Salts have formulas set by the capacity of their ions to form links with other ions. The topic Ions and salts has many examples of salts and their formulas. What is being investigated here is how these formulas are calculated from our knowledge of ions and their ability to attract and repel other ions.

The ability of an ion to attract or repel other ions is measured by the charge on the ion. It is this charge which is called electrovalency. "Electrovalency" and "covalency" are often confused. Although closely related, covalency is used for atoms in molecules and electrovalency is used for ionic solids or salts. Covalency is discussed in the topic Making molecules - the rules of covalency.

Electrovalency
Both metal and non-metal atoms form ions. Metal atoms lose electrons to form positive ions and non-metal atoms gain electrons to form negative ions. The number of electrons lost or gained determines the quantity of charge on the ion. Thus, the electrovalency of an ion is given by a positive or negative sign and a number.

Here is a list of common ions their electrovalency:

Note:- When a metal atom can form more than one ion, the positive charge on the ion is given by Roman numerals in brackets. The negative ions of non-metals lose the ending of the atom's name and replace it with the letters -ide. More complex ions will be considered later.

Using valencies
To write the formula for a salt, the valencies of the ions must be charge balanced. Take for instance the salt sodium chloride, made from sodium ions and chloride ions. The formula for sodium chloride is NaCl. Each sodium ion electrovalency +1 is charge balanced by a chloride ion each with electrovalency -1, ie +1 -1 = 0.

Hint:- The positive ion is written first in a formula and the charges are not shown. Keep a table of ions nearby!

Calcium fluoride is composed of calcium ions, electrovalency +2 and fluoride ions, electrovalency -1. The formula of calcium fluoride is therefore CaF₂.

Water is made of oxygen and hydrogen. Although water is considered a molecule it is one of the cases where a formula can be arrived at by using valencies or covalencies. If we assume that water is made of ions (which it is not) then we can use hydrogen, electrovalency +1 and oxide, electrovalency -2 to give us the formula. The central oxide ion will combine with two single hydrogen ions to form H₂O:-
ie -2 +1 +1 = 0. Hence water's other name, dihydrogen oxide.

More combinations
Combinations of valencies greater than one can be more complex, but follow the same rules, ie +2 -2 = 0 or (3 × +2) + (2 × -3) = 0. In each combination, the sum of the charges on the ions is zero.

Try to work out the formulas of the two salts copper(II) oxide and aluminium chloride and compare your formulas with the following illustrations.