Oxygen, an element, is a colourless, odourless gas that comprises about one-fifth of the Earth’s atmosphere. It is essential to life. Although oxygen is continually released into the atmosphere by the photosynthesis of plants, it is quite reactive and is removed at the same rate via respiration and oxidation. It is extracted from liquefied air by fractional distillation and used for a variety of purposes, such as oxywelding and health care. Liquid oxygen is light blue and is attracted to a magnet. It is commonly used as an oxidiser in rockets for spacecraft, when it is known as LOX.
The atoms in the molecule are connected by a double covalent bond. A covalent bond is formed when atoms share electrons and a single covalent bond has two shared electrons, one contributed by each atom. To form a double covalent bond, each oxygen atom contributes two electrons. Because the electrons in the bond are shared equally by the two oxygen atoms, the molecule does not have an electric dipole.
Oxygen atoms can bond together in two different ways, so the element is said to have two allotropes. The allotrope shown here is sometimes called dioxygen to distinguish it from ozone, the other allotrope. Ozone molecules, O₃, contain three oxygen atoms in an angular or bent shape.
The type of model shown here, called a ball-and-stick model, is one of several ways of modelling molecules. This type is useful because it clearly shows the geometry of the molecule and the order (single, double or triple) of the bonds. However, because 'sticks' are used to represent bonds, this type of model does not represent the actual shape of the molecule as well as 'space-filling' models do.