Group 18 Elements Revision Notes, Mnemonics and Trends

Group 18 Elements (Noble Gases): He, Ne, Ar, Kr, Xe, Rn

This quick revision Notes, Trends and Mnemonics are very important for CSIR-NET, GATE, SLET, NEET, JEE Main/Advanced, IIT-JAM.

Mnemonics: He: He | Ne: Never | Ar: Arrived | Kr: Krypton | Xe: Se | Rn: Radon

1. General Electronic Configuration

ns² np⁶ (except He: 1s²) → completely filled stable octet → chemically inert

2. Key Periodic Trends (Down the Group: He → Rn)

Property	Trend	Important Note/Exception
Atomic & Ionic Radius	Increases	He < Ne < Ar < Kr < Xe < Rn (Largest atomic radii among respective periods)
Ionisation Energy	Decreases	He > Ne > Ar > Kr > Xe > Rn He has highest IE in periodic table
Electron Affinity	≈ 0	Stable octet → no tendency to gain electron
Melting & Boiling Point	Increases	He < Ne < Ar < Kr < Xe < Rn (Due to increasing van der Waals forces)
Polarisability	Increases	He < Ne < Ar < Kr < Xe < Rn (Ease of distortion of electron cloud)
Adsorption on Surface	Increases	Rn > Xe > Kr > Ar > Ne > He
Chemical Reactivity	Increases	He ≈ Ne (none) < Ar (very few) < Kr < Xe < Rn Xe has maximum compounds
Density & Solubility in Water	Increases	Both increase down the group

3. Occurrence & Isolation

• Present in atmosphere (Ar ~0.93%, others in ppm)
• He from natural gas & radioactive decay (α-particles)
• Isolated by fractional distillation of liquid air (except He & Rn)
• Rn: radioactive, from radium decay

4. Important Compounds (Mainly Xe & some Kr)

• Xenon Fluorides:
  • XeF₂ (linear, sp³d)
  • XeF₄ (square planar, sp³d²)
  • XeF₆ (distorted octahedral, sp³d³, polymeric in solid)
• Xenon Oxyfluorides:
  • XeOF₄ (square pyramidal)
  • XeO₂F₂ (trigonal bipyramidal)
  • XeO₃F₂ (pentagonal planar)
• Xenon Oxides:
  • XeO₃ (pyramidal, explosive)
  • XeO₄ (tetrahedral, gaseous, most explosive)
• Xenon Oxo Salts:
  • Na₄XeO₆·8H₂O (perxenate, strong oxidiser)
• Krypton Compounds: Only KrF₂ (linear, unstable)
• No stable compounds of He, Ne, Ar

5. Reactions & Properties of Compounds

• XeF₂, XeF₄, XeF₆ → strong fluorinating & oxidising agents
• XeF₆ + H₂O → partial/complete hydrolysis → XeO₃ (explosive)
• XeF₄ + H₂O → XeOF₄ + 2HF (slow)
• Xe + O₂F₂ → XeF₂ + O₂
• Clathrates: Noble gases trapped in ice cages (e.g., Ar·6H₂O)
• Excimers: He₂*, Ar₂* used in lasers

6. Exam Tricks & Mnemonics

Reactivity Order: He ≈ Ne < Ar < Kr < Xe < Rn
Reason: Increasing size → lower IE → easier availability of electrons for bonding.

First Noble Gas Compound: XePtF₆ (Bartlett, 1962) → Xe⁺[PtF₆]⁻
(Similar IE of Xe & O₂ led to discovery)

Hybridisation Quick Recall:
XeF₂ → sp³d (linear)
XeF₄ → sp³d² (square planar)
XeF₆ → sp³d³ (distorted octahedral)
XeO₃ → sp³ (pyramidal)
XeO₄ → sp³ (tetrahedral)

Boiling Point Order: He < Ne < Ar < Kr < Xe < Rn
Exception: He has lowest BP among all substances.

Why no compounds of He/Ne?
Highest IE, smallest size, no d-orbitals, very stable configuration.

Most Powerful Oxidiser among Xe compounds: XeO₄ (gaseous) > XeF₆ > XeO₃

7. Frequently Asked Exam Questions

• Why are noble gases inert?
• Why does reactivity increase down the group?
• Structure & hybridisation of XeF₂, XeF₄, XeF₆, XeO₃, XeO₄?
• Which noble gas has maximum compounds? (Xe)
• Why He has lowest boiling point?
• Which has highest ionisation energy? (He)
• Preparation of XeF₆?
• Hydrolysis reactions of Xe fluorides?
• Why Xe forms compounds but Ne does not?

8. Quick Revision One-Liners

• Helium: lowest boiling point, used in cryogenics, balloons.
• Neon: glow discharge lamps (red colour).
• Argon: most abundant, used in welding, bulbs.
• Krypton/Xenon: flash lamps, lasers.
• Radon: radioactive, carcinogenic.
• XeF₂: mild fluorinating agent, stored in Ni vessel.
• XeO₃: explosive when dry, strong oxidiser in aqueous solution.