Oxygen is the third most abundant element in the universe after hydrogen and helium. It is an odorless, colorless and tasteless gas at room temperature and is the most reactive of all the non-metallic elements. Oxygen constitutes 49.2 percent of the earth's crust by mass and is also the major component of the world's oceans. Oxygen gas is the second most common element in the earth's atmosphere, accounting for 20.8 percent of its volume and 23.1 percent of its mass.
This atmospheric oxygen exists because of photosynthesis, in which plants and some bacteria manufacture food in the form of sugars and other energy-rich substances by absorbing water, carbon dioxide and sunlight and releasing oxygen as a byproduct. Oxygen is essential for nearly all organisms, both in the organic compounds that constitute their cells and in the form of ozone in the ozone layer, which shields life from the harmful effects of the sun's ultraviolet radiation.