Covalent bonds

Forming a covalent bond

A covalent bond is formed when two atoms share a pair of electrons. Covalent bonding occurs in most non-metal elements, and in compounds formed between non-metals.

These shared electrons are found in the outer shells of the atoms. Usually each atom contributes one electron to the shared pair of electrons.

The slideshow shows how a covalent bond forms between a hydrogen atom and a chlorine atom, making hydrogen chloride.

Structures of a hydrogen atom and a chlorine atom.

1. A hydrogen atom with one electron and a chlorine atom with 17 electrons


Most covalently bonded substances consist of small molecules. A molecule is a group of two or more atoms joined together by covalent bonds. Molecules of the same element or compound always contain the same number of atoms of each element.

The atoms in a molecule are always joined together by a covalent bond. Substances that are made up of ions do not form molecules.

Sizes of atoms and simple molecules

A small molecule contains only a few atoms, so atoms and small molecules have a similar range of sizes. They are very small, typically around 0.1 nm or 1 × 10-10 m across.

Individual atoms and molecules are too small to see even with the most powerful light microscope. Some electron microscopes can produce images of atoms and simple molecules.

A water molecule, H2O, is about 0.3 nm (3 × 10-10 m) across:

Structure of a water molecule

Formulae of substances with simple molecules

The chemical formula of a substance with small molecules shows the number of atoms of each element in one molecule of the substance.

SubstanceFormulaNumber of atoms
HydrogenH2Two hydrogen atoms
OxygenO2Two oxygen atoms
NitrogenN2Two nitrogen atoms
Hydrogen chlorideHClOne hydrogen atom and one chlorine atom
WaterH2OTwo hydrogen atoms and one oxygen atom
AmmoniaNH3One nitrogen atom and three hydrogen atoms

State the number of atoms of each type in one molecule of methane, CH4.

A methane molecule is made up of one atom of carbon and four atoms of hydrogen.


State the number and type of atoms in one molecule of chlorine, Cl2.

A chlorine molecule is made up of two chlorine atoms.

An explanation of covalent bonding