An element X combines with oxygen to form an oxide XO and it is electrically conducting. The formula of compound when X reacts with chlorine is ____________.

The formula of the compound when X reacts with chlorine is $\mathit{XC}{l}_2$.
The nature of oxide formed by an element X with oxygen is divalent because of the formula XO. Hence the element X must be divalent. The oxidation state of X is +2 while the oxidation state of Cl is -1. Hence their formula of chloride according to criss-cross method is $\mathit{XC}{l}_2$.