How do you define atomic number?

The , ##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 . For most hydrogen nuclei, the is ##"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 possess a few isotopes.