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

The chemical formulae will depend on the type of compound if it is covalent or ionic or other

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:

Compound	Formula
Ammonia	NH3
Carbon dioxide	CO2
Methane	CH4
Sulfur dioxide	SO2
Water	H2O

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.

Ions

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 ion	Formula of ion	Electrical charge(s)
Sodium	Na+	One positive
Magnesium	Mg2+	Two positive
Chloride	Cl-	One negative
Oxide	O2-	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 compound	Formula	Electrical charges
Sodium cloride	NaCl	One positive, one negative
Sodium oxide	Na2O	Two positive, two negative
Magnesium oxide	MgO	Two positive, two negative
Magnesium chloride	MgCl2	Two positive, two negative