(Easy 10 points) Which pair of ions will form a compound of formula MX2?

You're right, it is magnesium and iodide. Here's why: When ionic compounds form, the positive charges must be equal to the negative charges. Magnesium ions have a charge of +2, while iodide ions have a charge of -1. To make the charges equal, each magnesium ion will combine with 2 iodide ions so that the +2 of the magnesium is balanced out by the two -1 charges, one from each iodide.