Here are five of the six noble gases: helium, neon, argon, kypton and xeon. They're all colourless and transparent. Krypton and xeon form compounds only with difficulty. Helium, neon and argon don't form compounds at all.
As we descend the group in the periodic table the atomic number and relative atomic mass increases. The gasses get denser.
Helium - helium is lighter than air. Neon is just lighter. Argon and krypton - I've just got to unstick these from the anchorage... Argon and krypton are both heavier than air. And xeon, xeon is the heaviest of the lot, you've heard of a lead balloon, well this is it.
The noble gases, in order of their density, are helium, neon, argon, krypton, xenon and radon. They are called noble gases because they are so majestic that, in general, they don’t react with anything. For this reason they are also known as inert gases.
The noble gases are present in the atmosphere in small amounts:
- 0.934% Argon
- 0.0018% Neon
- 0.00052% Helium
- 0.00011% Krypton
- 0.000009% Xenon
Most of the noble gases are extracted from the air, except for helium. Helium is a product of radioactive decay (the alpha particle) and is found naturally in rocks. Most of the helium used nowadays comes from natural gas, of which it comprises 7%.
Helium, the lightest, is used for filling party balloons. It is also mixed with oxygen to create a mixture that divers can breathe at depth – the helium is not very soluble in the blood so it avoids the “bends”.
Neon is probably most famous for neon signs – a tube containing neon gas through which a current is passed which causes the gas to give out light. When mixed with helium it is used to make helium-neon lasers.
Argon is used as the inert atmosphere in many light bulbs – an electric current is passed through a wire to heat it up so that it gets so hot it emits light. At these temperatures the metal would react with any oxygen present which is why an inert gas is needed.
Krypton is used to produce white light for photography – again an electrical current is passed through the gas so it emits light.
Xenon is again used in high quality lamps such as those used in lighthouses and in lasers.
Radon is a radioactive element. Radon gas is formed by decay of other radioactive elements and the concentration in the environment depends upon the types of rocks on which you live. For example, the granite in Dartmoor contains small amounts of uranium that forms radon, which can accumulate in buildings and drinking water. This leads to an increased risk of cancer.