Ok, I'll spare you the sentences about taking a breath and thinking about it. Let's cut to the chase...
Nitrogen, N2 78%
Oxygen, O2 21%
Argon, Ar 1%
Water, H2O 0% yeah, z-e-r-o that's why it's called DRY air
That is it. The major components of dry air. I know, 1% for argon doesn't seem that major. I hear you, but there are so many other things in the air that are below 1%, it is worth putting argon there as a friendly reminder of the last of the major components.
Hey, while we are here, the 78/21 thing with nitrogen and oxygen is really close to 80/20 which is really just a percentage way of saying 4/1 or 4:1 or 4-to-1 or there are 4 parts nitrogen to every 1 part oxygen. I'm ok with that.
Well that is our big variable in the composition of air. If you have bone-dry air then the water content is truly 0%. This is what we call "dry" air. Now imagine air that is saturated with water - meaning water is free to evaporate and go to gas phase and reach its vapor pressure at a given temperature. Know this, the highest that will ever be under typical weather conditions is about 7% and that is for 100% relative humidity on a 104 °F day. More typical is around 3-5% for "humid" conditions. So if it is really humid, you could say that air is 74% N2, 20% O2, 5% H2O, and 1% Ar. Because of the water factor, most all calculations and dealings with "air" assume dry air.
It's good to wonder about CO2 and our air. It gets a lot of news due to its acclaimed position as the number one greenhouse gas for earth (not counting water vapor). Let's just say up front that CO2 is only about 0.04% of our troposphere. CO2 even has its own website to tell the world what its level is - go to co2.earth and find out.
Well the answer to that is air, duh - but let's go a bit further than that. Let's say you are in Austin, TX on an 85 °F day with about 73% humidity. When you breathe in, you are pulling an air composition of about 76% N2, 20% O2, 3% H2O, and 1% Ar (a trace, 0.04% CO2). But, the air you are breathing back out is about 75% N2, 15% O2, 5% H2O, 4% CO2, and 1% Ar. There are a couple of things to notice about those exhaled vs inhaled numbers.
First, notice that tiny amount of CO2 inhaled (0.04%) is now up at a full 4% which is a 100× jump in concentration. And second, there is still 15% O2 in your exhaled breath - which is good news for anyone on the receiving end of mouth-to-mouth resuscitation. Nice to know they are blowing 15% oxygen into your lungs to save you. Pretty much all mammals do this breathing thing... we pull in air and blow out air with a about 5% less oxygen and a jump up to about 4% CO2. Multiply this by all the living mammals on earth and integrate over a year and you get around 60 Gt (that's a gigatonne = 1012 kg) of CO2 pushed into the atmosphere per year by respiration. We will chat more about CO2 in chapter 4.
Interesting "Trivia" page about Air: https://climate.nasa.gov/news/2491/10-interesting-things-about-air/.