Answer:
We know based on the charges of each ion and the fact that they must always sum to 0
Explanation:
Na only has one outer electon to give so it is always +1
Mg has two outer electrons so it always gets +2
Halogens have one "missing" electron in their outer shell so they get a -1
For NaCl (+1) + (-1) = 0
For MgI2 (+2) + 2(-1) = 0
Its arbitrary. It was developed on the greek phoenetic system and never reallybwas explained.
