An element X forms an oxide XO3, what can be the valency of element X?
6
The compound XO3 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.