It's well-known that air is actually mostly nitrogen mixed with oxygen, and some traces, along with CO² as a byproduct of respiration. (Or the other way around.)
But what is its structure?
It's not a molecule, like NO, NO², or N²O, because those substances have significantly different properties and effects than air.
Any O that encounters H with likely bond and fall out, although water is not made this way, in fact water is rarely made on Earth at all. Most of the water on Earth comes from space, brought in on asteroids and other objects, so the water we drink is actually billions of years old, created on other planets, and has possibly be consumed and urinated out by millions of creatures before us.
Who knows, maybe by extraterrestrial life forms. In any event, the water in Earth existed way before the Earth even existed.
The water you shower with, splash around in, is likely unimaginably older than anything else on Earth. Not just any water, all water, the water you use.
(There are theories that water was created here on Earth, but the conditions required are so extreme that most scientists believe that it came from space.)
Yes, we, like all creatures, subsist largely on piss and shit (dirt that plants grow in that feed animals that we eat is significantly shit, along with rotten plants. That's why we use manure in gardens).
So anyway, if air is actually just free-floating, unbound N and O, how does it keep in a certain ratio and not separate into regions of more concentrated nitrogen and oxygen?
How does it "emulsify" and maintain the property mixture so that every breath any animal takes always contains the right proportion of the gases?
If the oxygen separated locally, the animals there would be greatly affected, plus fires would be more intense.
Breathing pure or concentrated O is extremely corrosive and not good for animals for extended periods. It also encourages a condition called "hyperoxia", which is too much of a good thing, causing your tissues to literally rust away.
If nitrogen separated locally, animals would experience hypoxia, as if they were suddenly transported to a great elevation.
Breathing pure N is perfectly benign, but without O you will quickly go to sleep and never wake up.
By contrast if you breathe pure CO², toxic chemicals will build up in your body immediately and instinctively make you seek O. This is how it feels when you think you're running out of O, but in fact you don't notice that deficiency, you just notice the buildup of CO².
Interestingly, you DON'T notice the buildup of CO, with just one less O, which will silently put you down like N, which is why there are carbon monoxide detectors.
Let's not even talk about how we can suffocate on gas that contains abundant oxygen, the carbon just makes it inaccessible to us.
Like all elements, O and N have different atomic weights, but they're adjacent on the periodic table at 7 and 8, so they're very close.
But any mix of substances with different weights should separate out eventually, just like salad dressings or tequila sunrises.
It seems kind of weird, but the answer is "wind". Kind of.
Basically heat energy keeps everything moving so constantly that it can't settle out.
It's like putting a bottle of salad dressing in a shaking machine, forever.
At this point it's almost natural to wonder why this effect results in a mixture that's perfect for living animals like us, but that's a teleological fallacy, like creationism or "fine-tuning", because WE are optimized for whatever the MIXTURE is, it's not optimized for us.
Like the famous Douglas Adams puddle that found the perfect-shaped hole to fit it.
Some millions of years ago the air was considerably richer in oxygen than it is now, and as a result animals were much different.
For instance, insects were gigantic, there were dragonflies with 6ft wingspans, and the cockroaches... don't think about it.
Imagine having to live in that world.
But we live in this one where the air just so happens to be what we need to breathe, and if it wasn't?
Well we'd be dead of course. There's no "necessity" for human beings or any animals.
That's right, no God. Oh well.