Chemical element

Tap: the periodic table o the chemical elements. Ablow: some examples o chemical elements. Frae left tae richt: hydrogen, barium, capper, uranium, bromine an helium.

A chemical element is a kind o atom defined bi the nummer o protons in its nucleus (that is, its atomic nummer, or Z).^[1]

For example, oxygen haes an atomic nummer o 8: ilka oxygen atom haes 8 protons in its nucleus. Atoms o the same element can hae different nummers o neutrons, an thir are cried the isotopes o the element. Atoms o ae element can be chynged intae atoms o anither in nuclear reactions, that chynge an atom's atomic nummer. The term "chemical element" is uised an aw for a pure chemical substance makkit o ae element anerly: oxygen gas, for example, is makkit o naething but oxygen atoms.

Historically, a "chemical element" meant a substance that canna be broken doun intae ither substances bi chemical reactions, an for maist practical purposes this definition still hauds. In the 1920s there wis some argiment ower whether isotopes shoud be coontit as separate elements gin thay coud be separatit bi chemical means.^[2] Bi November 2016 the International Union of Pure and Applied Chemistry (IUPAC) recognised 118 elements in aw. The first 94 occur naturally on Earth, an the ither 24 are synthetic elements makkit in nuclear reactions.

Muckle o the modren unnerstaundin o the elements comes frae the wirk o Dmitri Mendeleev, a Roushie chemist that published the first recognisable periodic table in 1869. The table pits the elements in order o atomic nummer, in rows cried periods, whaur the columns, or groups, share recurrin physical an chemical properties.