Isotopes are variants o a pairteecular chemical element that differ in neutron nummer, awtho aw isotopes o a gien element hae the same nummer o protons in ilk atom. The term isotope is formed frae the Greek ruits isos (ἴσος "equal") an topos (τόπος "place"), meanin "the same place"; sicweys, the meanin ahint the name is that different isotopes o a single element occupy the same poseetion on the periodic cairt.

The three naiturally-occurrin isotopes o hydrogen, frae left: protium (1H) wi zero neutrons, deuterium (2H) wi ane, an tritium (3H) wi twa. Aw are variants o hydrogen acause each haes ane proton; the identity o the isotope is gien bi the nummer o neutrons

The nummer o protons within the atom's nucleus is cried atomic nummer an is equal tae the nummer o electrons in the neutral (non-ionized) atom. Ilk atomic nummer identifees a speceefic element, but nae the isotope; an atom o a gien element mey hae a wide range in its nummer o neutrons. The nummer o nucleons (baith protons an neutrons) in the nucleus is the atom's mass nummer, an ilk isotope o a gien element haes a different mass number.

For ensaumple, caurbon-12, caurbon-13 an caurbon-14 are three isotopes o the element carbon wi mass numbers 12, 13 an 14 respectively. The atomic nummer o caurbon is 6, that means that ivery caurbon atom haes 6 protons, ssr that the neutron nummers o thir isotopes are 6, 7 an 8 respectively.