Why does air expand when heated and contracts when cooled?

Finding the answer to this question is quite easy if you know what, in fact, this air consists of. And it consists of molecules and atoms. What do we know about the behavior of these particles? And we know that when heated, the atoms expand, respectively, the molecules that consist of atoms also increase in size. And along with them, the gas itself begins to require more space. And the reverse process occurs for a similar reason. Atoms contract with decreasing temperature, electrons in their orbit begin to spin more slowly and atoms gather closer to each other. Accordingly, the air is also compressed. You may be aware of such items as oxygen cylinders. They are used, for example, by scuba divers to dive under water. It is the ability to compress air under the influence of temperature and pressure that helps them not to carry huge barrels of air on themselves, but only neat “backpacks”. With this treatment, the air takes up less space, nevertheless, there is much more air in the “backpack” of the scuba diver than in an ordinary barrel, where the air is in the natural environment.