Group 13 Elements Revision Notes, Mnemonics and Trends

Group 13 Elements (Boron Family): B, Al, Ga, In, Tl

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

Mnemonics: B: Baigan | Al: Aaloo | Ga: Gajar | In: In | Tl: Thaila

1. General Electronic Configuration

ns² np¹ (three valence electrons → generally +3 oxidation state)

2. Key Periodic Trends (Down the Group: B → Tl)

Property	Trend	Exception/Important Note
Atomic & Ionic Radius	Increases (generally)	B < Al < Ga < In < Tl Exception: Ga < Al (d-block contraction)
Ionisation Energy	Decreases (generally)	B > Al > Ga > In > Tl Exception: Al < Ga (poor shielding by 3d electrons)
Electronegativity	Decreases	B (2.0) > Al (1.5) > Ga ≈ In ≈ Tl
Metallic Character	Increases	B (non-metal) → Al (metalloid-like) → Ga, In, Tl (metals) +3 stability: B > Al > Ga > In > Tl (inert pair effect)
Melting Point	No regular trend	B (very high) > Al > Ga < In < Tl Ga lowest MP (29.8°C)
Boiling Point	Decreases	B > Al > Ga > In > Tl
Electropositive Character	Increases	B < Al < Ga < In < Tl
Density	Increases	B < Al < Ga < In < Tl

3. Anomalous Behaviour of Boron

• Small size, high IE, high electronegativity, absence of d-orbitals
• Non-metallic, forms covalent compounds
• Acidic oxides (B₂O₃), amphoteric for Al, basic for others
• Forms electron-deficient compounds (e.g., BH₃ dimerises to B₂H₆)
• Diagonal relationship with Si

4. Important Compounds & Reactions

• Boron Compounds:
  • Borax (Na₂B₄O₇·10H₂O) → bead test (coloured metaborates)
  • Boric acid (H₃BO₃): weak monobasic acid, antiseptic
  • B₂H₆ (diborane): 3-centre-2-electron bonds (banana bonds)
  • BF₃ (Lewis acid, trigonal planar), BCl₃ (monomeric)
  • Boron nitride (BN): diamond-like, high MP
• Aluminium Compounds:
  • Al₂O₃ (amphoteric, corundum/ruby/emerald)
  • AlCl₃ (dimeric in vapour, Lewis acid, Friedel-Crafts catalyst)
  • Potash alum (KAl(SO₄)₂·12H₂O)
  • Thermit reaction: Al + Fe₂O₃ → Fe (high heat)
• Hydrides: B (electron-deficient), Al (polymeric AlH₃), others unstable
• Halides: BX₃ monomeric covalent; AlX₃ dimeric → monomeric on heating
• Oxides: B₂O₃ acidic → Al₂O₃ amphoteric → Ga₂O₃, In₂O₃, Tl₂O basic

5. Exam Tricks & Mnemonics

Inert Pair Effect: +1 state stability: B < Al < Ga < In < Tl
Tl⁺ more stable than Tl³⁺ (strongest inert pair in group).

Acidity of Oxides: B₂O₃ > Al₂O₃ (amphoteric) > Ga₂O₃ > In₂O₃ > Tl₂O₃
Decreasing acidic → increasing basic character.

Low Melting Point of Ga: Due to unique structure (Ga₂ dimers) → melts in hand.

Diagonal Relationship B-Si:
Both non-metals, form acidic oxides, covalent hydrides, similar semiconductors.

Borax Bead Test Colours:
Cu (blue), Co (blue), Cr (green), Fe (brown), Ni (violet).

Lewis Acidity: BF₃ > BCl₃ > BBr₃ > BI₃
(Better π-back bonding in BF₃ → less Lewis acidic? Wait: Actually BCl₃ > BF₃ due to poorer overlap in larger halogens).
Correct order: BI₃ > BBr₃ > BCl₃ > BF₃ (poorer back bonding → stronger Lewis acid).

6. Frequently Asked Exam Questions

• Why is boron non-metallic while others metallic?
• Why Ga has smaller radius and higher IE than Al?
• Inert pair effect in group 13?
• Structure of diborane (3c-2e bonds)?
• Why AlCl₃ dimeric?
• Amphoteric nature of Al₂O₃ reactions?
• Why Tl⁺ compounds more stable than Tl³⁺?
• Borax bead test mechanism?

7. Quick Revision One-Liners

• Boron: non-metal, electron-deficient, hardest after diamond.
• Aluminium: most abundant metal in earth's crust, amphoteric.
• Gallium: lowest MP among metals, used in thermometers.
• Thallium: toxic, +1 state dominant.
• Al forms protective oxide layer → corrosion resistant.
• Boron never exhibits +3 ionic state.
• Al₂O₃ used in chromatography.