An element X forms an oxide XO3. What can be the valency of element X?
The compound XO3 has three atoms of oxygen.
Valency of one oxygen atom = 2
Valency of three oxygen atoms = 2 x 3 = 6
A molecule is overall neutral in charge. So, the valencies of the ions must balance each other while writing a chemical formula.
So, the valency of X can be 6.