You can determine the valency of a transitional element or a radical in a particular compound by noting how it combines with elements with known valency. This strategy is based on the octet rule, which tells us that elements and radicals combine so as to produce a stable outer shell of eight electrons. Simple illustrations of this strategy, note that sodium (Na), with a valency of +1, combines readily with chlorine (Cl), which has a valency of -1, to form sodium chloride (NaCl), or common salt. This is an example of an ionic compound in which an electron is donated by one atom and accepted by the other.
However, it takes two sodium atoms to combine ionically with sulfur (S) to form sodium sulfide (Na2S), Because it takes two sodium atoms to form this compound, the valency of sulfur must be -2.
Apply this strategy to more complex molecules, it's important to first realize that elements sometimes combine to form reactive radicals which haven't yet achieved a stable outer shell of eight electrons. An example is the sulfate radical (SO4)2-. This is a tetrahedral molecule in which the sulfur atom shares electrons with four oxygen atoms in what is called a covalent bond. In such a compound, you can't derive the valency of the atoms in the radical by looking at the formula. You can, however, determine the valency of the radical by the ionic compounds it forms. For example, the sulfate radical combines ionically with hydrogen to form sulfuric acid (H2SO4). This molecule contains two hydrogen atoms, each with a known valency of +1, so in this case, the valency of the radical is -2.
Once you have determined the valency of the radical, you can use it to calculate the valency of other elements and molecules with which it combines. For example, iron (Fe) is a transitional metal that can exhibit multiple valencies. When it combines with the sulfate radical to form ferrous sulfate, FeSO4, its valency must be +2, because the valency of the sulfate radical, as determined from the bond it forms with hydrogen, is -2.
