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

Compared with oxygen, the chemical properties of nitrogen are very stable, 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 a very high bond energy, which makes the nitrogen molecule the most stable diatomic molecule. Therefore, the triple bond of nitrogen and nitrogen is difficult to break and participate in chemical reactions.

In contrast, the two oxygen atoms in the oxygen molecule form a double bond, which has low bond energy and is easily broken and participates in chemical reactions. Therefore, oxygen is more active than nitrogen.

If human beings want to survive, they must consume energy for metabolism, which requires chemical reactions. Because oxygen is relatively active, it is easy to participate in chemical reactions, and can release energy after the reaction, so human breathing oxygen is used for chemical reactions in the body. Nitrogen is too stable, and the reactions they participate in tend to absorb energy, so humans will not use nitrogen.

However, there are a few organisms on the earth that do not depend on oxygen, and nitrogen-fixing bacteria can use nitrogen. In fact, the early earth creatures were all microorganisms, and they were all anaerobic. At that time, there was almost no oxygen on the earth.

About 2.6 billion years ago, a large number of cyanobacteria appeared on the earth. They produced a large amount of oxygen through photosynthesis, which led to the mass extinction of anaerobic microorganisms at that time, because oxygen was a toxic gas for them, which is the big oxidation. event. After that, most of the living things that appeared on the earth depended on oxygen.