Q.Why do astronauts have to wear protective suits when they are on the surface of moon?

The gaseous state is the simplest of the three states of matter, and the one we depend on most directly -- we can survive weeks without food and days without water, but only minutes without air. The atmosphere around us is roughly 78% nitrogen, 21% oxygen and 1% other gases by volume, and while different gases behave very differently chemically, they behave remarkably similarly physically -- which is precisely why a single set of gas laws can describe them all.

Only eleven elements exist as gases under ordinary atmospheric conditions: the diatomic molecules hydrogen (H$_2$), nitrogen (N$_2$), oxygen (O$_2$), fluorine (F$_2$) and chlorine (Cl$_2$); ozone (O$_3$), another elemental form of oxygen; and the six monatomic noble gases helium, neon, argon, krypton, xenon and radon. A handful of compounds -- CO, CO$_2$, NO$_2$, NO -- are also gases at room conditions.

Gas vs vapour. A gas is a substance normally in the gaseous state at room temperature and 1 atm; a vapour is the gaseous form of something that is normally a liquid or solid under those same conditions (water vapour, for instance, is the gaseous form of a substance -- water -- that is a liquid at room temperature). …