1 Mg 2 NO3 --- it is necessary to have one (+2) magnesium ion to cancel out two (-1) nitrate ions. Step 4: Mg(NO3)2 - the parenthesis around the NO3 indicate that you need 2 of the NO3 ions. Without the parenthesis, it would look like NO3 2, which appears like you need 1 nitrogen and 32 oxygens.