The name gas comes from the word chaos, which neatly summarises the main feature of the simplest state of matter.
A gas is a swarm of particles moving randomly and chaotically, constantly colliding with each other and the walls of any container. The real volume of the particles is minute compared to the total space which they occupy and this is why gases fill any available volume and are readily compressed. The average speeds of gas molecules are of the order of 100s of metres per second and they are colliding with each other billions of times per second. This is why gases mix rapidly and why they exert pressure.
This constant motion is easily demonstrated by releasing a small amount of odorous gas in a room. Within seconds the gas can be smelt in all parts of the room. These properties apply to substances, which are normally gaseous, and to vapours from evaporated liquids.
A volume of any gas at the same temperature and pressure contains the same number of molecules irrespective of what the gas is. This means that measuring gas by volume is very convenient. Gas measurements at higher levels are in % (volume) and at lower levels parts per million, ppm (volume).
Whilst different gases have different densities, they do not totally separate into layers according to their density. Heavy gases tend to sink and light gases tend to rise, but their constant motion means that there is continuous mixing (i.e they do not behave like liquids).
So in a room where there is a natural gas (methane) leak, the gas will tend to rise because it is lighter than air but the constant motion means that there will be a considerable concentration at floor level. This will happen in perfectly still conditions but if there are any air currents, the mixing will be increased.
Air is a mixture of gases, typically:
Nitrogen 77.2 %
Oxygen 20.9 %
Water Vapour 0.9 %
Argon 0.9 %
Carbon Dioxide 0.03 %
Other Gases 0.07 %
Because its composition is reasonably constant, air is usually considered as a single gas, which simplifies the measurement of toxic and flammable gases for safety and health applications.