products of the Big Bang, but the next three elements are rare since they had little time to form in the Big Bang and are not made in stars (they are, however, produced in small quantities by the breakup of heavier elements in interstellar dust, as a result of impact by cosmic rays). Beginning with carbon, elements are produced in stars by buildup from alpha particles (helium nuclei), resulting in an alternatingly larger abundance of elements with even atomic numbers (these are also more stable). In general, such elements up to iron are made in large stars in the process of becoming supernovas. Iron-56 is particularly common, since it is the most stable nuclide that can easily be made from alpha particles (being a product of decay of radioactive nickel-56, ultimately made from 14 helium nuclei). Elements heavier than iron are made in energy-absorbing processes in large stars, and their abundance in the universe (and on Earth) generally decreases with their atomic number.

The abundance of the chemical elements on Earth varies from air to crust to ocean, and in various types of life. The abundance of elements in Earth's crust differs from that in the Solar System (as seen in the Sun and massive planets like Jupiter) mainly in selective loss of the very lightest elements (hydrogen and helium) and also volatile neon, carbon (as hydrocarbons), nitrogen and sulfur, as a result of solar heating in the early formation of the Solar System. Oxygen, the most abundant Earth element by mass, is retained on Earth by combination with silicon. Aluminium at 8% by mass is more common in the Earth's crust than in the universe and solar system, but the composition of the far more bulky mantle, which has magnesium and iron in place of aluminium (which occurs there only at 2% of mass) more closely mirrors the elemental composition of the solar system, save for the noted loss of volatile elements to space, and loss of iron which has migrated to the Earth's core.

The composition of the human body, by contrast, more closely follows the composition of seawater—save that the human body has additional stores of carbon and nitrogen necessary to form the proteins and nucleic acids, together with phosphorus in the nucleic acids and energy transfer molecule adenosine triphosphate (ATP) that occurs in the cells of all living organisms. Certain kinds of organisms require particular additional elements, for example the magnesium in chlorophyll in green plants, the calcium in mollusc shells, or the iron in the hemoglobin in vertebrates' red blood cells.

Elements in our galaxy	Parts per million by mass
Hydrogen	739,000
Helium	240,000
Oxygen	10,400
Carbon	4,600
Neon	1,340
Iron	1,090
Nitrogen	960
Silicon	650
Magnesium	580
Sulfur	440
Potassium	210
Nickel	100

The concept of an "element" as an indivisible substance has developed through three major historical phases: Classical definitions (such as those of the ancient Greeks), chemical definitions, and atomic definitions.

Ancient philosophy posited a set of classical elements to explain observed patterns in nature. These elements originally referred to earth, water, air and fire rather than the chemical elements of modern science.

The term 'elements' (stoicheia) was first used by Greek philosopher Plato around 360 BCE in his dialogue Timaeus, which includes a discussion of the composition of inorganic and organic bodies and is a speculative treatise on chemistry. Plato believed the elements introduced a century earlier by Empedocles were composed of small polyhedral forms: tetrahedron (fire), octahedron (air), icosahedron (water), and cube (earth).

Aristotle, c. 350 BCE, also used the term stoicheia and added a fifth element, aether, which formed the heavens. Aristotle defined an element as:

Element – one of those bodies into which other bodies can decompose, and that itself is not capable of being divided into other.

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In 1661, in The Sceptical Chymist, Robert Boyle proposed his theory of corpuscularism which favoured the analysis of matter as constituted of irreducible units of matter (atoms); and, choosing to side with neither Aristotle's view of the four elements nor Paracelsus' view of three fundamental elements, left open the question of the number of elements. Boyle argued against a pre-determined number of elements—directly against Paracelsus' three principles (sulfur, mercury, and salt), indirectly against the "Aristotelian" elements (earth, water, air, and fire), for Boyle felt that the arguments against the former were at least as valid against the latter.

Much of what I am to deliver ... may be indifferently apply'd to the four Peripatetick Elements, and the three Chymical Principles ... the Chymical Hypothesis seeming to be much more countenanc'd by Experience then the other, it will be expedient to insist chiefly upon the disproving of that; especially since most of the Arguments that are imploy'd against it, may, by a little variation, be made ... at least as strongly against the less plausible, Aristotelian Doctrine.

Then Boyle stated his view in four propositions. In the first and second, he suggests that matter consists of particles, but that these particles may be difficult to separate. Boyle used the concept of "corpuscles"—or "atomes", as he also called them—to explain how a limited number of elements could combine into a vast number of compounds.

