Group 18 Elements - Noble Gases

Real-Life Connection: From Balloons to Lasers

Party balloons float because of helium! Neon lights up advertising signs in bright colors. Argon fills incandescent bulbs to prevent tungsten evaporation. Deep-sea divers breathe helium-oxygen mixtures to avoid nitrogen narcosis. And surprisingly, “inert” xenon forms compounds - breaking a century-old belief! These noble gases, once thought completely unreactive, have revolutionized lighting, medicine, and chemistry.

Group 18 Elements Overview

Members: Helium (He), Neon (Ne), Argon (Ar), Krypton (Kr), Xenon (Xe), Radon (Rn)

Common Names:

• Noble gases (chemically unreactive)
• Inert gases (old name, not entirely accurate)
• Rare gases (low abundance)

Discovery: Most were discovered by Ramsay (Nobel Prize 1904)

Electronic Configuration Pattern

• General configuration: ns² np⁶ (except He: 1s²)
• Helium: 1s²
• Neon: [He] 2s² 2p⁶
• Argon: [Ne] 3s² 3p⁶
• Xenon: [Kr] 4d¹⁰ 5s² 5p⁶

Complete octet/duplet (stable configuration)

Memory Trick - “He Never Argued, King Xerxes Ruled”: He, Ne, Ar, Kr, Xe, Rn

Key Trends Down the Group

Memory Trick for Reactivity: “Xenon eXtremely reactive, Helium Hardly reacts”

Interactive Demo: Visualize Group 18 in the Periodic Table

Explore the noble gases and their unique position in the periodic table.

Exception Alert: All have positive electron gain enthalpy (energy required to add electron)

General Properties of Noble Gases

Physical Properties

All are:

• Colorless, odorless, tasteless
• Monoatomic gases
• Non-flammable
• Sparingly soluble in water
• Good thermal and electrical insulators

Memory Trick for Abundance: “Argon = Abundant (1% of air), Others = Occasional”

Why Are Noble Gases Unreactive?

1. Complete octet (stable electronic configuration)
2. High ionization energy (difficult to remove electrons)
3. Positive electron gain enthalpy (won’t accept electrons)
4. No tendency to form covalent bonds (already stable)

Exception: Xe and Kr can form compounds (have accessible d-orbitals, lower IE)

Occurrence and Extraction

Occurrence

In atmosphere:

• Ar: 0.93% (most abundant)
• Ne: 0.0018%
• He: 0.0005%
• Kr, Xe: traces

He sources:

• Natural gas wells (up to 7%)
• Radioactive decay (α-particles are He²⁺)
• Sun and stars (fusion reactions)

Rn: Radioactive decay of ²²⁶Ra

²²⁶Ra → ²²²Rn + ⁴He (α-decay)

Extraction

From air (fractional distillation of liquid air):

1. Air liquefied at low temperature
2. N₂ boils off first (-196°C)
3. Ar, Ne, Kr, Xe remain
4. Separated by further fractional distillation

From natural gas (for He):

• Cooling and compression
• He doesn’t liquefy easily (lowest b.p. 4.2 K)
• Separated from other gases

Memory Trick: “FRACS = Fractional distillation Removes Air Components, Separates nobles”

Properties in Detail

Helium (He)

Unique Properties:

• Lightest noble gas (after H₂)
• Lowest boiling point (4.2 K, -269°C)
• Cannot be solidified at normal pressure (quantum effects)
• Only element with two liquid phases (He-I and He-II)
• He-II is superfluid (zero viscosity!)
• Non-flammable (unlike H₂)

Uses:

• Balloons, airships (lighter than air, non-flammable)
• Cryogenics (cooling superconducting magnets in MRI)
• Deep-sea diving (with O₂, prevents nitrogen narcosis)
• Leak detection (small atoms, penetrate tiny holes)
• Inert atmosphere for welding
• Breathing mixtures for asthma patients

Why He, not H₂ for balloons?

