Group 16 Elements Revision Notes, Mnemonics and Trends

Group 16 Elements (Chalcogens): O, S, Se, Te, Po

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

Mnemonics: O: ओ | S: एस | Se: से | Te: टे | Po: पो

1. General Electronic Configuration

ns² np⁴ (two electrons short of stable octet)

2. Key Periodic Trends (Down the Group: O → Po)

Property	Trend	Exception/Important Note
Atomic & Ionic Radius	Increases	O < S < Se < Te < Po
Ionisation Energy	Decreases	O > S > Se > Te > Po Exception: O > S (due to smaller size and higher effective nuclear charge in O)
Electronegativity	Decreases	O (3.5) > S (2.5) > Se (2.4) > Te (2.1) > Po (1.9) O is 2nd most electronegative element
Metallic Character	Increases	O, S (non-metals) → Se, Te (metalloids) → Po (metal)
Melting & Boiling Point	Increases (O to Se), then slight dip	O₂ low (van der Waals) S₈ high (puckered ring) Se & Te higher due to increasing metallic character
Catination	Decreases	S >> Se > Te > O > Po Sulphur shows maximum catenation
Oxidation States	-2, +2, +4, +6 common	-2 stability: O > S > Se > Te > Po +6 stability: S > Se > Te > Po (inert pair effect) O shows only -2 (no d-orbitals)
Nature of Oxides	Acidic nature decreases	SO₂/SO₃ strongly acidic → TeO₂ amphoteric → PoO₂ basic
Bond Energy (E=E)	Decreases	O=O (498 kJ/mol) > S=S > Se=Se > Te=Te

3. Important Compounds & Reactions

• Oxygen Allotropes: O₂ (paramagnetic), O₃ (ozone, strong oxidising agent)
• Sulphur Allotropes: Rhombic (α-S, most stable), Monoclinic (β-S), Plastic (amorphous)
• Hydrides: H₂O > H₂S > H₂Se > H₂Te > H₂Po
  
  • Thermal stability: H₂O > H₂S > H₂Se > H₂Te
  • Reducing character: H₂S < H₂Se < H₂Te < H₂Po
  • Bond angle: H₂O (104.5°) > H₂S (92°) > H₂Se > H₂Te (~90°)
  • Acidic strength: H₂O < H₂S < H₂Se < H₂Te < H₂Po
• Halides:
  • O forms OF₂ (only stable oxide-like halide)
  • SF₆ (most stable, octahedral, inert due to steric crowding)
  • SF₄ (see-saw), SCl₂, etc.
  • Te forms TeF₆
• Oxides:
  • SO₂ (acidic, reducing, bleaching agent)
  • SO₃ (strongest acidic oxide, pyramidal gas, forms H₂SO₄ with water)
  • SeO₂ sublimes, TeO₂ insoluble amphoteric
• Oxyacids:
  • Sulphuric acid series: H₂SO₄ > H₂SO₃ (acid strength)
  • Acid strength (+6 state): H₂SO₄ > H₂SeO₄ > H₆TeO₆
  • Oxidising power (+6): H₂SeO₄ > H₂SO₄ > H₆TeO₆
  • H₂SO₄ is king of chemicals, strongest dehydrating agent

4. Exam Tricks & Mnemonics

Anomalous Behaviour of Oxygen:
Reasons: Small size, high electronegativity, absence of d-orbitals, strong pπ-pπ bonding (O=O).

Acidic Strength of Hydrides: H₂O < H₂S < H₂Se < H₂Te
Mnemonic: Oh Such Severe Teasing → increasing acidity.

Reducing Nature of Hydrides: H₂S < H₂Se < H₂Te < H₂Po
Same order as acidity (easier to release H⁺).

Bond Angle Trend: Due to increasing size → lone pair in almost pure p-orbital → bond angle approaches 90°.

Volatility of Dioxides: SO₂ > SeO₂ > TeO₂
SO₂ gas, SeO₂ sublimes, TeO₂ non-volatile solid.

Oxidising Power of +6 Oxyacids: H₂SeO₄ > H₂SO₄ > H₂TeO₆
Se has intermediate electronegativity → maximum oxidising power.

Stability of Hexahalides: SF₆ > SeF₆ > TeF₆
SF₆ most inert due to perfect steric protection of S by 6F atoms.

5. Frequently Asked Exam Questions

• Why is oxygen anomalous in Group 16?
• Why does H₂O have higher boiling point than H₂S?
• Why is SF₆ inert while SCl₆ does not exist?
• Order of thermal stability/acid strength/reducing power of group 16 hydrides?
• Which element shows maximum catenation? (Sulphur)
• Why is SO₃ a stronger acid than SeO₃?
• Why does ozone act as a strong oxidising agent?
• Which oxyacid of sulphur is strongest oxidising agent in dilute form? (H₂SO₅ - Caro's acid)

6. Quick Revision One-Liners

• Oxygen: most abundant element, forms multiple bonds easily.
• Sulphur: maximum allotropes, maximum catenation, S₈ ring.
• H₂SO₄: dehydrating agent, oxidising agent (conc.), non-volatile.
• SF₆: hydrolytically stable, used as gaseous insulator.
• Polonium: radioactive, most metallic.
• Ozone: V-shaped, diamagnetic in gas phase, used for water purification.