(a) When atomic number increases, the difference of energy between the successive shells decreases. As a result of this, the M and N energy shells come so close together that they overlap. Due to this fact, the electrons start entering into N-shell when M-shell has acquired 8 electrons.
But when N-shell acquires 2 electrons, further M-shell comes to lower energy level and further electrons enter into the M-shell.
(b) The mass number is always a whole number (because protons and neutrons are always present in whole numbers), whereas atomic mass is usually not a whole number (because it is the relative mass as compared with mass of C-12 atom taken as 12). However, for most of the purposes, atomic mass is taken as equal to mass number.