Group 18 Elements — Revision Notes

⚡ 30-Second Revision

Electronic Configuration: — $ns^2np^6$ (except He: $1s^2$).
Inertness: — Due to stable, filled valence shell.
Ionization Enthalpy (IE): — Very high, decreases down group.
Electron Gain Enthalpy (EGE): — Highly positive (unfavorable).
Boiling Points: — Very low, increases down group (due to increasing London forces).
Physical State: — Monoatomic gases, colorless, odorless, tasteless.
Xenon Compounds: — XeF$_2$ (linear, $sp^3d$), XeF$_4$ (square planar, $sp^3d^2$), XeF$_6$ (distorted octahedral, $sp^3d^3$), XeO$_3$ (pyramidal, $sp^3$), XeOF$_4$ (square pyramidal, $sp^3d^2$).
Uses: — He (cryogenics, diving), Ne (neon signs), Ar (inert atmosphere, bulbs), Kr (flash lamps), Xe (high-intensity lamps), Rn (radiotherapy).

2-Minute Revision

Group 18 elements, the noble gases (He, Ne, Ar, Kr, Xe, Rn), are characterized by their exceptional chemical inertness. This stems from their stable electronic configuration ($ns^2np^6$, or $1s^2$ for He), which results in very high ionization enthalpies and positive electron gain enthalpies.

They exist as monoatomic gases with very low melting and boiling points due to weak London dispersion forces, which increase down the group, causing boiling points to rise from He to Rn. While traditionally considered unreactive, heavier noble gases, particularly Xenon, form compounds with highly electronegative elements like fluorine and oxygen.

Key Xenon compounds include XeF$_2$ (linear, $sp^3d$), XeF$_4$ (square planar, $sp^3d^2$), and XeF$_6$ (distorted octahedral, $sp^3d^3$), along with oxides like XeO$_3$ (pyramidal, $sp^3$) and oxyfluorides like XeOF$_4$ (square pyramidal, $sp^3d^2$).

These structures are crucial for NEET. Each noble gas has specific applications, such as Helium in cryogenics, Neon in advertising signs, Argon as an inert atmosphere, and radioactive Radon in cancer therapy.

5-Minute Revision

The Group 18 elements, known as noble gases, are He, Ne, Ar, Kr, Xe, and Rn. Their defining feature is chemical inertness, primarily due to their stable $ns^2np^6$ valence electron configuration (He is $1s^2$).

This full valence shell means they have no tendency to gain, lose, or share electrons, leading to very high ionization enthalpies (energy to remove an electron) and positive electron gain enthalpies (energy change on adding an electron).

These values decrease down the group as atomic size increases.

Physically, they are monoatomic gases, colorless, odorless, and tasteless. Their melting and boiling points are extremely low because only weak London dispersion forces exist between their atoms. These forces increase with atomic size, so boiling points increase from He to Rn. For example, Helium has the lowest boiling point of any substance ($4.2,\text{K}$).

The 'inert' label is not absolute. Neil Bartlett's work in 1962 showed that heavier noble gases, especially Xenon, can form compounds. Xenon forms various fluorides: XeF$_2$ (linear, $sp^3d$ hybridization), XeF$_4$ (square planar, $sp^3d^2$), and XeF$_6$ (distorted octahedral, $sp^3d^3$).

These fluorides can be hydrolyzed to form oxides like XeO$_3$ (pyramidal, $sp^3$) and oxyfluorides like XeOF$_4$ (square pyramidal, $sp^3d^2$). Understanding the VSEPR theory to predict these geometries is vital.

For instance, in XeF$_4$, Xe has 8 valence electrons, 4 form bonds with F, leaving 4 electrons (2 lone pairs). Total 6 electron domains ($4 \text{bp} + 2 \text{lp}$), leading to $sp^3d^2$ hybridization and square planar geometry.

