Ozone, or trioxygen, is a chemical with the symbol O. This means one molecule of ozone is made of three oxygen atoms. Ozone is rarely called "trioxygen", even though this is its IUPAC systematic name. Ozone is formed from oxygen gas (O) by the action of ultraviolet light and also atmospheric electrical discharges. It is present in low concentrations throughout the Earth's atmosphere. In total, ozone makes up only 0.6 ppm (parts per million) of the atmosphere by volume.