Propos. I. ... At the first Production of mixt Bodies, the Universal Matter whereof they ... consisted, was actually divided into little Particles. ... The Generation ... and wasting of Bodies ... and ... the Chymical Resolutions of mixt Bodies, and ... Operations of ... Fires upon them ... manifest their consisting of parts very minute... Epicurus ... as you well know, supposes ... all ... Bodies ... to be produc'd by ... Atomes, moving themselves to and fro ... in the ... Infinite Vacuum. ... Propos. II. ... These minute Particles ... were ... associated into minute ... Clusters ... not easily dissipable into such Particles as compos'd them. ... If we assigne to the Corpuscles, whereof each Element consists, a peculiar size and shape ... such ... Corpuscles may be mingled in such various Proportions, and ... connected so many ... wayes, that an almost incredible number of ... Concretes may be compos'd of them.

Boyle explained that gold reacts with aqua regia, and mercury with nitric acid, sulfuric acid, and sulfur to produce various "compounds", and that they could be recovered from those compounds, just as would be expected of elements. Yet, Boyle did not consider gold, mercury, or lead elements, but rather—together with wine—"perfectly mixt bodies".

Quicksilver ... with Aqua fortis will be brought into a ... white Powder ... with Sulphur it will compose a blood-red ... Cinaber. And yet out of all these exotick Compounds, we may recover the very same running Mercury. ... Propos. III. ... From most of such mixt Bodies ... there may by the Help of the Fire, be actually obtain'd a determinate number (whether Three, Four or Five, or fewer or more) of Substances ... The Chymists are wont to call the Ingredients of mixt Bodies, Principles, as the Aristotelians name them Elements. ... Principles ... as not being compounded of any more primary Bodies: and Elements, in regard that all mix'd Bodies are compounded of them.

Even though Boyle is primarily regarded as the first modern chemist, The Sceptical Chymist still contains old ideas about the elements, alien to a contemporary viewpoint. Sulfur, for example, is not only the familiar yellow non-metal but also an inflammable "spirit".

In 1724, in his book Logick, the English minister and logician Isaac Watts enumerated the elements then recognized by chemists. Watts' list of elements included two of Paracelsus' principles (sulfur and salt) and two classical elements (earth and water) as well as "spirit". Watts did, however, note a lack of consensus among chemists.

Elements are such Substances as cannot be resolved, or reduced, into two or more Substances of different Kinds. ... Followers of Aristotle made Fire, Air, Earth and Water to be the four Elements, of which all earthly Things were compounded; and they suppos'd the Heavens to be a Quintessence, or fifth sort of Body, distinct from all these : But, since experimental Philosophy ... have been better understood, this Doctrine has been abundantly refuted. The Chymists make Spirit, Salt, Sulphur, Water and Earth to be their five Elements, because they can reduce all terrestrial Things to these five :.. tho' they are not all agreed.

The first modern list of elements was given in Antoine Lavoisier's 1789 Elements of Chemistry, which contained 33 elements, including light and caloric. By 1818, Jöns Jacob Berzelius had determined atomic weights for 45 of the 49 then-accepted elements. Dmitri Mendeleev had 63 elements in his 1869 periodic table.

From Boyle until the early 20th century, an element was defined as a pure substance that cannot be decomposed into any simpler substance and cannot be transformed into other elements by chemical processes. Elements at the time were generally distinguished by their atomic weights, a property measurable with fair accuracy by available analytical techniques.

The 1913 discovery by English physicist Henry Moseley that the nuclear charge is the physical basis for the atomic number, further refined when the nature of protons and neutrons became appreciated, eventually led to the current definition of an element based on atomic number (number of protons). The use of atomic numbers, rather than atomic weights, to distinguish elements has greater predictive value (since these numbers are integers) and also resolves some ambiguities in the chemistry-based view due to varying properties of isotopes and allotropes within the same element. Currently, IUPAC defines an element to exist if it has isotopes with a lifetime longer than the 10 seconds it takes the nucleus to form an electronic cloud.

By 1914, eighty-seven elements were known, all naturally occurring (see Discovery of chemical elements). The remaining naturally occurring elements were discovered or isolated in subsequent decades, and various additional elements have also been produced synthetically, with much of that work pioneered by Glenn T. Seaborg. In 1955, element 101 was discovered and named mendelevium in honor of D. I. Mendeleev, the first to arrange the elements periodically.

Ten materials familiar to various prehistoric cultures are now known to be elements: Carbon, copper, gold, iron, lead, mercury, silver, sulfur, tin, and zinc. Three additional materials now accepted as elements, arsenic, antimony, and bismuth, were recognized as distinct substances before 1500 AD. Phosphorus, cobalt, and platinum were isolated before 1750.

