Question 8
What is the chemical formula of magnesium oxide?
The right answer is the one marked above.
MgO. Magnesium has a valency of 2 and so does oxygen. Crossing over gives Mg2O2 — but the two valencies already balance, so the formula is written in its simplest ratio.
Calcium oxide is the same case: Ca is 2, O is 2, and the answer is CaO, not Ca2O2. Whenever the crossed-over numbers share a factor, reduce them.