Since the atmosphere has a mass of approximately 5.15 x 10^18 kilograms, the total mass of oxygen in the atmosphere is approximately 1.08 x 10^18 kilograms. Oxygen makes up approximately 20.9 percent of the Earth's atmosphere.
While there is no definitive dividing line, the Kaman line at 100 kilometers above the Earth's surface is often used a border between the Earth's atmosphere and outer space. The Earth's atmosphere becomes thinner with increasing altitude, and about 75 percent of the atmosphere's mass is found within 11 kilometers of the Earth's surface. While the atmosphere is composed of over a dozen different gases, almost 99 percent of it is made up of only two elements: nitrogen and oxygen.