Why oxygen and carbon dioxide are not the most abundant gases in the atmosphere?

Answer 1

Oxygen readily combines with many other elements, carbon (CO2), hydrogen (water), iron (rust), etc. So absent photosynthetic life we would not expect there to be much oxygen in any atmosphere.

CO2, on the other hand, ought to be fairly high, and it most probably was early in earth's history. However, living organisms began pulling the CO2 out of the atmosphere, cracking it, and releasing the O2. The carbon taken up by these marine organisms was incorporated into their shells and exoskeletons. When they died they sank to the sea floor, forming vast chalk and limestone deposits over hundreds of millions of years. Much of the early earth atmospheric CO2 ended up in limestone deposits.

This left nitrogen as the most predominant gas.

Answer 2

Nitrogen is the most abundant gas in the atmosphere at about 78%, followed by oxygen at around 21%. While both oxygen and carbon dioxide play crucial roles in sustaining life on Earth, nitrogen dominates due to its inert nature and stable abundance. Carbon dioxide is present in much lower concentrations, around 0.04%, due to its involvement in biological processes like photosynthesis and respiration.