Bond formation in oxygen molecule. Illustration of the sharing of electrons (dots and crosses) between two oxygen atoms (O) to form a molecule of oxygen (O2). This is an example of covalent bonding, with the double bond formed by two shared electron pairs, each pair consisting of an electron from each atom. The outer electron shell of each atom is shown as a red ring. Oxygen is a gas at room temperature and pressure. For the atoms before the bonds form, see image C028/6488.