• H₂ is flammable (Hindenburg disaster 1937)
• He is non-flammable and safe

Neon (Ne)

Properties:

• Reddish-orange glow in discharge tubes
• Low reactivity (no compounds known)

Uses:

• Neon signs (advertising, distinctive colors)
• Gas lasers (He-Ne laser, red light 632.8 nm)
• High-voltage indicators
• Television tubes

Argon (Ar)

Properties:

• Most abundant noble gas in atmosphere
• Obtained as by-product in O₂ production

Uses:

• Incandescent light bulbs (prevents W filament oxidation)
• Welding (inert atmosphere)
• Museum preservation (prevents oxidation)
• Wine preservation
• Electronics manufacturing

Krypton (Kr)

Properties:

• White glow in discharge tubes
• More polarizable than Ar

Uses:

• High-performance light bulbs
• Flash lamps for photography
• Lasers (excimer lasers)
• Definition of meter (1960-1983): 1,650,763.73 wavelengths of Kr-86 orange line

Xenon (Xe)

Properties:

• Blue glow in discharge tubes
• Forms compounds (most reactive noble gas)
• Anesthetic properties

Uses:

• High-intensity lamps (xenon arc lamps)
• Flash lamps
• Medical anesthetic
• Ion propulsion (space engines)
• Excimer lasers (eye surgery)

Radon (Rn)

Properties:

• Radioactive (α-emitter)
• Longest-lived isotope: ²²²Rn (t₁/₂ = 3.8 days)
• Health hazard (accumulates in basements)

Uses:

• Cancer radiotherapy (limited, due to radioactivity)
• Earthquake prediction (Rn emission changes)

Compounds of Noble Gases

Historical Background

1962: Neil Bartlett’s breakthrough

O₂⁺[PtF₆]⁻ (known compound, orange-red)
Xe + PtF₆ → Xe⁺[PtF₆]⁻ (similar color!)

Actually forms: XePtF₆ or Xe⁺[Pt₂F₁₁]⁻ (complex mixture)

Reasoning:

• IE of Xe (1170 kJ/mol) ≈ IE of O₂ (1175 kJ/mol)
• If O₂ can be oxidized by PtF₆, so can Xe!

Result: Shattered the myth that noble gases are “inert”

Memory Trick: “BART = Bartlett’s Argument: Reactivity Tested” (Xe compounds discovered)

Xenon Fluorides

Why only fluorine?

• Most electronegative element
• Small size, strong oxidizer
• Can oxidize Xe

Why Xe, not He, Ne, Ar?

• Xe largest, lowest IE
• d-orbitals available (5d)
• More polarizable

1. Xenon Difluoride (XeF₂)

Preparation:

Xe + F₂ → XeF₂ (1:1 ratio, 673 K, Ni vessel)
or 2Xe + 2F₂ → 2XeF₂ (electrical discharge)

Structure:

• Linear (sp³d hybridization)
• Three lone pairs on equatorial positions (VSEPR: AX₂E₃)
• F-Xe-F angle: 180°

Properties:

• Colorless crystalline solid
• Powerful fluorinating agent
• Hydrolyzes readily

Reactions:

1. Hydrolysis:

2XeF₂ + 2H₂O → 2Xe + 4HF + O₂

2. With hydrogen:

XeF₂ + H₂ → Xe + 2HF

3. Fluorinating agent:

XeF₂ + 2HCl → Xe + 2HF + Cl₂
XeF₂ + I₂ → Xe + 2IF

2. Xenon Tetrafluoride (XeF₄)

Preparation:

Xe + 2F₂ → XeF₄ (1:2 ratio, 673 K, 6 atm, Ni vessel)

Structure:

• Square planar (sp³d² hybridization)
• Two lone pairs on axial positions (VSEPR: AX₄E₂)
• F-Xe-F angles: 90°

Properties:

• Colorless crystalline solid
• More reactive than XeF₂
• Strong fluorinating and oxidizing agent

Reactions:

