Glossary · Vật lý hạt nhân
Proton
The positively charged particle in an atomic nucleus. The number of protons, the atomic number Z, determines which element an atom is; change it and the element itself changes.
An atomic nucleus is made of protons and neutrons, together called nucleons. The two have roughly the same mass, but the proton carries one unit of positive charge while the neutron is neutral. The CODATA 2022 value for the proton mass is 1.672 621 925 95 times 10 to the minus 27 kg.
The proton count Z identifies the element: every atom with Z = 6 is carbon, whatever its neutron count. Atoms with the same Z but different neutron numbers are isotopes of each other.
The proton is treated as stable: no experiment has observed a free proton decaying, and measurements only set lower bounds on its lifetime. Some grand unified theories predict that protons do decay, but the search — at Super-Kamiokande, with its 50,000-tonne tank of water — had turned up nothing as of 2014.