Valency of magnesium is 2,and that of chlorine is 1,then what is the molecular formula?

To balance the molecule (so that it has no charge), you need twice as many Chlorine atoms as Magnesium so the simplest formula would be MgCl2. But Mg2Cl4 is also valid. It would still have the empirical formula of MgCl2, though, which is magnesium chloride. That's your answer.