Periodic Classification of Elements — Science STD 10 — Question

1. The valency of an element is determined by the number of electrons present in the last shell.

If a number of valence electrons are less than or equal to 4 then, valency = a number of valence electrons.

If, the number of valence electrons greater than 4 then, valency = 8 - number of valence electrons.

2. The electronic configuration of element X with atomic number 9 is 2, 7

Valency of X = 8 - 7 =1

Hence, the valency of the element with atomic number 9 is 1.