Final answer:
The crisscross method for determining the formula of a compound involves crossing over the absolute values of the ion charges of the elements involved, using them as subscripts in the empirical formula, and then simplifying if necessary to ensure the compound is electrically neutral.
Step-by-step explanation:
The question seeks an explanation for the crisscross method used to determine the correct formula for a compound by crossing the oxidation states (oxidation numbers) of the involved ions. The crisscross method involves taking the absolute value of the charge from one ion and using it as the subscript for the other ion in the chemical formula. In the case of Mg2+ and P3-, the oxidation state for magnesium (Mg) is +2, and for phosphorus (P) is -3, without considering the signs. When we crisscross the oxidation states, we get Mg3P2 (ignoring the signs), where the subscripts in the empirical formula are the crossed-over absolute values of the oxidation states, reduced to their simplest ratio if necessary. This method ensures that the sum of the oxidation numbers in a compound is zero, satisfying the rule of electrical neutrality.