Chemical formulae of compounds

A substance formed by combining 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 formed between non-metal elements, are held together by covalent bonds. All compounds that exist as molecules are covalent. Here are some examples:

CompoundFormula
AmmoniaNH3
Carbon dioxideCO2
MethaneCH4
Sulfur dioxideSO2
WaterH2O

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 one or more electrons 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 of elements in groups of the periodic table

Sodium is in group 1 of the periodic table, so it has one electron in its outer shell. When sodium forms an ion, it loses this one electron. Oxygen is in group 6 of the periodic table, so it has six electrons in its outer shell. When oxygen forms an ion, it gains two electrons. Therefore, the charge of simple ions is related to the group that an element is in:

GroupIon chargeExample
1+1Na+
2+2Mg2+
3+3Al3+
6-2O2-
7-1Cl-

Compound ions

As well as simple ions of elements, there are some common ions which contain two elements combined together. These ions are known as compound ions.

Here are the names and formulae of some compound ions:

Name of ionIon formulaElements combined
SulfateSO42-Sulfur, oxygen
CarbonateCO32-Carbon, oxygen
NitrateNO3-Nitrogen, oxygen
HydroxideOH-Oxygen, hydrogen
AmmoniumNH4+Nitrogen, hydrogen

Simple formulae

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

CompoundIons presentElectrical chargesFormula
Sodium chlorideNa+ and Cl-One positive, one negativeNaCl
Sodium oxideNa+ and O2-One positive, two negativeNa2O
Magnesium oxideMg2+ and O2-Two positive, two negativeMgO
Magnesium chlorideMg2+ and Cl-Two positive, one negativeMgCl2