What’s air made of? Strictly speaking, air is mostly nitrogen (about 78 percent) and oxygen (about 21 percent), which leaves about 1 percent of other stuff, such as water vapor and carbon dioxide.
But on the whole, air is mostly empty space. In fact, if you ever condensed a gallon of air into a solid, you’d end up with a little over one-tenth of 1 percent of the air’s original volume.
And the reason we don’t notice all this emptiness is because air molecules are constantly moving around at the speed of sound, and bouncing against any surface that happens to fall in their path.
The force these air molecules exert is what we measure when we measure air pressure.
“Are There Spaces Between the Air We Breathe” (Mad Sci Network)