The number of electrons that an atom of an element loses or gains to form a electrovalent bond is called it’s electrovalency.
The electronic configuration of Na (at. no. 11) = 2, 8, 1 and in order to attain the nearest noble gas electronic configuration i.e., Ne (2,8) it tends to lose 1 electron, hence, it is said that it has electropositive valency of +1.
The electronic configuration of Cl (at. no. 17) = 2, 8, 7 and in order to attain the nearest noble gas electronic configuration i.e., Ar (2, 8, 8) it tends to gain 1 electron, hence, it is said that it has electronegative valency of -1.
