What are the molecules that make up the most of atmosphere?

Nearly 79% of the atmosphere is made up of nitrogen, which is mostly inert but can be broken down by certain types of bacteria. These bacteria are known as nitrogen-fixing bacteria, and they are vital to life because they extract nitrogen from the atmosphere and use it to make proteins, enzymes, DNA, RNA, and other molecules that are needed for survival.

The majority of the remaining 21% is made up of oxygen, which is produced both as a byproduct of photosynthesis and from the breakdown of water in the atmosphere. A higher percentage of oxygen would be dangerous for life because it would cause oxidation, which would break down most organic molecules, whereas a lower percentage would make it more difficult for cellular respiration to occur.

The molecules that make up the majority of Earth's atmosphere are nitrogen (N2) and oxygen (O2).