How is the atomic number of an atom defined?

The of an atom can be represented as below :

##""_X^Z A## where ##X## is the proton number, ##Z## is the nucleon number (atomic number) and ##A## is the symbol of the element. ##Z## represents the total number of particles that are present in the nucleus of the atom (protons and neutrons). The neutron number of an atom is also referred as the atomic number as it is equal to the relative (r.a.m) of the atom. For example : ##""_12^25 Mg^(2+)## has a relative atomic mass of 25.