Covalent bonds

Forming a covalent bond

A covalent bond is formed when a pair of electrons is shared between two atoms, usually 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


A molecule consists of a group of two or more atoms joined together by covalent bonds. Molecules of the same element or compound have a fixed size - in other words, they always contain the same number of atoms of each element. For example, a molecule of methane, CH4, always contains one carbon atom and four hydrogen atoms.

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

An explanation of covalent bonding