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.
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.
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: