Oxygen comprises about 21% of the atmosphere. Without the presence of Oxygen emitting plants, there will be very little Oxygen in the air. The early atmosphere had very little Oxygen but this value rose upon the development of photosynthesis where Carbon Dioxide, water and energy are converted into sugar and Oxygen. The generation of Oxygen allowed Oxygen breathing life to take hold on the planet.

Oxygen is element number 8 on the Periodic Table with an average atomic mass of 16.00 amu. The atomic mass of 16 comes from the presence of 8 protons and typically 8 neutrons in the nucleus of the atom. Both Carbon Dioxide (CO2) and water (H2O) contain Oxygen as one of the elements in the compound. Thus breaking these compounds will produce Carbon, Oxygen and Hydrogen. Oxygen is found naturally as a diatomic molecule. Oxygen is highly reactive. Thus, single Oxygen atoms will quickly combine with other elements or with each other instead of existing as free floating individual Oxygen atoms. A diatomic molecule is denoted as O2.

Besides being critical for Oxygen breathing organisms (respiration), the presence of Oxygen allows fires to burn more aggressively. Without photosynthesis to continually recharge Oxygen levels, the Oxygen level in the atmosphere would decrease over time. Oxygen is also the constituent within Ozone (O3). Ozone is responsible for shielding a large amount of harmful UV (Ultraviolet) radiation from reaching the Earth’s surface. This ozone forms high aloft in the stratosphere and is called the Ozone Layer. This shielding makes it easier for life to inhabit the land surface without being quickly damaged from excessive UV radiation.