The elements of group 18, such as helium, \mathrm{He}, neon, \mathrm{Ne}, argon, \mathrm{Ar}, krypton, \mathrm{Kr}, and xenon, \mathrm{Xe}, are called the noble gases.
Helium has 2 electrons in the K shell, and each of the other elements has 8 electrons in its outermost shell, which is a very stable arrangement of electrons. For this reason, they do not readily give, take or share electrons with other atoms, and their number of valence electrons is taken to be 0.
The noble gases hardly react, but xenon is known to form compounds with fluorine.
Noble gas atoms do not bond to other atoms, so each single atom behaves as a molecule by itself. Molecules like this are called monatomic molecules.
The larger the atom, the stronger the intermolecular forces, so the boiling point is higher for elements further down the group. The boiling point of helium is about -269 °C, the lowest of all substances.
Argon makes up about 0.9 % of the air by volume, and it is the most abundant component after nitrogen and oxygen.
Because it is so unreactive, it is put inside light bulbs to keep the filament from being damaged, and it is used as a gas that shields metals from the air while they are welded.
The name argon comes from the Greek word for lazy.
When a high voltage is applied to a glass tube containing a noble gas so that an electric discharge passes through it, the gas glows with a colour that depends on which gas it is.
Neon glows red-orange, and argon glows blue-violet. The light of neon has been used in neon signs.
Helium is the lightest gas after hydrogen, and its density is about one-seventh that of air. Unlike hydrogen, it does not burn, so it is used to make balloons and airships float.
Liquid helium is extremely cold, at about -269 °C, so it is also used to cool the powerful magnets of MRI scanners in hospitals.