Glossary · Vật lý hạt nhân
Atomic number
The number of protons in a nucleus, written Z. It decides which element an atom is: change Z and you change the element, while changing the neutron count at fixed Z leaves the same element, only a different isotope.
A nucleus is made of protons and neutrons, collectively nucleons. The proton count is the atomic number Z; the neutron count is the neutron number N; the total nucleon count is the mass number A.
The three are locked together: N = A − Z. And it is Z, not A, that identifies the element. Every atom with Z = 6 is carbon, whether A is 12, 13 or 14.
Atoms with the same Z but different N are isotopes of each other. Hydrogen has three: ordinary hydrogen (one proton, no neutrons), deuterium (one proton, one neutron) and tritium (one proton, two neutrons).