Group 18 Elements: The Noble Gases -- General Trends and Uses

Group 18, the noble gases, comprises helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon

(Xe) and the radioactive radon (Rn). Every member except helium has the general outer

configuration $ns^2np^6$, a fully complete octet; helium itself has the fully complete, though

smaller, $1s^2$ configuration. Because every one of these elements already possesses a filled

valence shell, none has any inherent "urge" either to gain or to lose electrons in the way every

other p-block group does, and this is the structural root of the entire group's defining

property: chemical inertness.

Physically, the noble gases are monatomic gases at room temperature (the only elements that

exist as single, uncombined atoms rather than as diatomic or polyatomic molecules under ordinary

conditions), and their inter-atomic forces are limited to very weak, instantaneous-dipole van der

Waals attractions. Consequently they have exceptionally low melting and boiling points --

the lowest of any elements in the corresponding period -- rising down the group as atomic size

(and hence polarisability, and hence the strength of these weak van der Waals forces) increases

from helium to radon. Down the group, ionisation enthalpy is very high throughout (the

highest in each respective period, since a full octet resists electron removal strongly) but

falls steadily from helium (which has the highest first ionisation enthalpy of any element)

to radon, as the outer electrons sit progressively farther from the nucleus.

Because they are so unreactive, the noble gases find their principal uses in situations that

specifically require an inert, non-combustible, non-reactive atmosphere or medium, or that

exploit their light weight or characteristic emission colours. Helium, non-flammable and far

lighter than air, has replaced hazardous hydrogen in weather balloons and airships; mixed with

oxygen, it is used as a breathing gas for deep-sea divers, since it is much less soluble in blood

than nitrogen and so avoids the painful "bends" (decompression sickness) and dangerous nitrogen

narcosis associated with compressed-air diving; and, liquefied, it provides the very low

temperatures needed to maintain the superconducting magnets used in MRI scanners and other

cryogenic apparatus. Neon is used in discharge tubes and advertising signs, glowing with its

characteristic reddish-orange light when an electric current is passed through it at low

pressure, and in lightning-arrestor/surge-protection devices. Argon, being cheap, plentiful

(the most abundant noble gas in air) and totally inert, is used to provide a protective,

oxidation-free atmosphere for arc welding of reactive metals and inside incandescent electric …