There is the most nitrogen in the air, why can humans survive only by absorbing oxygen?

Nitrogen is chemically very stable compared to oxygen because the two nitrogen atoms that make up the nitrogen molecule form a very stable covalent triple bond. Due to the existence of the nitrogen-nitrogen triple bond, the 3p orbital of the nitrogen atom is completely filled, so this covalent bond has extremely high bond energy, which makes the nitrogen gas molecule the most stable diatomic molecule. Therefore, the nitrogen-nitrogen triple bond is difficult to break and participate in chemical reactions.

In contrast, the two oxygen atoms in the oxygen molecule form a double bond with low bond energy and are easily broken to participate in chemical reactions, so oxygen is more active than nitrogen.

To survive, human beings must expend energy for metabolism, which requires chemical reactions to occur. Because oxygen is more active, it is easy to participate in chemical reactions, and can release energy after the reaction, so humans breathe oxygen for chemical reactions in the body. And nitrogen is too stable, and the reactions they participate in tend to absorb energy, so humans will not use nitrogen.