Reacting Substances
To determine what would occur when two substances react, we must identify their charges.
There is what's called the criss-cross method where we put the substances with their charges side by side, and criss-cross their charges to get the number of atoms there are for each substance.
Solving the Question
We're given:
Mg has a charge of +2 and Cl has a charge of -1:
⇒
Criss-cross the charges:
⇒
Therefore, for every 1 atom of magnesium, there would be 2 atoms of chlorine.
Answer
For every 1 atom of magnesium, there would be 2 atoms of chlorine.