There is more to writing formulas than crossing over the oxidation states. This Powerpoint presentation examines the bonding ratios of elements bonding both ionically and covalently based on the valence structure. It also shows how the valence structure relates to the oxidation state. Only after examining bonding through Bohr diagrams and electron dot diagrams do the students realize the sum of the oxidation states in a neutral compound is always zero. Finally, the crossover rule is introduced as the easiest way to find the lowest common multiple of oxidation states that add up to zero. This is not just crossover rule. This is crossover rule with understanding.
Free worksheets are available for this topic on my website at http://www.evanschemistrycorner.com.