Gaseous State

The atmosphere is a mixture of gases such as oxygen, carbon dioxide, nitrogen, ozone, water vapour, etc. The gaseous state is the simplest of all states of matter, but only 11 gases in the periodic table behave as gases under standard temperature and pressure conditions ( STP i.e. 1 atm and 273K). These are Helium, Neon, Argon, Krypton, Xenon, Radon, Hydrogen, Nitrogen, Oxygen, Fluorine and Chlorine. These gases are called pure gases.