What are Noble Gases?
Noble gases are chemically unreactive gases (at least under normal earth-like conditions) that exist in trace quantities in our atmosphere and to varying extents dissolved in the ocean. They are unreactive because their outermost electron shells are completely filled, so they don't form chemical bonds very easily. If you look at the table of elements, you will find them in the far right hand column. The first 5 noble gases are
Helium: This is the lightest of the gases, and has the elemental symbol He. It is the second most abundant element in the universe, but exists in our atmosphere at only 5 parts per million. The reason why it is so rare in our atmosphere is because it is so light: a lot of it has been lost to outer space over geologic time. It is very insoluble in water compared to most other gases.
Neon: The second lightest noble gas has the elemental symbol Ne. It is much less abundant in our universe than He, but it is actually about 4 times more abundant (18 parts per million) than He in our atmosphere. Ne is also very insoluble in water, but a little more than He.
Argon: Argon's elemental symbol is Ar, and it is the most abundant noble gas in the atmosphere. There's so much Ar in the atmosphere because most of it was produced by a long-lived radioactive isotope of potassium in the earth's crust and released over geologic time. Ar is more soluble in seawater than He and Ne.
Krypton: This has nothing to do with superman! This is a heavier noble gas with the elemetnal symbol Kr. It is rarer than He in the atmosphere, having an abundance slightly more than 1 part per million. It is also more soluble than Ar in water.
Xenon: This is the heaviest of the stable noble gases, with the elemental symbol Xe. It is also the least abundant in our atmosphere, being 10 times less common than Kr. It is the most soluble of the 5 stable noble gases in water.