The burning of Magnesium metal in oxygen has no 'net ionic equation'. At no point in the reaction are the species in an ionic form. As was said earlier, the product of the reaction is an ionic compound, but the reaction itself is not. Therefore, there is no net ionic equation. Even if this were an aqueous solution, there would be no net ionic equation because the O(2-) ion does not exist in an aqueous solution. It immediately attacks a water molecule, or any other proton donator, to form OH-.