A 15.33 g sample of magnesium oxide is found to contain 12.67 g of magnesium. What is the percent (by mass) composition of the magnesium in the compound?

the number of valence electrons for each molecule or ion is shown beneath the structure. remember hydrogen will not have more than two electrons. this hydrogen is part of a covalent bond (sharing two electrons). a double bond here would cause hydrogen to share four electrons with phosphorus.