1. Hydrolysis (in different conditions):

6XeF₄ + 12H₂O → 4Xe + 2XeO₃ + 24HF + 3O₂ (complete)
3XeF₄ + 6H₂O → Xe + XeO₃ + 12HF + 1.5O₂ (partial)

2. With SiO₂ (glass attack):

2XeF₄ + SiO₂ → 2Xe + SiF₄ + O₂

3. Xenon Hexafluoride (XeF₆)

Preparation:

Xe + 3F₂ → XeF₆ (1:5 ratio, 573 K, 60-70 atm, Ni vessel)

Structure:

• Distorted octahedral (sp³d³ hybridization)
• One lone pair causes distortion (VSEPR: AX₆E)
• Fluxional (rapidly changing shape)

Properties:

• Colorless solid
• Most reactive xenon fluoride
• Very strong fluorinating agent

Reactions:

1. Complete hydrolysis:

XeF₆ + 3H₂O → XeO₃ + 6HF

2. Partial hydrolysis:

XeF₆ + H₂O → XeOF₄ + 2HF
XeF₆ + 2H₂O → XeO₂F₂ + 4HF

3. With quartz:

2XeF₆ + SiO₂ → 2XeOF₄ + SiF₄

4. Forming complex salts:

XeF₆ + MF → M⁺[XeF₇]⁻ (M = Na, K, Rb, Cs)
XeF₆ + 2MF → M₂⁺[XeF₈]²⁻

Comparison of Xenon Fluorides:

Memory Trick: “2-4-6 = Linear-Square-Octahedral” (structure trend)

Xenon Oxides

Xenon Trioxide (XeO₃)

Preparation:

6XeF₄ + 12H₂O → 2XeO₃ + 4Xe + 24HF + 3O₂
XeF₆ + 3H₂O → XeO₃ + 6HF

Structure:

• Trigonal pyramidal (sp³ hybridization)
• One lone pair (VSEPR: AX₃E)

Properties:

• Colorless, explosive solid
• Highly dangerous (explodes on contact with organic matter)
• Powerful oxidizing agent
• Acidic (forms perxenic acid)

Reactions:

1. With water:

XeO₃ + H₂O → H₂XeO₄ (perxenic acid, unstable)

2. Oxidizing agent:

XeO₃ + 6HI → Xe + 3I₂ + 3H₂O

Xenon Tetroxide (XeO₄)

Preparation:

Ba₂XeO₆ + 2H₂SO₄ → 2BaSO₄ + XeO₄ + 2H₂O

Structure:

• Tetrahedral (sp³ hybridization)
• No lone pairs (VSEPR: AX₄)

Properties:

• Colorless, highly explosive gas
• Most unstable xenon compound
• Explodes above -36°C

Xenon Oxyfluorides

Types: XeOF₂, XeOF₄, XeO₂F₂, XeO₃F₂

Example - XeOF₄:

Preparation:

XeF₆ + H₂O → XeOF₄ + 2HF

Structure:

• Square pyramidal (sp³d² hybridization)
• One lone pair (VSEPR: AX₅E)

Krypton Compounds

Very few compounds known (higher IE than Xe)

Example: KrF₂ (krypton difluoride)

Preparation:

Kr + F₂ → KrF₂ (electrical discharge at -183°C)

Structure: Linear (like XeF₂)

Properties:

• Colorless solid
• Decomposes at -10°C
• Very unstable

Why No Compounds of He, Ne, Ar?

