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

(i) The noble gases except helium `color{red}(1s^2)` have completely filled `color{red}(ns^2 np^6)` electronic configuration in their valence shell.

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

`=>` In March `1962`, Neil Bartlett, then at the University of British Columbia, observed the reaction of a noble gas.

● First, he prepared a red compound which is formulated as `color{red}(O_2^+PtF_6^–)`. He, then realised that the first ionisation enthalpy of molecular oxygen `(1175 kJmol^(–1))` was almost identical with that of xenon `(1170 kJ mol^(–1))`.

● He made efforts to prepare same type of compound with `color{red}(Xe)` and was successful in preparing another red colour compound `color{red}(Xe^+PtF_6^–)` by mixing `color{red}(PtF_6)` and xenon.

● After this discovery, a number of xenon compounds mainly with most electronegative elements like fluorine and oxygen, have been synthesised.

`=>` The compounds of krypton are fewer. Only the difluoride `color{red}(KrF_2)` has been studied in detail.

`=>` Compounds of radon have not been isolated but only identified (e.g., `color{red}(RnF_2)`) by radiotracer technique.

`=>` No true compounds of `color{red}(Ar, Ne)` or `color{red}(He)` are yet known.

`=>` Xenon forms three binary fluorides, `color{red}(XeF_2, XeF_4)` and `color{red}(XeF_6)` by the direct reaction of elements under appropriate experimental conditions.

`color{red}(undersettext{(xenon in excess)}(Xe(g)) +F_2 (g) overset(673 , text(1 bar))→ XeF_2 (s))`

`color{red}(undersettext{(1:5 ratio)} (Xe (g)) +2F_2 (g) overset(873K, text(7 bar))→ XeF_4 (s))`

`color{red}(undersettext{(1:20 ratio)} (Xe (g)) +3F_2 (g) oversettext(573 K, 60−70bar)→ XeF_6 (s))`

`=>` `color{red}(XeF_6)` can also be prepared by the interaction of `color{red}(XeF_4)` and `color{red}(O_2F_2)` at `143K`.

`color{red}(XeF_4 +O_2F_2 → XeF_6 +O_2)`

`=>` `color{red}(XeF_2`, `XeF_4)` and `color{red}(XeF_6)` are colourless crystalline solids and sublime readily at `298 K`.

`=>` They are powerful fluorinating agents.

`=>` They are readily hydrolysed even by traces of water. For example, `color{red}(XeF_2)` is hydrolysed to give `color{red}(Xe, HF)` and `color{red}(O_2)`.

`color{red}(2XeF_2 (s) + 2H_2O (l) → 2Xe (g)+4HF(aq) +O_2 (g))`

`=>` The structures of the three xenon fluorides can be deduced from VSEPR and these are shown in Fig. 7.9.

● `color{red}(XeF_2)` and `color{red}(XeF_4)` have linear and square planar structures respectively.

● `color{red}(XeF_6)` has seven electron pairs (6 bonding pairs and one lone pair) and would, thus, have a distorted octahedral structure as found experimentally in the gas phase.

`=>` Xenon fluorides react with fluoride ion acceptors to form cationic species and fluoride ion donors to form fluoroanions.

`color{red}(XeF_2+PF_5 → [XeF]^+ [PF_6]^(-) ; \ \ \ \ \ XeF_4 + SbF_5 → [XeF_3]^(+) [ SbF_6]^(-))`

`color{red}(XeF_6 + MF → M^(+) [XeF_7]^(-) \ \ (M = Na , K , Rb text(or) Cs))`

`=>` Hydrolysis of `color{red}(XeF_4)` and `color{red}(XeF_6)` with water gives `color{red}(XeO_3)`

`color{red}(6XeF_4 +12 H_2O → 4Xe +2XeO_3 +24 HF +3O_2)`

`color{red}(XeF_6+3H_2O → XeO_3 +6HF)`

`=>` Partial hydrolysis of `color{red}(XeF_6)` gives oxyfluorides, `color{red}(XeOF_4)` and `color{red}(XeO_2F_2)`.

`color{red}(XeF_6+H_2O → XeOF_4+2HF)`

`color{red}(XeF_6+2H_2O → XeO_2F_2+4HF)`

`=>` `color{red}(XeO_3)` is a colourless explosive solid and has a pyramidal molecular structure (Fig. 7.9).

`=>` `color{red}(XeOF_4)` is a colourless volatile liquid and has a square pyramidal molecular structure (Fig.7.9).

`=>` `color{green}("Helium ")`

● It is a non-inflammable and light gas.

● It is used in filling balloons for meteorological observations.

● It is also used in gas-cooled nuclear reactors.

● Liquid helium (b.p. `4.2 K`) finds use as cryogenic agent for carrying out various experiments at low temperatures.

● It is used to produce and sustain powerful superconducting magnets which form an essential part of modern NMR spectrometers and Magnetic Resonance Imaging (MRI) systems for clinical diagnosis.

● It is used as a diluent for oxygen in modern diving apparatus because of its very low solubility in blood.

`=>` `color{green}("Neon ")`

● It is used in discharge tubes and fluorescent bulbs for advertisement display purposes.

● Neon bulbs are used in botanical gardens and in green houses.

`=>` `color{green}("Argon ")`

● It is used mainly to provide an inert atmosphere in high temperature metallurgical processes (arc welding of metals or alloys) and for filling electric bulbs.

● It is also used in the laboratory for handling substances that are air-sensitive.

`=>` There are no significant uses of Xenon and Krypton. They are used in light bulbs designed for special purposes.