# How do you define atomic number?

The atomic number, $Z$, is the number of massive, positively charged NUCLEAR particles.
And $Z$ defines elemental identity. $Z = 1$, hydrogen, $Z = 2$, helium, $Z = 3$, lithium,...............$Z = 46$, palladium.
Note that the nucleus will also contain differing numbers of neutrons, massive, neutrally charged nuclear particles. This gives rise to the existence of isotopes. For most hydrogen nuclei, the atomic mass is $\text{1 amu}$, which we would denote by ""^1H. Some few hydrogen nuclei contain a neutron to give ""^2H, the deuterium isotope, and some fewer number contain 2 neutrons to give ""^3H, the tritium isotope. Most of the elements possess a few isotopes.