Chemical formulae of compounds

A substance formed by the chemical union of two or more elements is called a compound. The chemical formula for a compound shows:

  • the symbols for each element in the compound
  • the number of atoms of each element in a unit of the compound

Covalent compounds

A bond formed between two atoms, where electrons are shared, is called a covalent bond.Covalent compounds usually form between non-metal elements and are held together by covalent bonds. All compounds that exist as molecules are covalent. Here are some examples:

Carbon dioxideCO2
Sulfur dioxideSO2
A diagram showing one symbol of nitrogen and two symbols of hydrogen.

The subscript numbers in formulae show how many atoms of that element appear in the molecule.

So, the formula NH3 shows that ammonia contains one nitrogen and three hydrogen atoms.

Ionic compounds

A bond formed between two atoms where an electron is transferred from one atom to the other is called an ionic bond. Ionic compounds are made up of atoms joined together by ionic bonds. They usually (but not always) contain at least one metal element and one non-metal element.


An ion is a charged particle formed when an atom, or a group of atoms, loses or gains electrons. The number and sign of its electrical charges are shown in superscript text.

Names and formulae of some common ions:

Name of ionFormula of ionElectrical charge(s)
SodiumNa+One positive
MagnesiumMg2+Two positive
ChlorideCl-One negative
OxideO2-Two negative

Simple formulae

The formula of an ionic compound can be predicted by using the formulae of its ions. The numbers of ions in a formula must give an equal number of positive and negative charges.

Name of compoundFormulaElectrical charges
Sodium clorideNaClOne positive, one negative
Sodium oxideNa2OTwo positive, two negative
Magnesium oxideMgOTwo positive, two negative
Magnesium chlorideMgCl2Two positive, two negative