What are the molecules that make up the most of atmosphere?
Nitrogen and Oxygen gases.
Nitrogen makes up almost 79% of the atmosphere. Nitrogen is generally inert but can be metabolized by some forms of bacteria. These nitrogen fixing bacteria are essential for life. The Nitrogen that the bacteria pull out of the atmosphere provide the building blocks for proteins, enzymes, DNA, RNA and other molecules necessary for life.
Oxygen makes up most of the remaining 21%. Oxygen is formed from the break down of water in the atmosphere and as a by product of photosynthesis. A higher percentage of Oxygen would be harmful to life as Oxidation would degrade most organic molecules. A lower percentage would make it hard for cellular respiration to occur.