1. Very high ionization energy (He = 2372 kJ/mol)
2. Very small size (difficult to accommodate electronegative atoms)
3. No d-orbitals (in He, Ne; Ar has 3d but very high energy)
4. Very stable configuration (difficult to disturb)

Theoretical: ArF₂ predicted but not synthesized yet

Uses of Noble Gases

Memory Trick: “BALANCE = Balloons (He), Advertising (Ne), Lamps (Ar), Anesthesia (Xe), Nuclear therapy (Rn), Cryogenics (He), Electronics (Ar)”

Common Mistakes to Avoid

1. Mistake: Noble gases are completely inert

   • Correct: Xe and Kr form compounds (especially with F)

2. Mistake: All noble gases form compounds

   • Correct: Only Xe and Kr (mainly Xe) form stable compounds

3. Mistake: XeF₂ is bent like H₂O

   • Correct: XeF₂ is linear (3 lone pairs on equatorial positions)

4. Mistake: XeF₄ is tetrahedral

   • Correct: XeF₄ is square planar (2 lone pairs on axial positions)

5. Mistake: Noble gases have negative electron gain enthalpy

   • Correct: Positive (endothermic, won’t accept electrons)

6. Mistake: He can be easily liquefied

   • Correct: Lowest b.p. (4.2 K), very difficult to liquefy

7. Mistake: Xe forms compounds with all halogens equally

   • Correct: Mainly with F (most electronegative), limited with Cl

8. Mistake: XeO₃ is safe to handle

   • Correct: Highly explosive, dangerous!

Practice Problems

Level 1: JEE Main Basics

1. Why are Group 18 elements called noble gases?

2. Arrange in order of: a) Increasing boiling point: He, Ne, Ar, Kr, Xe b) Decreasing ionization energy: He, Ne, Ar, Kr, Xe

3. Write the electronic configuration of: a) Ne b) Ar c) Xe

4. Why is helium preferred over hydrogen in balloons?

5. Draw the structure of: a) XeF₂ b) XeF₄

Level 2: JEE Main Advanced

6. Explain why: a) Noble gases have positive electron gain enthalpy b) Xe forms compounds but He doesn’t c) Helium is used in deep-sea diving

7. Complete and balance: a) XeF₂ + H₂O → b) XeF₆ + 3H₂O → c) Xe + F₂ (1:2 ratio, heat) →

8. What is the hybridization and shape of: a) XeF₂ b) XeF₄ c) XeF₆ d) XeO₃

9. Why does XeF₂ have linear structure despite having lone pairs?

10. Neil Bartlett prepared Xe⁺[PtF₆]⁻. What was his reasoning based on ionization energies?

Level 3: JEE Advanced

11. Using VSEPR theory, predict the shapes of: a) XeF₂ (5 electron pairs) b) XeF₄ (6 electron pairs) c) XeOF₄ (6 electron pairs) d) XeO₃ (4 electron pairs) Explain the role of lone pairs.

12. The bond angles in XeF₄ are exactly 90° and 180°. Explain why there is no distortion despite the presence of lone pairs.

13. Calculate the oxidation state of Xe in: a) XeF₂ b) XeF₆ c) XeO₃ d) XeO₄ e) H₂XeO₄

14. Why is the F-Xe-F bond in XeF₂ longer than expected for a normal Xe-F single bond? (Hint: Consider 3-center-4-electron bonding)

15. Xenon forms XeF₆ but not XeCl₆. Explain based on: a) Electronegativity b) Size of halogen c) Bond energy considerations d) Ability to oxidize Xe

Cross-Links to Other Topics

Related to Periodic Classification

• Periodic Trends - Exceptional electron gain enthalpy
• Noble Gas Configuration - Stability

Related to Chemical Bonding

• VSEPR Theory - XeF₂, XeF₄, XeF₆ structures
• Hybridization - sp³d, sp³d², sp³d³
• 3-Center-4-Electron Bonds - XeF₂ bonding

Related to Atomic Structure

• Electronic Configuration - Octet rule
• Ionization Energy - Trends

Related to Other Chapters

• Qualitative Analysis - No reactions (inert)
• Nuclear Chemistry - Radon radioactivity

Memory Palace for Group 18

Imagine a Noble Gas Palace:

Palace Gates: Sign “He Never Argued, King Xerxes Ruled”

Hall of Stability (entrance):

• Complete octet models hanging (ns² np⁶)
• “No Entry” sign for extra electrons
• Positive electron gain enthalpy meter

Helium Wing (floating):