Most of the remaining naturally occurring elements were identified and characterized by 1900, including:

• Such now-familiar industrial materials as aluminium, silicon, nickel, chromium, magnesium, and tungsten
• Reactive metals such as lithium, sodium, potassium, and calcium
• The halogens fluorine, chlorine, bromine, and iodine
• Gases such as hydrogen, oxygen, nitrogen, helium, argon, and neon
• Most of the rare-earth elements, including cerium, lanthanum, gadolinium, and neodymium
• The more common radioactive elements, including uranium, thorium, and radium

Elements isolated or produced since 1900 include:

• The three remaining undiscovered stable elements: hafnium, lutetium, and rhenium
• Plutonium, which was first produced synthetically in 1940 by Glenn T. Seaborg, but is now also known from a few long-persisting natural occurrences
• The three incidentally occurring natural elements (neptunium, promethium, and technetium), which were all first produced synthetically but later discovered in trace amounts in geological samples
• Four scarce decay products of uranium or thorium (astatine, francium, actinium, and protactinium), and
• All synthetic transuranic elements, beginning with americium and curium

The first transuranium element (element with an atomic number greater than 92) discovered was neptunium in 1940. Since 1999, the IUPAC/IUPAP Joint Working Party has considered claims for the discovery of new elements. As of January 2016, all 118 elements have been confirmed by IUPAC as being discovered. The discovery of element 112 was acknowledged in 2009, and the name copernicium and the chemical symbol Cn were suggested for it. The name and symbol were officially endorsed by IUPAC on 19 February 2010. The heaviest element that is believed to have been synthesized to date is element 118, oganesson, on 9 October 2006, by the Flerov Laboratory of Nuclear Reactions in Dubna, Russia. Tennessine, element 117 was the latest element claimed to be discovered, in 2009. On 28 November 2016, scientists at the IUPAC officially recognized the names for the four newest elements, with atomic numbers 113, 115, 117, and 118.

The following sortable table shows the 118 known elements.

• Atomic number, Element, and Symbol all serve independently as unique identifiers.
• Element names are those accepted by IUPAC.
• Block indicates the periodic table block for each element: red = s-block, yellow = p-block, blue = d-block, green = f-block.
• Group and period refer to an element's position in the periodic table. Group numbers here show the currently accepted numbering; for older numberings, see Group (periodic table).

