Question

An element X forms an oxide $X{O}_3$, the valency of element X is ___

Answer 1

The compound $X{O}_3$ has three atoms of oxygen

Valency of one O atom = 2

Valency of three O atoms = 2 x 3 = 6

Since valencies of the ions must balance while writing a chemical formulae.

So, valency of X should be 6; otherwise the reaction is not possible.