Covalent bonds

Molecules

A molecule consists of two or more non-metal atoms joined together by covalent bonds. A molecule can be:

  • an element, if its atoms are of the same non-metal
  • a compound, if its atoms are of different non-metals

A covalent bond is formed when a pair of electrons is shared between two atoms. These shared electrons are found in the outer shells of the atoms. In general, each atom contributes one electron to the shared pair of electrons.

Dot and cross diagrams

A simple molecule consists of a small number of atoms joined together by covalent bonds. The bonding in these molecules can be modelled using dot and cross diagrams, in which:

  • the outer shell of each atom is drawn as a circle
  • circles overlap where there is a covalent bond
  • electrons from one atom are drawn as dots, and electrons from another atom as crosses

For example, a hydrogen molecule forms when a hydrogen atom shares its outer electron with another hydrogen atom.

Dot and cross diagram of a hydrogen bondingA dot and cross diagram to model the bonding in hydrogen, H2

Drawing structures

A simple molecule can be modelled by drawing its structure. In these structures:

  • show each atom by its element symbol
  • show each covalent bond as a straight line

For example, a hydrogen molecule has two H atoms and one covalent bond. The diagram shows its structure.

Two letter Hs connected by a lineThe structure of a hydrogen molecule

Some examples of models

Table of models of hydrogen chloride, water and methaneTable of models of hydrogen chloride, water and methane

Chemical formulae

Simple molecules contain fixed numbers of atoms. This means that the chemical formula used for a simple molecular substance is its molecular formula.

A molecular formula shows the actual numbers of atoms of each element in a molecule. For example, the formula for methane is CH4. It shows that each methane molecule contains one carbon atom and four hydrogen atoms.

curriculum-key-fact
A covalent bond is a shared pair of electrons.

An explanation of covalent bonding