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.
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:
For example, a hydrogen molecule forms when a hydrogen atom shares its outer electron with another hydrogen atom.
A simple molecule can be modelled by drawing its structure. In these structures:
For example, a hydrogen molecule has two H atoms and one covalent bond. The diagram shows its structure.
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.