1) If the alkaline-earth metal will react with chlorine we have the following reaction:
M + Cl₂ → MCl₂
Because the alkaline-earth metal have a valence of 2 and the chlorine atom in this compound a valence of 1 the formula will be:
MCl₂
2) How many atoms of each of the following elements are present in Hg₃(BO₃)₃?
Hg = 3 atoms
B = 1 × 3 = 3 atoms
O = 3 × 3 = 9 atoms