Group-18: Inert Gases

What are Inert Gases?

Members of group 18 in the modern periodic table are known as noble gases or inert gases. They are colourless, odourless gases at room temperature. These elements were isolated by William Ramsay in 1898 from the air. The members of the group are:

1. Helium (He)
2. Neon (Ne)
3. Argon (Ar)
4. Krypton (Kr)
5. Xenon (Xe)
6. Radon (Rn)

The members of the group have eight electrons in their outermost orbit (except helium which has two electrons). Thus, they have a stable configuration. Group 18 elements are gases and chemically unreactive, which means they don't form many compounds. Thus, the elements are known as inert gases. Like the other group elements, noble gas elements also exhibit trends in their physical and chemical properties. The general characteristics of noble gases are discussed below. 

Although noble gases were earlier considered completely inert, the discovery of compounds of xenon and krypton showed that heavier noble gases can form stable compounds under suitable conditions.

General/Physical Characteristics of Group 18 Elements:

1. Electronic configuration: 

   Their valence shell electronic configuration is ns² np⁶ except He (Helium) which is 1s².
   

2. Physical state: 

   They are all gases under ordinary conditions of temperature and pressure.

3. Abundance in Air: 

   Among the noble gases present in the atmosphere, the order of abundance is Ar > Ne > He > Kr > Xe.

4. Atomicity: 

   The Cp/Cv = 1.67 shows their monoatomic nature for an ideal monoatomic gas. 
   

5. Melting and boiling points: 

   Due to the increase in the magnitude of Van der Waals' forces, the melting point and boiling point increase from He to Rn i.e. down the group.

6. Atomic and Van der Waals' radii: 

   The atomic radii increase from He to Rn i.e. down the group. It corresponds to the Van der Waals' radii. 

7. Density: 

   The density of noble gases increases down the group i.e. from helium to radon because of increasing atomic mass. 

8. Heat of vaporisation: 

   They have very low values of heat of vaporisation due to weak Van der Waals' forces of attraction. The value increases down the group with stronger dispersion forces for larger atoms. 

9. Solubility in water: 

   They are slightly soluble in water and solubility increases from He to Rn i.e. down the group as polarisation increases.

10. Liquefication: 

    It is extremely difficult to liquefy inert gases due to weak Van der Waals' forces of attraction among their molecules. Hence, they possess a low value of critical temperature also.

11. Ionisation energy: 

    All noble gases possess a very stable (ns² and ns² np⁶) electronic configuration. Therefore, the ionisation energy of noble gases is very high and decreases down the group due to increase in atomic size and shielding. 

12. Electron affinity: 

    Due to the presence of stable electronic configuration, they have no tendency to accept an additional electron. Therefore, electron affinity is almost zero.

Chemical Properties of Group 18 Elements

The noble gases are inert in nature because of their completely filled subshells. However, under suitably extreme conditions, particularly for the heavier noble gases (krypton and xenon), compounds can be formed. In 1962, the first compound of noble gases was prepared. It is hexafluoroplatinate (prepared by Bartlett).

Xe + PtF₆ → Xe[PtF₆]

Now, many compounds of Xe and Kr are known with fluorine and oxygen. 

Preparation of Compounds of Xenon:

Xenon reacts with fluorine under suitable conditions (e.g., heating under pressure, electrical discharge or presence of powerful fluorinating agents) to give xenon fluorides.

Krypton forms one well-established fluoride, KrF₂, usually prepared by the reaction of krypton with fluorine using electrical discharge or irradiation.

Chemical Reactions of Xenon Compounds:

Important chemical behaviour and reactions of xenon fluorides and related compounds:

• Hydrolysis: Xenon fluorides hydrolyse to form oxides and oxyacids; for example, hydrolysis of xenon hexafluoride produces xenon trioxide and hydrofluoric acid: XeF₆ + 3 H₂O → XeO₃ + 6 HF. Xenon trioxide (XeO₃) and xenon tetroxide (XeO₄) are powerful oxidising agents.
• Oxidation states of xenon: Xenon exhibits oxidation states +2, +4, +6, and +8 in different compounds: XeF₂ (+2), XeF₄ (+4), XeF₆ (+6) and XeO₄ (+8).
• Structures (VSEPR-based): XeF₂ is linear (AX₂E₃), XeF₄ is square planar (AX₄E₂), and XeF₆ is distorted octahedral (AX₆E₁).
• Redox and complex formation: Xenon forms salts with strongly oxidising anions, e.g., xenon hexafluoroplatinate. Many xenon compounds are strong oxidising agents and must be handled with care.

Occurrence, Isolation and Commercial Production

Occurrence: Noble gases are present in the atmosphere in small amounts; argon is the most abundant noble gas in air. Helium is obtained commercially from natural gas.

Isolation: Noble gases such as argon, neon, krypton and xenon are obtained by fractional distillation of liquid air.

Summary

Noble gases (Group 18) are characterised by filled valence shells, very low chemical reactivity under normal conditions, low boiling and melting points and monoatomic gaseous nature. The heavier members (krypton and xenon) can form stable compounds, particularly with fluorine and oxygen; these discoveries enlarged the classical idea of "inertness". Noble gases have important practical uses-from cryogenics (helium) to lighting (neon, xenon), and in specialised industrial and scientific applications-but require handling precautions where they are reactive (xenon compounds) or radioactive (radon).