According to formulas posted by Crescent High School, there are nine oxygen atoms in a molecule of Al2(CO3)3, aluminum carbonate, because there are three sets of carbonate, CO3, ions. There are also two aluminum and three carbon atoms in total.
To count the total number of atoms in a chemical formula, multiply each portion in parentheses by the subscript after it. By this method, Al2(CO3)3 is equivalent to Al2C3O9. If there is a coefficient, for example, in a chemical equation that must be balanced, the quantity of each atom is multiplied by this coefficient as well. For example, 2H2O, or two molecules of water, contains a total of four hydrogen and two oxygen atoms.