The most common chemical element in Earth’s atmosphere is nitrogen. In ordinary dry air, nitrogen gas exists mainly as diatomic molecules, N₂, and makes up about 78.1% of the atmosphere by volume. Oxygen follows at roughly 20.9%, while argon and carbon dioxide account for most of the remainder.
Nitrogen’s abundance is not the same as biological usefulness in its atmospheric form. The strong triple bond between the two nitrogen atoms makes N₂ relatively unreactive, so most plants and animals cannot use atmospheric nitrogen directly. Specialized bacteria, lightning, and industrial processes convert it into compounds such as ammonia and nitrates that living organisms can use.
Nitrogen is essential to amino acids, proteins, and DNA. It moves through ecosystems in the nitrogen cycle, passing among the atmosphere, soil, water, plants, animals, and decomposers. Human activity has significantly altered this cycle through fertilizer production and fossil-fuel combustion.
A common mistake is answering oxygen because humans need it for respiration. Oxygen is vital to many organisms, but nitrogen is more abundant in the atmosphere. Water vapor also varies greatly by location and is not a chemical element; it is a compound made from hydrogen and oxygen.