Group 18
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Group 18 is the rightmost column of the periodic table of elements. It contains elements known as noble gases: helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn). The 118th element, oganesson (Og), is also classified in this group based on its electronic structure, though its physical properties are not yet fully understood.
The defining characteristic of this group is that the outermost electron shell is complete. This causes these elements to be highly resistant to bonding with other elements. Under normal conditions they exist as gases, are colorless, and are odorless.
Chemical properties
Because the outer shell is full, the energy required to remove an electron (ionization energy) is very high, while electron affinity is very low. For this reason this group is considered to have low reactivity. However, the heavier noble gases such as xenon have been made into compounds with fluorine and oxygen, for example xenon hexafluoroplatinate and xenon tetrafluoride.
Physical properties
The forces between atoms are weak (London forces). For this reason melting points and boiling points are very low and increase as atomic mass increases. Helium has the lowest melting point of all elements and does not solidify under ordinary pressure.
Occurrence and applications
Argon is the most abundant of the noble gases in Earth's atmosphere. Most of these gases are obtained by fractional distillation of liquefied air. Helium is usually obtained from natural gas wells. Applications include: helium for filling balloons and for cooling powerful magnets; argon for protecting metals and lamps; neon for light-producing lamps; krypton for lamps and for certain medical procedures.