v t e List of chemical elements
Element			Origin of name	Group	Period	Block	Standard atomic weight Ar°(E)	Density	Melting point	Boiling point	Specific heat capacity	Electro­negativity	Abundance in Earth's crust	Origin	Phase at r.t.
Atomic number Z	Symbol	Name					(Da)	(⁠g/cm⁠)	(K)	(K)	(⁠J/g · K⁠)		(⁠mg/kg⁠)
1	H	Hydrogen	Greek roots hydro- + -gen, 'water-forming'	1	1	s-block	1.0080	0.00008988	14.01	20.28	14.304	2.20	1400	primordial	gas
2	He	Helium	Greek hḗlios 'sun'	18	1	s-block	4.0026	0.0001785	–	4.22	5.193	–	0.008	primordial	gas
3	Li	Lithium	Greek líthos 'stone'	1	2	s-block	6.94	0.534	453.69	1560	3.582	0.98	20	primordial	solid
4	Be	Beryllium	Beryl, mineral (ultimately after Belur, Karnataka, India?)	2	2	s-block	9.0122	1.85	1560	2742	1.825	1.57	2.8	primordial	solid
5	B	Boron	Borax, mineral (from Arabic: bawraq, Middle Persian: *bōrag)	13	2	p-block	10.81	2.34	2349	4200	1.026	2.04	10	primordial	solid
6	C	Carbon	Latin carbo 'coal'	14	2	p-block	12.011	2.267	>4000	4300	0.709	2.55	200	primordial	solid
7	N	Nitrogen	Greek nítron + -gen, 'niter-forming'	15	2	p-block	14.007	0.0012506	63.15	77.36	1.04	3.04	19	primordial	gas
8	O	Oxygen	Greek oxy- + -gen, 'acid-forming'	16	2	p-block	15.999	0.001429	54.36	90.20	0.918	3.44	461000	primordial	gas
9	F	Fluorine	Latin fluo 'to flow'	17	2	p-block	18.998	0.001696	53.53	85.03	0.824	3.98	585	primordial	gas
10	Ne	Neon	Greek néon 'new'	18	2	p-block	20.180	0.0009002	24.56	27.07	1.03	–	0.005	primordial	gas
11	Na	Sodium	Coined by Humphry Davy who first isolated it, from English soda (specifically caustic soda), via Italian from Arabic ṣudāʕ 'headache'  ·  Symbol Na, from Neo-Latin natrium, coined from German Natron 'natron'	1	3	s-block	22.990	0.968	370.87	1156	1.228	0.93	23600	primordial	solid
12	Mg	Magnesium	Magnesia region, eastern Thessaly, Greece	2	3	s-block	24.305	1.738	923	1363	1.023	1.31	23300	primordial	solid
13	Al	Aluminium	Alumina, from Latin alumen (gen. aluminis) 'bitter salt, alum'	13	3	p-block	26.982	2.70	933.47	2792	0.897	1.61	82300	primordial	solid
14	Si	Silicon	Latin silex 'flint' (originally silicium)	14	3	p-block	28.085	2.3290	1687	3538	0.705	1.9	282000	primordial	solid
15	P	Phosphorus	Greek phōsphóros 'light-bearing'	15	3	p-block	30.974	1.823	317.30	550	0.769	2.19	1050	primordial	solid
16	S	Sulfur	Latin	16	3	p-block	32.06	2.07	388.36	717.87	0.71	2.58	350	primordial	solid
17	Cl	Chlorine	Greek chlōrós 'greenish yellow'	17	3	p-block	35.45	0.0032	171.6	239.11	0.479	3.16	145	primordial	gas
18	Ar	Argon	Greek argós 'idle' (it is inert)	18	3	p-block	39.95	0.001784	83.80	87.30	0.52	–	3.5	primordial	gas
19	K	Potassium	Neo-Latin potassa 'potash', from pot + ash  ·  Symbol K, from Neo-Latin kalium, from German	1	4	s-block	39.098	0.89	336.53	1032	0.757	0.82	20900	primordial	solid
20	Ca	Calcium	Latin calx 'lime'	2	4	s-block	40.078	1.55	1115	1757	0.647	1.00	41500	primordial	solid
21	Sc	Scandium	Latin Scandia 'Scandinavia'	3	4	d-block	44.956	2.985	1814	3109	0.568	1.36	22	primordial	solid
22	Ti	Titanium	Titans, children of Gaia and Ouranos	4	4	d-block	47.867	4.506	1941	3560	0.523	1.54	5650	primordial	solid
23	V	Vanadium	Vanadis, a name for Norse goddess Freyja	5	4	d-block	50.942	6.11	2183	3680	0.489	1.63	120	primordial	solid
24	Cr	Chromium	Greek chróma 'color'	6	4	d-block	51.996	7.15	2180	2944	0.449	1.66	102	primordial	solid
25	Mn	Manganese	Corrupted from magnesia negra; see magnesium	7	4	d-block	54.938	7.21	1519	2334	0.479	1.55	950	primordial	solid
26	Fe	Iron	English, from Proto-Celtic *īsarnom 'iron', from a root meaning 'blood'  ·  Symbol Fe, from Latin ferrum	8	4	d-block	55.845	7.874	1811	3134	0.449	1.83	56300	primordial	solid
27	Co	Cobalt	German Kobold, 'goblin'	9	4	d-block	58.933	8.90	1768	3200	0.421	1.88	25	primordial	solid
28	Ni	Nickel	Nickel, a mischievous sprite in German miner mythology	10	4	d-block	58.693	8.908	1728	3186	0.444	1.91	84	primordial	solid
29	Cu	Copper	English, from Latin cuprum, after Cyprus	11	4	d-block	63.546	8.96	1357.77	2835	0.385	1.90	60	primordial	solid
30	Zn	Zinc	Most likely German Zinke, 'prong, tooth', but some suggest Persian sang 'stone'	12	4	d-block	65.38	7.14	692.88	1180	0.388	1.65	70	primordial	solid
