Atomic number is the number of protons.
The number of protons determines the type of atom. For example, all carbon atoms have 6 protons, all nitrogen atoms have 7 protons. If the number of protons changes, the element changes. If the number of neutrons does not exactly equal the number of protons, then the atom is an isotope of that element. The emission of alpha radiation causes the atomic number of the nucleus to decrease by two and the atomic mass to decrease by four.