Science - valency of magnesium is 2 and that of .chlorine is 1 then what is the molecular formula of

Sadly, I don't think you're going to fool many people into doing your chem homework for you. I'd suggest asking a friend in the class to explain the answer and why it is the way it is, you're more likely to understand the explanation in person when you can ask questions and get answers in real time. Your teacher might also be willing to help.

But I'll give you a hint. Magnesium has two electrons it wants to give away and Chlorine has a home for one electron. You can figure out which elements want to give and which want to take and how many by looking at the periodic table as it is very much based on which column they're in for the ones at each end.