31	Ga	Gallium	Latin Gallia 'France'	13	4	p-block	69.723	5.91	302.9146	2673	0.371	1.81	19	primordial	solid
32	Ge	Germanium	Latin Germania 'Germany'	14	4	p-block	72.630	5.323	1211.40	3106	0.32	2.01	1.5	primordial	solid
33	As	Arsenic	Middle English, from Middle French arsenic, from Greek arsenikón 'yellow arsenic' (influenced by arsenikós 'masculine, virile'), from a West Asian wanderword ultimately from Old Persian: *zarniya-ka, lit. 'golden'	15	4	p-block	74.922	5.727	1090	887	0.329	2.18	1.8	primordial	solid
34	Se	Selenium	Greek selḗnē 'moon'	16	4	p-block	78.971	4.81	453	958	0.321	2.55	0.05	primordial	solid
35	Br	Bromine	Greek brômos 'stench'	17	4	p-block	79.904	3.1028	265.8	332.0	0.474	2.96	2.4	primordial	liquid
36	Kr	Krypton	Greek kryptós 'hidden'	18	4	p-block	83.798	0.003749	115.79	119.93	0.248	3.00	1×10	primordial	gas
37	Rb	Rubidium	Latin rubidus 'deep red'	1	5	s-block	85.468	1.532	312.46	961	0.363	0.82	90	primordial	solid
38	Sr	Strontium	Strontian, a village in Scotland, where it was found	2	5	s-block	87.62	2.64	1050	1655	0.301	0.95	370	primordial	solid
39	Y	Yttrium	Ytterby, Sweden, where it was found; see terbium, erbium, ytterbium	3	5	d-block	88.906	4.472	1799	3609	0.298	1.22	33	primordial	solid
40	Zr	Zirconium	Zircon, mineral, from Persian zargun 'gold-hued'	4	5	d-block	91.224	6.52	2128	4682	0.278	1.33	165	primordial	solid
41	Nb	Niobium	Niobe, daughter of king Tantalus in Greek myth; see tantalum	5	5	d-block	92.906	8.57	2750	5017	0.265	1.6	20	primordial	solid
42	Mo	Molybdenum	Greek molýbdaina 'piece of lead', from mólybdos 'lead', due to confusion with lead ore galena (PbS)	6	5	d-block	95.95	10.28	2896	4912	0.251	2.16	1.2	primordial	solid
43	Tc	Technetium	Greek tekhnētós 'artificial'	7	5	d-block	[97]	11	2430	4538	–	1.9	~ 3×10	from decay	solid
44	Ru	Ruthenium	Neo-Latin Ruthenia 'Russia'	8	5	d-block	101.07	12.45	2607	4423	0.238	2.2	0.001	primordial	solid
45	Rh	Rhodium	Greek rhodóeis 'rose-colored', from rhódon 'rose'	9	5	d-block	102.91	12.41	2237	3968	0.243	2.28	0.001	primordial	solid
46	Pd	Palladium	Pallas, asteroid, then considered a planet	10	5	d-block	106.42	12.023	1828.05	3236	0.244	2.20	0.015	primordial	solid
47	Ag	Silver	English, from Proto-Germanic  ·  Symbol Ag, from Latin argentum	11	5	d-block	107.87	10.49	1234.93	2435	0.235	1.93	0.075	primordial	solid
48	Cd	Cadmium	Neo-Latin cadmia 'calamine', from King Cadmus, mythic founder of Thebes	12	5	d-block	112.41	8.65	594.22	1040	0.232	1.69	0.159	primordial	solid
49	In	Indium	Latin indicum 'indigo', the blue color named after India and observed in its spectral lines	13	5	p-block	114.82	7.31	429.75	2345	0.233	1.78	0.25	primordial	solid
50	Sn	Tin	English, from Proto-Germanic  ·  Symbol Sn, from Latin stannum	14	5	p-block	118.71	7.265	505.08	2875	0.228	1.96	2.3	primordial	solid
51	Sb	Antimony	Latin antimonium, of unclear origin: folk etymologies suggest Greek antí 'against' + mónos 'alone', or Old French anti-moine 'monk's bane', but could be from or related to Arabic ʾiṯmid 'antimony'  ·  Symbol Sb, from Latin stibium 'stibnite'	15	5	p-block	121.76	6.697	903.78	1860	0.207	2.05	0.2	primordial	solid
52	Te	Tellurium	Latin tellus 'ground, earth'	16	5	p-block	127.60	6.24	722.66	1261	0.202	2.1	0.001	primordial	solid
53	I	Iodine	French iode, from Greek ioeidḗs 'violet'	17	5	p-block	126.90	4.933	386.85	457.4	0.214	2.66	0.45	primordial	solid
54	Xe	Xenon	Greek xénon, neuter of xénos 'strange, foreign'	18	5	p-block	131.29	0.005894	161.4	165.03	0.158	2.60	3×10	primordial	gas
55	Cs	Caesium	Latin caesius 'sky-blue'	1	6	s-block	132.91	1.93	301.59	944	0.242	0.79	3	primordial	solid
56	Ba	Barium	Greek barýs 'heavy'	2	6	s-block	137.33	3.51	1000	2170	0.204	0.89	425	primordial	solid
57	La	Lanthanum	Greek lanthánein 'to lie hidden'	f-block groups	6	f-block	138.91	6.162	1193	3737	0.195	1.1	39	primordial	solid
58	Ce	Cerium	Ceres (dwarf planet), then considered a planet	f-block groups	6	f-block	140.12	6.770	1068	3716	0.192	1.12	66.5	primordial	solid
59	Pr	Praseodymium	Greek prásios dídymos 'green twin'	f-block groups	6	f-block	140.91	6.77	1208	3793	0.193	1.13	9.2	primordial	solid
