The distribution of electrons In K, L, M, N shells and the number of neutrons in the nuclei of the following elements are given below:
$$\begin{array}{cccccc}\text{Elements}& \text{K}& \text{L}& \text{M}& \text{N}& \text{No. of Neutrons}\\ \text{A}& 2& \text{6}& \text{-}& \text{-}& \text{8}\\ \text{B}& 2& \text{8}& \text{2}& \text{-}& \text{1}2\\ \text{C}& 2& \text{8}& \text{5}& \text{-}& \text{1}6\\ \text{D}& 2& \text{8}& \text{8}& \text{-}& \text{2}2\end{array}$$
The mass number of A, B, C, and W respectively is:

The correct option is A 16, 24, 31, 40
Total number of electrons in an element = No. of electrons in K shell + No. of electrons in L shell +No. of electrons in M shell +No. of electrons in N shell
Total number of electrons in element A = 2 +6 +0 +0 = 8
Total number of electrons in element B = 2 +8 +2 +0 = 12
Total number of electrons in element C = 2 +8 +5 +0 = 15
Total number of electrons in element D = 2 +8 +8 +0 = 18
As we know total number of protons is equal to total number of electrons, therefore
Total number of protons in element A = 8
Total number of protons in element B = 12
Total number of protons in element C = 15
Total number of protons in element D = 18
Atomic mass number = Total number of protons + Total number of neutrons
Atomic mass number of element A = 8 + 8 =16
Atomic mass number of element A = 12 + 12 =24
Atomic mass number of element A = 16 + 15 =31
Atomic mass number of element A = 22 + 18 =40