If x stands for a metal how can you tell from the following formulas xcl and xcl2 which compound contains an alkali metal and which contains an alkaline earth metal?

You have to balance the valence between the metal and Cl. Cl wants one electron, so it has a charge of -1. It takes one alkali metal to balance that charge, since alkali metals only give up one electron. This means XCl is made of an alkali metal. Alkaline earth metals give up two electrons, and so it can give one of those two electrons to two Cl. Therefore, XCl2 contains an alkaline earth metal