Each noble gas has practical uses: Helium for cryogenics, balloons, and deep-sea diving mixtures; Neon for advertising signs; Argon for inert atmospheres in welding and light bulbs; Krypton for flash lamps; Xenon for high-intensity lamps; and radioactive Radon for cancer therapy. Remember to differentiate between the properties and uses of each element, and pay close attention to the conditions required for Xenon compound formation.

Prelims Revision Notes

Group 18 Elements (Noble Gases) - NEET Revision Notes

1. General Characteristics:

Elements: — Helium (He), Neon (Ne), Argon (Ar), Krypton (Kr), Xenon (Xe), Radon (Rn).
Electronic Configuration: — $ns^2np^6$ (except He: $1s^2$). This is the primary reason for their stability.
Physical State: — Monoatomic gases at room temperature, colorless, odorless, tasteless.
Solubility: — Sparingly soluble in water.

2. Trends in Physical Properties (Down the Group He to Rn):

Atomic Radii: — Increases (due to increasing number of shells).
Ionization Enthalpy (IE): — Very high, but decreases down the group (due to increasing size and shielding effect).
Electron Gain Enthalpy (EGE): — Highly positive (or less negative), indicating no tendency to accept electrons. Decreases (becomes less positive) down the group.
Melting & Boiling Points: — Very low, but increases down the group (due to increasing strength of London dispersion forces with increasing atomic size/polarizability).
Density: — Increases down the group.

3. Chemical Properties & Compounds:

Inertness: — Generally unreactive due to stable electronic configuration.
Reactivity: — Increases down the group (Xe > Kr > Ar > Ne > He) due to decreasing IE and increasing atomic size, making valence electrons more accessible.
Xenon Compounds (Most Important for NEET):

* Fluorides: * **XeF$_2$:** Linear, $sp^3d$ hybridization (2 bp, 3 lp). * **XeF$_4$:** Square planar, $sp^3d^2$ hybridization (4 bp, 2 lp). * **XeF$_6$:** Distorted octahedral, $sp^3d^3$ hybridization (6 bp, 1 lp).

* Oxides: * **XeO$_3$:** Pyramidal, $sp^3$ hybridization (3 bp, 1 lp). Formed by complete hydrolysis of XeF$_4$ or XeF$_6$. Explosive. * **XeO$_4$:** Tetrahedral, $sp^3$ hybridization. Highly explosive.

* Oxyfluorides: * **XeOF$_4$:** Square pyramidal, $sp^3d^2$ hybridization (5 bp, 1 lp). Formed by partial hydrolysis of XeF$_6$. * **XeO$_2$F$_2$:** Trigonal bipyramidal, $sp^3d$ hybridization (4 bp, 1 lp).

Formed by partial hydrolysis of XeF$_6$.

Krypton Compounds: — Only KrF$_2$ is known, less stable than Xe compounds.
He, Ne, Ar: — No true stable compounds known under normal conditions.

4. Uses of Noble Gases:

Helium (He): — Non-flammable, lightest, used in balloons, cryogenics (superconductors, MRI), deep-sea diving (diluent for O$_2$).
Neon (Ne): — Used in discharge tubes, 'neon signs' (reddish-orange glow), beacon lights.
Argon (Ar): — Most abundant noble gas in air, used for inert atmosphere in welding, filling electric bulbs (prevents filament oxidation), metallurgical processes.
Krypton (Kr): — Used in some photographic flash lamps, high-efficiency incandescent bulbs.
Xenon (Xe): — Used in high-intensity discharge lamps (car headlights, projection), specialized lasers.
Radon (Rn): — Radioactive, used in radiotherapy for cancer treatment. Naturally occurring radioactive gas.

Vyyuha Quick Recall

To remember the order of noble gases and their increasing boiling points: Heavy New Army Kry Xenophobes Really Need. (He, Ne, Ar, Kr, Xe, Rn). For Xenon compounds and their shapes, visualize the central Xe atom and count electron pairs for VSEPR: 'Two Fs, Linear; Four Fs, Square; Six Fs, Distorted Octa; Three Os, Pyramidal; Four Fs and One O, Square Pyramidal.'