Question
Explain the anomalous behaviour of lithium in Group 1 (alkali metals). List the properties in which lithium differs from sodium, potassium, and other alkali metals.
Solution — Step by Step
Lithium is anomalous because it has:
- Smallest atomic and ionic radius in Group 1 (Li⁺ = 76 pm vs Na⁺ = 102 pm)
- Highest charge density (charge/volume ratio of Li⁺ is very high)
- No d-orbitals in the valence shell (only 2s)
- High polarising power due to high charge density
High charge density means Li⁺ distorts neighbouring anions strongly, leading to more covalent character in its compounds — unlike the purely ionic compounds of Na, K, Rb, Cs.
1. Hardness and melting point: Lithium is harder and has a higher melting point (180°C) than other alkali metals (Na: 98°C, K: 63°C). Stronger metallic bonding due to smaller atomic size.
2. Reaction with oxygen: Li burns in air to form mainly the normal oxide (Li₂O). Na forms mainly the peroxide (Na₂O₂). K, Rb, Cs form mainly superoxides (KO₂, RbO₂, CsO₂).
3. Reaction with nitrogen: Lithium is the only alkali metal that reacts with N₂ directly at room temperature to form lithium nitride (Li₃N). Other alkali metals do not form nitrides directly.
4. Thermal stability of carbonate: Li₂CO₃ readily decomposes on heating (lower thermal stability) unlike Na₂CO₃, K₂CO₃ which are thermally stable. This is because the high charge density of Li⁺ polarises the carbonate ion, weakening it.
5. Solubility of salts: Many lithium salts are sparingly soluble or insoluble: LiF is sparingly soluble, LiOH is sparingly soluble. Na and K analogues are highly soluble. Exception: LiCl, LiBr, LiI are highly soluble (even soluble in organic solvents due to covalent character).
6. Lithium hydroxide (LiOH): LiOH is the weakest base among alkali metal hydroxides. NaOH, KOH are stronger. LiOH decomposes on heating, unlike NaOH and KOH.
7. No peroxide or superoxide formation: Li does NOT form peroxides or superoxides because Li⁺ is too small to stabilise the large peroxide (O₂²⁻) or superoxide (O₂⁻) ions.
8. Diagonal relationship with Magnesium: Li resembles Mg (Period 3, Group 2) more than it resembles Na. Both have similar size (Li: 152 pm metal, Mg: 160 pm). Both form nitrides directly (Mg₃N₂ and Li₃N). Both form oxide (not peroxide). Both react slowly with water (Li is the slowest-reacting alkali metal, like Mg). Both form slightly soluble carbonates and hydroxides.
| Property | Li | Mg |
|---|---|---|
| Nitride formation | Li₃N (direct) | Mg₃N₂ (direct) |
| Oxide type | Li₂O (normal) | MgO (normal) |
| Carbonate stability | Low | Low |
| Hydroxide solubility | Slightly soluble | Slightly soluble |
| Reaction with water | Slow | Slow (or no reaction with cold water) |
| Chloride solubility in organic solvents | Soluble | Soluble |
Why This Works
The anomalous behaviour of the first member of any group in the periodic table follows from their small atomic size and high charge density. This applies to Be in Group 2, B in Group 13, C in Group 14, N in Group 15, O in Group 16, F in Group 17. In each case, the first member shows more covalent character and resembles the second element of the next group (diagonal relationship).
Alternative Method — Quick Checklist
Whenever a JEE question asks about lithium’s anomalous properties, run through this checklist: (1) only normal oxide, (2) reacts with N₂, (3) diagonal similarity with Mg, (4) high melting point, (5) LiOH decomposes, (6) Li₂CO₃ less stable.
Common Mistake
Students sometimes say “lithium is the most reactive alkali metal.” In fact, lithium is the least reactive alkali metal in terms of reaction with water (even though it has the highest reduction potential due to high hydration energy). Reactivity with water follows: Cs > Rb > K > Na > Li (Li reacts calmly without catching fire; Cs explodes). Don’t confuse electrode potential with reaction rate.
The diagonal relationship is a standard 2-mark JEE Main question. Memorise three Li-Mg similarities: (1) both form normal oxides (not peroxides), (2) both react with N₂ to form nitrides, (3) both react slowly with water (unlike Na which reacts vigorously). These three points cover most exam questions.