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 + 3H₂O → XeO₃ (explosive) + 6HF (Complete hydrolysis)
• XeF₄ + H₂O → XeOF₂ + 2HF (Partial hydrolysis)
• 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?
  Noble gases have completely filled valence shells (ns²np⁶), making them stable and chemically non-reactive under normal conditions.
• Why does reactivity increase down the group?
  Larger atomic size and lower ionisation energy down the group make heavier noble gases (like Xe, Rn) more polarizable and reactive compared to lighter ones.
• Structure & hybridisation of Xe compounds:
  XeF₂: Linear, sp³d hybridisation (3 lone pairs, 2 bond pairs)
  XeF₄: Square planar, sp³d² hybridisation (2 lone pairs, 4 bond pairs)
  XeF₆: Distorted octahedral, sp³d³ hybridisation (1 lone pair, 6 bond pairs)
  XeO₃: Trigonal pyramidal, sp³ hybridisation (1 lone pair, 3 bond pairs)
  XeO₄: Tetrahedral, sp³ hybridisation (no lone pairs, 4 bond pairs)
• Which noble gas has maximum compounds?
  Xenon (Xe) forms the maximum number of compounds due to its relatively low ionisation energy and large size.
• Why He has lowest boiling point?
  Helium exists as monoatomic gas with weak van der Waals forces, leading to extremely low boiling point (−268.9 °C).
• Which has highest ionisation energy?
  Helium (He) has the highest ionisation energy due to its small size and strong effective nuclear charge.
• Preparation of XeF₆?
  XeF₆ is prepared by heating xenon and fluorine at 573 K under 50–60 atm pressure.
• Hydrolysis reactions of Xe fluorides?
  XeF₂ + H₂O → Xe + O₂ + HF
  XeF₄ + H₂O → XeO₃ + HF
  XeF₆ + H₂O → XeO₂F₂ / XeO₃ + HF
• Why Xe forms compounds but Ne does not?
  Xenon has lower ionisation energy and larger atomic size, allowing it to be oxidized by strong oxidizing agents like fluorine. Neon has very high ionisation energy and small size, making it chemically inert.

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.