A chemical element is a species of atoms whose nuclei contain the same number of protons. This number, called the atomic number and represented by Z, uniquely identifies the element. Hydrogen has atomic number 1, carbon 6, and oxygen 8. The term also refers to a pure substance composed entirely of atoms of one element. Elements are fundamental categories in chemistry: ordinary chemical processes can change their combinations and physical forms but cannot change one element into another. (goldbook.iupac.org)
Atomic identity and isotopes
An atomic nucleus contains protons and, except in ordinary hydrogen, neutrons. Surrounding electrons determine much of an atom’s chemical behavior. A neutral atom has equal numbers of protons and electrons; gaining or losing electrons produces an ion without changing the element’s identity. Thus, a sodium atom and a sodium ion remain forms of the same element. (openstax.org)
Atoms of one element can contain different numbers of neutrons. These variants are isotopes, distinguished by their mass number, A, the total number of protons and neutrons. Carbon-12 and carbon-13 both contain six protons but contain six and seven neutrons, respectively. Isotopes generally have similar chemical properties, although their masses and nuclear stability differ. (openstax.org)
An element’s atomic weight reflects the masses and relative abundances of its isotopes rather than simply counting nuclear particles. Consequently, tabulated values are often nonintegers. IUPAC expresses some standard atomic weights as intervals because natural isotopic composition varies among terrestrial materials. For elements lacking a characteristic natural isotopic abundance, periodic tables commonly provide a selected isotope’s mass number in brackets instead. (openstax.org)
Elements, compounds, and elemental forms
An element must be distinguished from a chemical compound, which contains atoms of two or more elements chemically combined in definite proportions. Water, H₂O, is a compound of hydrogen and oxygen. Oxygen gas, O₂, is an elemental substance even though its particles are molecules containing two atoms. A mixture, by contrast, contains substances whose relative amounts can vary and which retain their individual chemical identities. (openstax.org)
Some elements exhibit allotropy: they occur in different structural forms. Diamond and graphite are both elemental carbon, but their different atomic arrangements produce markedly different properties. Allotropes should not be confused with isotopes: allotropy concerns structure, whereas isotopic variation concerns neutron number. An element’s identity therefore does not specify a single material with one fixed set of bulk properties. (goldbook.iupac.org)
During a chemical reaction, atoms are rearranged as chemical bonds form or break. The identities and numbers of atoms of each element are conserved in ordinary chemical reactions. Transforming one element into another requires a nuclear process that changes proton number; such transformations can occur through radioactive decay or nuclear reactions. (openstax.org)
Periodic classification
The periodic table organizes elements by increasing atomic number. Its rows are periods, and its columns are groups. Recurrent patterns in electron configuration, particularly among outer electrons, explain why elements in the same group often have related chemical properties. The familiar arrangement contains seven periods and eighteen numbered groups. (openstax.org)
Elements are broadly classified as metals, nonmetals, and metalloids. Metals commonly conduct electricity and can be shaped without fracturing; nonmetals show a wider range of contrasting properties. Metalloids have combinations of characteristics associated with both categories. These classifications are useful descriptions rather than definitions as precise as atomic number. (openstax.org)
Periodic trends include changes in atomic size and ionization energy, the energy required to remove an electron from a gaseous atom or ion. Across many periods, atomic radius generally decreases while first ionization energy generally increases. Down a group, atoms generally become larger. Such trends have exceptions arising from electron arrangements and interactions, but they help explain differences in chemical reactivity. (openstax.org)
Historical development
The modern concept emerged through experimental distinctions between simple substances and compounds. In 1789, Antoine Lavoisier published a list of substances regarded as elementary because available methods could not decompose them. His list also included light, caloric, and substances subsequently identified as compounds, illustrating the provisional character of early classification. John Dalton later connected elements with distinct kinds of atoms and their relative masses. (rsc.org)
In 1869, Dmitri Mendeleev arranged elements principally by atomic weight and recurring properties, leaving gaps for undiscovered elements. Henry Moseley’s measurements of characteristic X-rays in 1913 established atomic number as the physically meaningful ordering principle. This resolved discrepancies between chemical similarities and strict atomic-weight order. (periodic-table.rsc.org)
Origins and recognition
Elements originate through nucleosynthesis, the formation of atomic nuclei. Processes associated with the Big Bang produced mainly hydrogen and helium, with smaller quantities of other light nuclei. Nuclear fusion in stars subsequently formed heavier nuclei. Neutron-capture processes also build heavy nuclei, with rapid capture occurring in environments including neutron-star mergers. Different processes contribute to the observed distribution of elements and isotopes. (energy.gov)
The IUPAC periodic table recognizes 118 named elements, from hydrogen to oganesson. Many occur naturally, while others were first identified through artificial production. Recognition of a new element requires convincing experimental evidence of nuclei with a previously unestablished atomic number. Discovery assessment is distinct from naming: evidence must establish identity before an official name and symbol are adopted. The names of elements 113, 115, 117, and 118 received final approval on November 28, 2016. (iupac.org)