Give reasons for the following:
$(a)$ In potassium and calcium,the filling of electrons in the $N$-shell starts after the $M$-shell has acquired $8$ electrons,although the $M$-shell can accommodate up to $18$ electrons. Why?
$(b)$ There is a difference between mass number and atomic mass.

(N/A) As the atomic number increases,the energy difference between successive shells decreases. Consequently,the $M$ and $N$ energy shells come close enough to overlap. Due to this,electrons begin entering the $N$-shell once the $M$-shell has acquired $8$ electrons. However,once the $N$-shell acquires $2$ electrons,the $M$-shell shifts to a relatively lower energy level,allowing further electrons to enter and complete the $M$-shell.
$(b)$ The mass number is always a whole number because it represents the sum of protons and neutrons,which are discrete particles. In contrast,atomic mass is typically not a whole number because it is a relative mass calculated by comparing the average mass of an atom to $1/12$th the mass of a $C-12$ atom. For most practical purposes,atomic mass is approximated as equal to the mass number.