Mass Number

Mass number (A) = Number of protons or Atomic number (Z) + Number of neutrons or Number of neutrons = A – Z .

(i) Since mass of a proton or a neutron is not a whole number (on atomic weight scale), weight is not necessarily a whole number.

(ii) The atom of an element X having mass number (A) and atomic number (Z) may be represented by a symbol,_ZX^A.