The formulae of many simple ionic compounds, covered in 'Bonding'   can be found from the charges on the ions, and the rules can be extended to work quite well for many simple compounds which are not ionic too.

Simple Binary Ionic Compounds

A binary compound   is one made from two elements   only. Its name ends in -ide.

The charge on an ion can be identified from its group in the periodic table (shown below). When atoms react to form ionic compounds, the total charge on the 'formula' must add to zero.

For example, the formula of calcium chloride can be worked out by finding calcium and chlorine on a periodic table.

The calcium ion must be Ca²⁺ since it is in group II; the chloride ion must be Cl^- since it is in group VII. The formula of calcium chloride must therefore show one Ca²⁺ ion and two Cl^- ions. This is done by writing Ca²⁺Cl^-₂.  

The formula CaCl₂ means the compound calcium chloride is made up of calcium 2+ ions and chloride 1- ions. The little ₂ in CaCl₂ (called a subscript) shows that there are two chloride ions for every calcium ion in the compound. Subscripts apply only to the element in front of the subscript.

Simple Covalent Compounds

The table can be used to work out the formulae of some simple covalent compounds, such as those found at the beginning of GCSE, for example, ammonia is made of nitrogen and hydrogen. If the ionic rules are used it gives a formula of H₃N (which is correct, although it is usually written NH₃ for historical reasons.) In a similar way the formulae of CO₂, and CH₄ can be found (the '±4' means that there are no group IV elements with either a positive or negative charge of four.)

More Complicated Compounds

Copper sulfate is a well-known compound and its formula is CuSO₄. It is composed of countless millions of repeating units of CuSO₄. The copper has a 2+ ion, and the sulfate is made up of one atom of sulphur and four atoms of oxygen covalently bonded together as shown, but there is a 2- charge on the sulfate ion. The sulfate ion is always SO₄^2-. (Therefore formula of sodium sulfate must be Na₂SO₄. Two Na⁺ ions are needed to balance the charge of the sulfate.)

Understanding formulae will develop in the course, for now they simply have to be learned.

The 'complex ions', like sulfate, must also be known because they aren't on the periodic table.

Table of complex ions

	Ammonium		Nitrate		Carbonate		Sulfate		Hydroxide
	NH4+		NO3-		CO32-		SO42-		OH-

Once it is accepted that the formulae of compounds have to be learned, their use in equations becomes quite simple.

The naming of compounds can get difficult. There are rules, but they are covered later in the course.