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.
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 will have a set size - in other words, they will always contain the same number of atoms of each element. For example, a molecule of methane, CH4, will always contain one carbon atom and four hydrogen atoms.
A simple molecule contains only a few atoms, so the sizes of atoms and simple molecules have a similar range of sizes - they are very small, typically around 0.1 nm or 10-10 m across.
Individual atoms and molecules are too small to see even with the strongest light microscope. Some electron microscopes can produce images of atoms and simple molecules.