Answer: (b) Mg
We can see from the formula XCl2 that one atom of X combines with two atoms of Cl. It implies that X has a valency of two.
- Na has 1 valency, so it forms NaCl
- Mg has 2 valency electrons, so it forms MgCl2
- Al has 3 valencies, as a result it forms AlCl3
- Si has 4 valencies, so it forms SiCl4