Oxygen is a naturally occurring element that can be found in abundance on Earth. It is present in the atmosphere as a gas, and can also be found in almost all living things, in water, and in the soil. Oxygen is also found in many minerals and rocks, making up nearly half of the Earth’s crust. The percent of oxygen present in the atmosphere is approximately 21%, making it the most abundant element in the atmosphere. Oxygen makes up about 65% of the Earth’s total mass. In the oceans, oxygen is present in concentrations of about 6-8%. In plants, oxygen is a necessary component of photosynthesis, and is present in concentrations of up to 40%. In animals, oxygen is necessary for respiration, and is present in concentrations of 15-20%.