60	Nd	Neodymium	Greek néos dídymos 'new twin'	f-block groups	6	f-block	144.24	7.01	1297	3347	0.19	1.14	41.5	primordial	solid
61	Pm	Promethium	Prometheus, a Titan	f-block groups	6	f-block	[145]	7.26	1315	3273	–	1.13	2×10	from decay	solid
62	Sm	Samarium	Samarskite, a mineral named after V. Samarsky-Bykhovets, Russian mine official	f-block groups	6	f-block	150.36	7.52	1345	2067	0.197	1.17	7.05	primordial	solid
63	Eu	Europium	Europe	f-block groups	6	f-block	151.96	5.244	1099	1802	0.182	1.2	2	primordial	solid
64	Gd	Gadolinium	Gadolinite, a mineral named after Johan Gadolin, Finnish chemist, physicist and mineralogist	f-block groups	6	f-block	157.25	7.90	1585	3546	0.236	1.2	6.2	primordial	solid
65	Tb	Terbium	Ytterby, Sweden, where it was found; see yttrium, erbium, ytterbium	f-block groups	6	f-block	158.93	8.23	1629	3503	0.182	1.2	1.2	primordial	solid
66	Dy	Dysprosium	Greek dysprósitos 'hard to get'	f-block groups	6	f-block	162.50	8.540	1680	2840	0.17	1.22	5.2	primordial	solid
67	Ho	Holmium	Neo-Latin Holmia 'Stockholm'	f-block groups	6	f-block	164.93	8.79	1734	2993	0.165	1.23	1.3	primordial	solid
68	Er	Erbium	Ytterby, where it was found; see yttrium, terbium, ytterbium	f-block groups	6	f-block	167.26	9.066	1802	3141	0.168	1.24	3.5	primordial	solid
69	Tm	Thulium	Thule, the ancient name for an unclear northern location	f-block groups	6	f-block	168.93	9.32	1818	2223	0.16	1.25	0.52	primordial	solid
70	Yb	Ytterbium	Ytterby, where it was found; see yttrium, terbium, erbium	f-block groups	6	f-block	173.05	6.90	1097	1469	0.155	1.1	3.2	primordial	solid
71	Lu	Lutetium	Latin Lutetia 'Paris'	3	6	d-block	174.97	9.841	1925	3675	0.154	1.27	0.8	primordial	solid
72	Hf	Hafnium	Neo-Latin Hafnia 'Copenhagen' (from Danish havn, harbor)	4	6	d-block	178.49	13.31	2506	4876	0.144	1.3	3	primordial	solid
73	Ta	Tantalum	King Tantalus, father of Niobe in Greek myth; see niobium	5	6	d-block	180.95	16.69	3290	5731	0.14	1.5	2	primordial	solid
74	W	Tungsten	Swedish tung sten 'heavy stone'  ·  Symbol W, from Wolfram, from Middle High German wolf-rahm 'wolf's foam' describing the mineral wolframite	6	6	d-block	183.84	19.25	3695	6203	0.132	2.36	1.3	primordial	solid
75	Re	Rhenium	Latin Rhenus 'Rhine'	7	6	d-block	186.21	21.02	3459	5869	0.137	1.9	7×10	primordial	solid
76	Os	Osmium	Greek osmḗ 'smell'	8	6	d-block	190.23	22.59	3306	5285	0.13	2.2	0.002	primordial	solid
77	Ir	Iridium	Iris, Greek goddess of rainbow	9	6	d-block	192.22	22.56	2719	4701	0.131	2.20	0.001	primordial	solid
78	Pt	Platinum	Spanish platina 'little silver', from plata 'silver'	10	6	d-block	195.08	21.45	2041.4	4098	0.133	2.28	0.005	primordial	solid
79	Au	Gold	English, from same Proto-Indo-European root as 'yellow'  ·  Symbol Au, from Latin aurum	11	6	d-block	196.97	19.3	1337.33	3129	0.129	2.54	0.004	primordial	solid
80	Hg	Mercury	Mercury, Roman god of commerce, communication, and luck, known for his speed and mobility  ·  Symbol Hg, from Latin hydrargyrum, from Greek hydrárgyros 'water-silver'	12	6	d-block	200.59	13.534	234.43	629.88	0.14	2.00	0.085	primordial	liquid
81	Tl	Thallium	Greek thallós 'green shoot / twig'	13	6	p-block	204.38	11.85	577	1746	0.129	1.62	0.85	primordial	solid
82	Pb	Lead	English, from Proto-Celtic *ɸloudom, from a root meaning 'flow'  ·  Symbol Pb, from Latin plumbum	14	6	p-block	207.2	11.34	600.61	2022	0.129	1.87 (2+) 2.33 (4+)	14	primordial	solid
83	Bi	Bismuth	German Wismut, via Latin and Arabic from Greek psimúthion 'white lead'	15	6	p-block	208.98	9.78	544.7	1837	0.122	2.02	0.009	primordial	solid
84	Po	Polonium	Latin Polonia 'Poland', home country of discoverer Marie Curie	16	6	p-block	[209]	9.196	527	1235	–	2.0	2×10	from decay	solid
85	At	Astatine	Greek ástatos 'unstable'; it has no stable isotopes	17	6	p-block	[210]	(8.91–8.95)	575	610	–	2.2	3×10	from decay	unknown phase
86	Rn	Radon	Radium emanation, originally the name of Rn	18	6	p-block	[222]	0.00973	202	211.3	0.094	2.2	4×10	from decay	gas
87	Fr	Francium	France, home country of discoverer Marguerite Perey	1	7	s-block	[223]	(2.48)	281	890	–	>0.79	~ 1×10	from decay	unknown phase
88	Ra	Radium	Coined in French by discoverer Marie Curie, from Latin radius 'ray'	2	7	s-block	[226]	5.5	973	2010	0.094	0.9	9×10	from decay	solid
