Why is the atmosphere 80% Nitrogen and 20% Oxygen (roughly)?

.If the oxygen content was higher then fires and explosions would be the norm. the planet would be unlivable. Since nitrogen is inert it dilutes the oxygen and also holds down rust and corrosion.

And of course this perfect mix happened when a ball of mud randomly exploded in space.

The earth is about 4 billion years old. Its early atmosphere was actually methane and ammonia gases. Virtually no oxygen or nitrogen. Then about 3.5 billion years ago, a life form evolved -- cyanobacteria. These were photosynthetic bacteria that formed colonies called stromatolites. For the next billion years, these bacteria gradually built up the oxygen level to a few percent. It was not until about a billion years ago that things really got interesting with the evolution of multicellular photosynthetic lifeforms such as phytoplankton, algae, and plants that made the atmosphere to where it is today. The Carboniferous Period which occurred from about 354 to 290 million years ago when the earth was predominantly swamps was important in locking up the remaining methane into coal and converting ammonia into nitrogen.

That is the scary thing about "global warming". We are releasing the carbon that took hundreds of millions of years to take out from our atmosphere. The earth may turn into a hot swampy planet (or a hot desert as in the Permian Period) in a matter of centuries.

For the past billion years, due to living organisms, the amount of oxygen in the atmosphere has been increasing. It may continue to increase as long as there is life, but the increase of solar radiation due to the Sun's evolving into a brighter main sequence star may start killing off life, and hence oxygen, before it gets up to 53 %.

There's another problem. Fire. If there is more oxygen in the Earth's atmosphere, more fires will occur, until the whole planet is up in flames. The fires convert the oxygen to carbon dioxide, so it may not be possible to get to 53% because oxygen will burn up things and get out of the atmosphere before then.

Actually, the kinetic theory of gases makes no predictions about atmospheric composition whatever. The nitrogen has been around since primordial time; the oxygen mostly accumulated during Carboniferous. The balance between plant and animal life, and decay processes, has kept the O2 percentage nearly constant for millions of years. The principal minor component, argon, is a decay product of potassium-40, which is slightly radioactive.

life silly ( the atmosphere has had many influences but over time life has by far been the greatest and is responsible for 100% of the current oxygen and the ratio of course )

there are other things in the atmosphere. if there is 2 much O2 than the atmosphere is literally a powder keg, and life wouldn't exist

And the same question shows up again

I don't really know how to answer this