Answer
According to the nuclear model, an atom having Z number of protons consists of a nucleus of charge +Ze surrounded by electrons, same in number as Z and of charge –e. These electrons occupy atomic orbitals oor shells around the nucleus, with no more than two electrons in any one orbital. Each shell being characterized by the principal quantum number, n = 1, 2, …. A shell consists of $n^{2}$ individual orbitals, which are grouped together into n subshells; these subshells, and the orbitals they contain, are denoted s, p, d, and f.
The atomic number, Z is the number of protons in an atom.
The nucleon number A or atomic mass number is the sum of protons and neutrons of the atom.