89	Ac	Actinium	Greek aktís 'ray'	f-block groups	7	f-block	[227]	10	1323	3471	0.12	1.1	5.5×10	from decay	solid
90	Th	Thorium	Thor, the Norse god of thunder	f-block groups	7	f-block	232.04	11.7	2115	5061	0.113	1.3	9.6	primordial	solid
91	Pa	Protactinium	English prefix proto- (from Greek prôtos 'first, before') + actinium; protactinium decays into actinium.	f-block groups	7	f-block	231.04	15.37	1841	4300	–	1.5	1.4×10	from decay	solid
92	U	Uranium	Uranus, the seventh planet	f-block groups	7	f-block	238.03	19.1	1405.3	4404	0.116	1.38	2.7	primordial	solid
93	Np	Neptunium	Neptune, the eighth planet	f-block groups	7	f-block	[237]	20.45	917	4273	–	1.36	≤ 3×10	from decay	solid
94	Pu	Plutonium	Pluto, dwarf planet, then considered a planet	f-block groups	7	f-block	[244]	19.85	912.5	3501	–	1.28	≤ 3×10	from decay	solid
95	Am	Americium	Americas, where the element was first synthesized, by analogy with its homolog europium	f-block groups	7	f-block	[243]	12	1449	2880	–	1.13	–	synthetic	solid
96	Cm	Curium	Pierre and Marie Curie, physicists and chemists	f-block groups	7	f-block	[247]	13.51	1613	3383	–	1.28	–	synthetic	solid
97	Bk	Berkelium	Berkeley, California, where it was first synthesized	f-block groups	7	f-block	[247]	14.78	1259	2900	–	1.3	–	synthetic	solid
98	Cf	Californium	California, where it was first synthesized in LBNL	f-block groups	7	f-block	[251]	15.1	1173	(1743)	–	1.3	–	synthetic	solid
99	Es	Einsteinium	Albert Einstein, German physicist	f-block groups	7	f-block	[252]	8.84	1133	(1269)	–	1.3	–	synthetic	solid
100	Fm	Fermium	Enrico Fermi, Italian physicist	f-block groups	7	f-block	[257]	(9.7)	(1125) (1800)	–	–	1.3	–	synthetic	unknown phase
101	Md	Mendelevium	Dmitri Mendeleev, Russian chemist who proposed the periodic table	f-block groups	7	f-block	[258]	(10.3)	(1100)	–	–	1.3	–	synthetic	unknown phase
102	No	Nobelium	Alfred Nobel, Swedish chemist and engineer	f-block groups	7	f-block	[259]	(9.9)	(1100)	–	–	1.3	–	synthetic	unknown phase
103	Lr	Lawrencium	Ernest Lawrence, American physicist	3	7	d-block	[266]	(14.4)	(1900)	–	–	1.3	–	synthetic	unknown phase
104	Rf	Rutherfordium	Ernest Rutherford, chemist and physicist from New Zealand	4	7	d-block	[267]	(17)	(2400)	(5800)	–	–	–	synthetic	unknown phase
105	Db	Dubnium	Dubna, Russia, where it was discovered in JINR	5	7	d-block	[268]	(21.6)	–	–	–	–	–	synthetic	unknown phase
106	Sg	Seaborgium	Glenn Seaborg, American chemist	6	7	d-block	[267]	(23–24)	–	–	–	–	–	synthetic	unknown phase
107	Bh	Bohrium	Niels Bohr, Danish physicist	7	7	d-block	[270]	(26–27)	–	–	–	–	–	synthetic	unknown phase
108	Hs	Hassium	Neo-Latin Hassia 'Hesse', a state in Germany	8	7	d-block	[271]	(27–29)	–	–	–	–	–	synthetic	unknown phase
109	Mt	Meitnerium	Lise Meitner, Austrian physicist	9	7	d-block	[278]	(27–28)	–	–	–	–	–	synthetic	unknown phase
110	Ds	Darmstadtium	Darmstadt, Germany, where it was first synthesized in the GSI labs	10	7	d-block	[281]	(26–27)	–	–	–	–	–	synthetic	unknown phase
111	Rg	Roentgenium	Wilhelm Röntgen, German physicist	11	7	d-block	[282]	(22–24)	–	–	–	–	–	synthetic	unknown phase
112	Cn	Copernicium	Nicolaus Copernicus, Polish astronomer	12	7	d-block	[285]	(14.0)	(283±11)	(340±10)	–	–	–	synthetic	unknown phase
113	Nh	Nihonium	Japanese Nihon 'Japan', where it was first synthesized in Riken	13	7	p-block	[286]	(16)	(700)	(1400)	–	–	–	synthetic	unknown phase
114	Fl	Flerovium	Flerov Laboratory of Nuclear Reactions, part of JINR, where it was synthesized; itself named after Georgy Flyorov, Russian physicist	14	7	p-block	[289]	(11.4±0.3)	(284±50)	–	–	–	–	synthetic	unknown phase
115	Mc	Moscovium	Moscow, Russia, where it was first synthesized in JINR	15	7	p-block	[290]	(13.5)	(700)	(1400)	–	–	–	synthetic	unknown phase
116	Lv	Livermorium	Lawrence Livermore National Laboratory in Livermore, California	16	7	p-block	[293]	(12.9)	(700)	(1100)	–	–	–	synthetic	unknown phase
117	Ts	Tennessine	Tennessee, US, home to ORNL	17	7	p-block	[294]	(7.1–7.3)	(700)	(883)	–	–	–	synthetic	unknown phase
118	Og	Oganesson	Yuri Oganessian, Russian physicist	18	7	p-block	[294]	(7)	(325±15)	(450±10)	–	–	–	synthetic	unknown phase

