Matter is everywhere: we are made of matter, the pencils you’ve used are made of matter, the water you drink is made of matter, the air we breathe is also a form of matter (gas). If matter didn’t exist, nothing would. If we are matter ourselves, how would we exist if matter did not? Without air, how would we breathe? This dependence of humans on air makes the gaseous state of matter, extremely important to us.
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.
Gases have a lower density and are highly compressible as compared to solids and liquids.
They exert an equal amount of pressure in all directions.
The space between gas particles is a lot, and they have high kinetic energy.
The inter-molecular forces between these gas particles are negligible.
These particles move at high speeds in all directions and hit each other, thus causing the gas to spread throughout the container they are kept in, evenly. This also causes them to exert pressure on the walls of the container.
So, gases take the volume and shape of the container.
While a real gas has negligible inter-molecular forces of attraction, an ideal gas has zero inter-molecular forces of attraction because the molecules of an ideal gas move so fast and they are so far away from each other that they do not interact at all. There is no ideal gas that exists naturally. However, gases behave most ideally at high temperatures and low pressure conditions.