Atomic Number
The number of protons in the nucleus of an atom, which uniquely identifies the element.
Formula / equation
Z = number of protons
Unit
Dimensionless
In depth
Symbolised Z, the atomic number equals the nuclear charge and, in a neutral atom, the electron count. Moseley's X-ray studies showed that ordering elements by Z rather than by atomic mass removes every anomaly in the periodic table, establishing it as the true organising principle of chemistry.
Examples in the real world
Hydrogen has Z = 1, carbon Z = 6, iron Z = 26 and uranium Z = 92.