When atoms of a metal and nonmetal combine to form a compound, the metal atoms tends to donate one or more electrons to the nonmetal atoms. This electron transfer results in the conversion of the atoms to ions, or charged atoms. Electrons possess a negative charge. In a charge-neutral atom, the positively charged protons in the atom’s nucleus balance the electrons’ negative charges on a one-to-one basis. An atom of iron, for example, contains 26 protons and 26 electrons. But if iron forms a compound and donates three electrons to another atom, it assumes a +3 charge because it now contains three more protons than electrons. Determining the charges of atoms in compounds requires only a cursory understanding of electron configurations and how elements are arranged in the periodic table.
Find the elements composing the compound on the periodic table of the elements and verify that the compound consists of at least one metal and one nonmetal. Generally, metals reside on the left side of the table and nonmetals reside on the right side. Most periodic tables include a dark line in a zig-zag pattern down the table that represents the metal/nonmetal dividing line.
Find the group numbers of the elements by looking at the number above the column in which the element resides. The group number should be represented with a Roman numeral and the letter A or B. For example, consider the compound calcium chloride, CaCl2. Calcium resides in group IIA and chloride resides in group VIIA.
Calculate the charge of the anion by subtracting eight from its group number. In the case of chloride, 7 - 8 = -1; thus chloride assumes a charge of negative one. The charge of the metal will be equal to its group number. Calcium, in group IIA, therefore assumes a charge of positive two.
Calculate the charge of B-group metals by examining the formula of the compound and the charge of the anions. Many B-group elements can assume more than one possible charge. Therefore, first calculate the charge on the anion according to step 2. Then multiply the charge by the number of anions in the compound’s formula and divide by the number of cations. For example, consider Fe2O3. Oxygen resides in Group VIA and therefore assumes a charge of -2. The formula indicates that the compound contains three oxygen atoms, so the total negative charge is -2 * 3 = -6. For a compound to be stable, it must possess overall charge neutrality, meaning that in this case the metal atoms must provide a total charge of +6, because -6 + 6 = 0. However, because the formula contains two iron atoms, this charge is assumed to be evenly divided between them. Thus, the charge on each iron atom is +6 / 2 = +3.
In many, but not all, cases, charge can be discerned from the formula of the compound by assuming that the charge of the atom is equal to the subscript of the opposite atom. For example, in AlCl3, the charge on aluminum is +3 and the charge on chloride is -1.