The name of a compound usually gives you all the information you need to write its chemical formula. The first part of the name denotes the cation, or positively charged ion that forms the molecule, while the second part denotes the anion, or negative ion. A balanced chemical formula also has subscripts to show the number of each ion in the compound. These subscripts depend on the valencies of the ions, which you look up in the periodic table. The problem with transition metals, which always form cations, is that they can can lose different numbers of electrons because of the nature of the outer orbital the electrons occupy. They therefore have different valencies and can form ions with different charges. The name of the chemical formula usually includes a number in Roman numerals to tell you what valency the transition metal displays in the compound.
Modern and Traditional Naming Systems
The transition metals are those elements that occupy groups 3 through 12 in the periodic table. They include such familiar metals as copper (Cu), silver (Ag), gold (Au) and iron (Fe). When you see the name of one of these metals in the name of a chemical formula, you'll probably also see the number in Roman numerals written after it to tell you the ionic charge the metal displays in the compound.
This isn't the only system in use, however. You may also see the name of the ion followed by "ic" or "ous." The "ic" suffix indicates the ion has its most common positive charge, and the "ous" suffix indicates it has one less than that. For example, iron usually forms the ferric (+3) ion, but it can also form the ferrous (+2) ion. Copper, on the other hand, has a standard ionic charge of +2, so a cupric ion has a charge of +2 and cuprous ion has a charge of +1.
Writing the Chemical Formula
The procedure for writing a chemical formula for a compound that contains a transition metal, given the name of the compound, involves three steps.
Write the Elemental Symbols
Write the Ionic Charge
Balance the Charges
Look up the symbols in the periodic table if you don't know them. If the anion is polyatomic, enclose its chemical formula in brackets. For example, the elements in iron (III) chloride are Fe and Cl, while those in iron (III) sulfate are Fe and (SO4).
Indicate the charge on each ion as a superscript that follows its symbol. This is an intermediate step to make balancing the formula easier. These superscripts don't appear in the chemical formula.
For example in iron (III) chloride, the iron atom has a charge of +3, as indicated in the name, and the chlorine atom always has a charge of -1. Write Fe+3Cl-1. In iron (III) sulfate, iron has a charge of +3 and sulfate has a charge of -2, so you would write Fe+3(SO4)-2.
Change the superscripts to subscripts to indicate a net charge of 0. For example, because the iron atom in iron (II) chloride has a charge of +3 and the chlorine atom has a charge of -1, it takes three chlorine atoms for every iron atom to create a net charge of 0. So the chemical formula for iron (III) chloride is FeCl3. Similarly, it takes three sulfate ions and two iron (III) ions to create a balanced formula for iron (III) sulfate, so its formula is Fe2(SO4)3.
One More Example
What is the formula of cuprous oxide?
The word "cuprous" means the charge on the copper ion is +1. The charge of the oxygen anion is always -2. Write the elemental symbols with their charges: Cu+1O-2, which leads directly to the balanced formula: