Group 1 Elements Revision Notes, Mnemonics and Trends

Group 1 Elements (Alkali Metals): Li, Na, K, Rb, Cs, Fr

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

Mnemonics: Li: ली | Na: ना | K: के | Rb: रब | Cs: से | Fr: फ़रीयाद

1. General Electronic Configuration

ns¹ (one valence electron → highly reactive, strong reducing agents)

2. Key Periodic Trends (Down the Group: Li → Fr)

Property	Trend	Exception/Important Note
Atomic & Ionic Radius	Increases	Li < Na < K < Rb < Cs < Fr (Largest in respective periods)
Ionisation Energy	Decreases	Li > Na > K > Rb > Cs > Fr Li has highest IE in group
Electronegativity	Decreases	Li (1.0) > Na (0.9) > K (0.8) > Rb > Cs (0.7) Lowest EN in periodic table (Cs)
Electropositive Character	Increases	Li < Na < K < Rb < Cs Strongest reducing agents
Melting & Boiling Point	Decreases	Li > Na > K > Rb > Cs (Weak metallic bonding due to single valence e⁻)
Density	Increases (generally)	Li < Na < K < Rb < Cs Exception: K < Na (K has loosely packed bcc structure)
Hydration Energy	Decreases	Li⁺ > Na⁺ > K⁺ > Rb⁺ > Cs⁺ (Smaller ion → higher charge density)
Flame Colour	—	Li (crimson red), Na (yellow), K (violet), Rb (red-violet), Cs (blue)
Reactivity	Increases	Li < Na < K < Rb < Cs (Ease of losing ns¹ electron)

3. Anomalous Behaviour of Lithium

• Small size, high IE, high electronegativity, high polarising power
• Forms covalent compounds (e.g., LiCl soluble in alcohol, Li alkyls)
• Li₂CO₃, LiOH, LiF less soluble; decompose on heating (unlike others)
• LiHCO₃ exists in solution only
• Li₃N stable nitride (others decompose)
• Diagonal relationship with Mg (similar charge density)

4. Important Compounds & Reactions

• Oxides:
  • Li₂O (normal oxide)
  • Na₂O₂ (peroxide)
  • K, Rb, Cs → O₂⁻ (superoxides)
• Hydroxides: Strong bases; solubility & thermal stability increases down group
• Carbonates & Bicarbonates:
  • Li₂CO₃ insoluble, decomposes on heating
  • Others soluble & stable
• Nitrates:
  • LiNO₃ → Li₂O + NO₂ + O₂ (decomposes)
  • Others → nitrite + O₂
• Halides: Ionic; LiX covalent character
• Reactions:
  • With O₂: Li (oxide), Na (peroxide), K-Rb-Cs (superoxide)
  • With H₂O: Vigorous → MOH + ½H₂ (reactivity increases)
  • With NH₃ (liq): Solvated e⁻ → blue solution (paramagnetic, conducting)

5. Exam Tricks & Mnemonics

Reactivity Order: Li < Na < K < Rb < Cs
Mnemonic: Lazy Naughty Kids Run Crazy → increasing reactivity.

Flame Test Colours: Li (Crimson), Na (Yellow), K (Violet/Lilac), Rb (Red-violet), Cs (Blue)
Mnemonic: LiNa (Lina) wears Kurta of Red & Cyan colour.

Oxide Type: Li₂O → Na₂O₂ → KO₂ (superoxide)
Reason: Stability of O₂⁻ increases with cation size (larger cation stabilises larger anion).

Density Exception: Na > K (remember K floats on water but denser than expected due to structure).

Diagonal Relationship Li-Mg: Both form nitride, carbide (C₂²⁻), similar solubility trends.

Thermal Decomposition:

• Carbonates: Only Li₂CO₃ decomposes
• Nitrates: Only LiNO₃ gives oxide

6. Frequently Asked Exam Questions

• Why is lithium anomalous?
• Order of reactivity, reducing power, hydration energy?
• Why Na₂O₂ is peroxide but KO₂ superoxide?
• Why Li compounds are covalent?
• Flame colour mechanism?
• Which alkali metal has lowest density? (Li)
• Why K density less than Na?
• Photoelectric effect: Cs used (lowest IE)

7. Quick Revision One-Liners

• Lithium: hardest, highest MP/BP, anomalous behaviour.
• Sodium: most abundant alkali metal, NaOH (caustic soda), Na₂CO₃ (washing soda).
• Potassium: essential for plants, KO₂ used in breathing masks.
• Cesium: lowest IE, used in photoelectric cells.
• Francium: radioactive, least stable.
• All stored in kerosene (except Li in paraffin wax).
• Strongest reducing agent in aqueous solution: Li (due to high hydration energy).