When Ag (silver) and O (oxygen) react, they form an ionic compound with a 1:1 ratio of Ag to O ions. The silver ion has a +1 charge, while the oxide ion has a -2 charge. To form a neutral compound, one Ag ion with a +1 charge will combine with one oxide ion with a -2 charge. Therefore, the chemical formula for the ionic compound formed by Ag and O is Ag2O.