• Boyle, R. (1661). The Sceptical Chymist: or Chymico-Physical Doubts & Paradoxes, Touching the Spagyrist's Principles Commonly call'd Hypostatical; As they are wont to be Propos'd and Defended by the Generality of Alchymists. Whereunto is præmis'd Part of another Discourse relating to the same Subject. Printed by J. Cadwell for J. Crooke.

• Ball, P. (2004). The Elements: A Very Short Introduction. Oxford University Press. ISBN 978-0-19-284099-8.

• Emsley, J. (2003). Nature's Building Blocks: An A–Z Guide to the Elements. Oxford University Press. ISBN 978-0-19-850340-8.
• Gray, T. (2009). The Elements: A Visual Exploration of Every Known Atom in the Universe. Black Dog & Leventhal Publishers Inc. ISBN 978-1-57912-814-2.
• Scerri, E.R. (2007). The Periodic Table, Its Story and Its Significance. Oxford University Press. ISBN 978-0-19-530573-9.
• Strathern, P. (2000). Mendeleyev's Dream: The Quest for the Elements. Hamish Hamilton Ltd. ISBN 978-0-241-14065-9.
• Kean, Sam (2011). The Disappearing Spoon: And Other True Tales of Madness, Love, and the History of the World from the Periodic Table of the Elements. Back Bay Books.
• A.D. McNaught; A. Wilkinson, eds. (1997). Compendium of Chemical Terminology (2nd ed.). Oxford: Blackwell Scientific Publications. doi:10.1351/goldbook. ISBN 978-0-9678550-9-7. XML on-line corrected version: created by M. Nic, J. Jirat, B. Kosata; updates compiled by A. Jenkins

• Videos for each element by the University of Nottingham
• "Chemical Elements", In Our Time, BBC Radio 4 discussion with Paul Strathern, Mary Archer and John Murrell (25 May 2000)