Group 18 consists of six elements: helium, neon, argon, krypton, xenon and radon. All these are gases and chemically unreactive. They form very few compounds. Because of this they are termed noble gases.

All noble gases except Radon are found in the atmosphere. Their abundance in dry air is less than 1% in which argon is the major constituent. The main source of helium is natural gas. Helium and Neon are also found in radioactive minerals. Radon is obtained in a radioactive decay. Xenon and Radon are the rarest elements of the group.

All noble gases have general electronic configuration ns2 np6 except helium which has ns2 configuration. As they have closed shell structures they are inactive in nature.

Order of Atomic radii: Xe>Kr>Ar>Ne>He

Due to the stable electronic configuration these elements have high ionization enthalpy values.

Order of Ionization enthalpy: He>Ne>Ar>Kr>Xe>Rn

Since noble gases have stable electronic configuration they have no tendency to accept an electron so they have high positive electron gain enthalpy.

Order of electron gain enthalpy: Ne>Ar=Kr>Xe>Rn>He

All the noble gases are monoatomic. They are colorless, odorless and tasteless. The interatomic interaction between them is weak dispersion force hence these elements have low melting and boiling points.

Helium has the lowest boiling point (4.2 K) of any known substance. It has an unusual property of diffusing through most commonly used laboratory materials such as rubber, glass or plastics.

In general, noble gases are least reactive. Their inertness to chemical reactivity is attributed to the following reasons:

(i) The noble gases except helium (1s2 ) have completely filled ns2 np6 electronic configuration in their valence shell.

(ii) They have high ionization enthalpy and more positive electron gain enthalpy.

Since they are not reactive attempts to force them to react to form the compounds, were unsuccessful for many few years.

Neil Bartlett, at the University of British Columbia, noticed the reaction of a noble gas in March 1962. First, he created a red compound with the formula O2 + PtF6 -. He then discovered that the first ionization enthalpy of molecular oxygen (1175 kJmol-1) was nearly identical to that of xenon (1170 kJmol-1). He attempted to make the same type of compound using Xe and was successful in making another red color compound Xe+ PtF6 - by combining PtF6 and xenon.

After this discovery, a number of xenon compounds mainly with most electronegative elements like fluorine and oxygen, have been synthesized. The compounds of krypton are fewer. Only the difluoride (KrF2 ) is known.

Compounds of radon have not been isolated but only identified (e.g., RnF2 ) by radiotracer technique.

No true compounds of Ar, Ne or He are yet known.

Xenon Compounds

• Xenon- Fluorine Compounds

Xenon forms three binary fluorides, XeF2 , XeF4 and XeF6 by the direct reaction of elements under appropriate experimental conditions.

Xe(in excess)+F2→XeF2 [673K, 1bar]

Xe+2F2→XeF4 [873K, 7bar]

Xe+3F2→XeF6 [573K, 60-70bar]

XeF6 can also be synthesized by reaction between XeF4 and O2F2.

XeF2 has linear structure, XeF4 has square planar structure and XeF6 has distorted octahedral structure since it has 6 bonding pairs and a lone pair.

XeF2 , XeF4 and XeF6 are colorless crystalline solids. They are powerful fluorinating agents and are hydrolyzed readily even in small traces of water.

For example: 2XeF2+2H2O→2Xe+4HF+O2

• Xenon-Oxygen Compounds

Hydrolysis of XeF4 and XeF6 with water gives XeO3.

6XeF4 + 12 H2O → 4Xe + 2Xe03 + 24 HF + 3 O2

XeF6 + 3 H2O → XeO3 + 6 HF

Partial hydrolysis of XeF6 gives oxyfluorides, XeOF4 and XeO2F2 .

XeF6 + H2O → XeOF4 + 2 HF

XeF6 + 2 H2O → XeO2F2 + 4HF

XeO3 is a colorless explosive solid and has a pyramidal molecular structure.

XeOF4 is a colorless volatile liquid and has a square pyramidal molecular structure.

Liquid helium is used as cryogenic agent for carrying out various experiments at low temperatures. It is used to produce superconducting magnets which form an essential part of modern NMR spectrometers and Magnetic Resonance Imaging (MRI) systems. It is used as a diluent for oxygen in modern diving apparatus because of its very low solubility in blood.

Neon is used in discharge tubes and fluorescent bulbs for displays.

Argon is used mainly to provide an inert atmosphere in high temperature.

There are no significant uses of Xenon and Krypton.