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 M shell of the atom (X) is
Seven
Valency of Sodium is 1. Since the element X forms 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.