Occurrence of Oxygen

Oxygen is the most abundant element on the earth’s crust. It occurs in both free and combined states in nature. In combination with other elements, oxygen consists of about 50% of the mass of the earth’s crust, the oceans and the air.

Oxygen occurs as O₂ molecules and, to a limited extent, as O₃ (ozone) molecules in air. It forms about 20% of the mass of air and 89% of the mass of water, in combination with carbon, hydrogen, and nitrogen.

Oxygen is a large part of plants and animals.