An atom of an element (X) has its K, L and M shells filled with some electrons. It reacts with sodium metal to form a compound NaX. The number of electrons in the valence shell of the atom (X) will be

The correct option is B

Seven

By Bohr's Bury rule number of electrons in a particular orbital is calculated by the formula $2n^2$.

For K=1, $2\times 1^2$ = 2 electrons

For L=2, $2\times 2^2$ = 8 electrons

For M=3, $2\times 3^2$ = 18 electrons.

The atomic number of sodium $Na$ is 11 which means it has 11 protons and 11 electrons. So K=2, L=8, and M=1.

Valency of Sodium is 1. Since the element X forms the compound with chemical formula NaX, it means, the valency of X =1. The number of electrons in the M shell of an atom of X will be seven.