• Balloons everywhere (lighter than air)
• Cryogenic chamber at 4.2 K
• MRI machine (superconducting magnets)
• Diving suit with He-O₂ mixture

Neon Gallery:

• Colorful neon signs on walls
• Red He-Ne laser beams
• Advertising displays

Argon Chambers:

• Light bulbs with Ar gas
• Welding workshop (inert atmosphere)
• Museum artifacts in Ar cases

Xenon Laboratory (most active wing):

• Bartlett’s original apparatus (1962 display)
• PtF₆ orange crystals

Fluoride Section:

• XeF₂ model: Linear, 3 LP equatorial
• XeF₄ model: Square planar, 2 LP axial
• XeF₆ model: Distorted octahedral, 1 LP

Oxide Section (danger zone):

• XeO₃ behind thick glass (explosive warning!)
• Oxidizing power demonstration
• Trigonal pyramidal model

Oxyfluoride Gallery:

• XeOF₄ model: Square pyramidal

Radioactive Basement:

• Radon chamber (sealed, radiation warning)
• ²²²Rn decay counter
• Medical radiotherapy equipment (historical)

Quick Revision Checklist

• Group 18 configuration: ns² np⁶ (He: 1s²)
• Complete octet = stable = unreactive
• Positive electron gain enthalpy (all nobles)
• Boiling point increases down group
• He: Lowest b.p. (4.2 K), balloons, cryogenics
• Ne: Neon signs, red-orange glow
• Ar: Most abundant (0.93% air), light bulbs
• Xe: Forms compounds (most reactive noble)
• Rn: Radioactive, α-emitter
• Bartlett 1962: Xe⁺[PtF₆]⁻ (first noble gas compound)
• XeF₂: Linear, sp³d, 3 LP
• XeF₄: Square planar, sp³d², 2 LP
• XeF₆: Distorted octahedral, sp³d³, 1 LP
• XeO₃: Explosive, trigonal pyramidal
• Only F forms stable Xe compounds (most electronegative)

Important Equations Summary

1. XeF₂ preparation: Xe + F₂ → XeF₂ (673 K, 1:1)
2. XeF₄ preparation: Xe + 2F₂ → XeF₄ (673 K, 6 atm, 1:2)
3. XeF₆ preparation: Xe + 3F₂ → XeF₆ (573 K, 60 atm, 1:5)
4. XeF₂ hydrolysis: 2XeF₂ + 2H₂O → 2Xe + 4HF + O₂
5. XeF₄ hydrolysis: 6XeF₄ + 12H₂O → 4Xe + 2XeO₃ + 24HF + 3O₂
6. XeF₆ hydrolysis: XeF₆ + 3H₂O → XeO₃ + 6HF
7. XeF₆ partial: XeF₆ + H₂O → XeOF₄ + 2HF
8. XeO₃ from XeF₄: 6XeF₄ + 12H₂O → 2XeO₃ + 4Xe + 24HF + 3O₂
9. Bartlett: Xe + PtF₆ → Xe⁺[PtF₆]⁻
10. Complex: XeF₆ + MF → M⁺[XeF₇]⁻
11. Rn decay: ²²⁶Ra → ²²²Rn + ⁴He
12. KrF₂: Kr + F₂ → KrF₂ (discharge, -183°C)

Visualization: Lone Pair Positions

Key Concept: Lone pairs occupy positions to minimize repulsion (VSEPR)

XeF₂ (AX₂E₃):
    LP
    |
LP--Xe--LP  (3 LP equatorial)
   / \
  F   F     (2 F axial) → Linear

XeF₄ (AX₄E₂):
    LP
    |
F---Xe---F  (2 LP axial)
    |
    LP
   / \
  F   F     (4 F equatorial) → Square planar

XeF₆ (AX₆E):
Octahedral with 1 LP → Distorted

Memory Rule: Lone pairs prefer equatorial in 5-coordination, axial in 6-coordination

Last updated: August 2025 Previous: Group 17 Elements | Next: Oxoacids of p-Block