That's a tough one. It seems like all of those are true. The atmosphere does have 2 atom molecules like N2 and O2, however there are other sized molecules like H2o and CO2 which has 3 atoms, Neon which has 1 atom. It let's sunlight through which allow photosynthesizes to occur, without which there would be no life on earth. Some parts of the electromagnetic radiation spectrum are not visible because of our atmosphere. We can only see certain ranges with telescopes in space. Many, but not all. of the gases in the atmosphere react with other molecules.