Group 18 Elements: The Noble Gases
Group 18 consists of six elements: Helium ($He$), Neon ($Ne$), Argon ($Ar$), Krypton ($Kr$), Xenon ($Xe$), and Radon ($Rn$). They are known as Noble Gases because they are chemically inert due to their stable, completely filled valence shell configuration ($ns^2np^6$, except Helium $1s^2$).
1. General Characteristics
- Electronic Configuration: Stable octet ($ns^2 np^6$).
- Ionization Enthalpy: Very high (Highest in their respective periods).
- Electron Gain Enthalpy: Large positive values (They have no tendency to accept electrons).
- Physical State: All are monoatomic, colorless, odorless, and tasteless gases.
- Forces: Only weak dispersion forces (London forces) exist between atoms, leading to very low melting and boiling points.
2. Discovery of Reactivity (Neil Bartlett)
The Breakthrough
In 1962, Neil Bartlett observed that $PtF_6$ reacts with Oxygen to form an ionic solid $O_2^+ [PtF_6]^-$.
He realized that the First Ionization Enthalpy of molecular oxygen ($1175 \ kJ/mol$) is almost identical to that of Xenon ($1170 \ kJ/mol$).
He successfully prepared the first noble gas compound:
$$ Xe + PtF_6 \rightarrow Xe^+[PtF_6]^- \text{ (Red solid)} $$
3. Compounds of Xenon
Xenon forms fluorides, oxides, and oxyfluorides. Krypton forms only $KrF_2$. Radon is radioactive.
A. Xenon Fluorides ($XeF_2, XeF_4, XeF_6$)
Prepared by direct reaction of Xenon and Fluorine under specific conditions.
B. Reactions of Xenon Fluorides
- Hydrolysis:
- $2XeF_2 + 2H_2O \rightarrow 2Xe + 4HF + O_2$
- $XeF_6 + 3H_2O \rightarrow XeO_3 + 6HF$ (Complete Hydrolysis)
- $XeF_6 + H_2O \rightarrow XeOF_4 + 2HF$ (Partial Hydrolysis)
- With Fluoride Ion Acceptors: $XeF_2 + PF_5 \rightarrow [XeF]^+ [PF_6]^-$
- With Fluoride Ion Donors: $XeF_6 + NaF \rightarrow Na^+ [XeF_7]^-$
4. Structures (VSEPR Theory) - Exam Critical
| Compound | Hybridization | Lone Pairs | Shape (Geometry) |
|---|---|---|---|
| $XeF_2$ | $sp^3d$ | 3 | Linear |
| $XeF_4$ | $sp^3d^2$ | 2 | Square Planar |
| $XeF_6$ | $sp^3d^3$ | 1 | Distorted Octahedral |
| $XeO_3$ | $sp^3$ | 1 | Pyramidal |
| $XeOF_4$ | $sp^3d^2$ | 1 | Square Pyramidal |
5. Uses
- Helium: Used in filling balloons (light, non-flammable), in diving apparatus (to prevent bends, low solubility in blood), and in NMR machines (cryogenic agent).
- Argon: Used to provide an inert atmosphere in welding and electric bulbs.
- Neon: Used in discharge tubes and fluorescent bulbs for advertisement display.
Noble Gases Quiz
Test your concepts on Group 18. 10 MCQs with explanations.
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