Introduction

We can go without food for weeks and without water for days, but without air we last only minutes -- a reminder that the gaseous state, though the simplest of the three states of matter, is also the one we depend on most directly and continuously.

The air around us is a mixture of gases, roughly 78% nitrogen, 21% oxygen and 1% other gases by volume. Even though different gases differ enormously in their chemical behaviour, all gases share remarkably similar physical behaviour -- which is exactly what makes a single set of gas laws possible.

Of all the known elements, only eleven exist as gases under ordinary atmospheric conditions. Five of these are diatomic molecules: hydrogen (H$_2$), nitrogen (N$_2$), oxygen (O$_2$), fluorine (F$_2$) and chlorine (Cl$_2$). Another form of elemental oxygen, ozone (O$_3$), is also a gas at room temperature. The six noble gases of group 18 -- helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe) and radon (Rn) -- exist as monatomic gases. A handful of compounds are also gaseous at room conditions, including carbon monoxide (CO), carbon dioxide (CO$_2$), nitrogen dioxide (NO$_2$) and nitric oxide (NO). …