The atomic number Z is the number of protons in the atomic nucleus
of an element The mass number A is the total number of protons and
neutrons in the atomic nucleus; A = Z+N; Here N denotes the number
of neutrons in the nucleus A nuclear species or a nuclide is represented as  
ZAX
, where X is the
chemical symbol of the species Nuclides with the same atomic number Z, but different neutron number
N are called isotopes