Alkali Metals | Anomalous Properties of Lithium#
Anomalous behaviour of lithium#
The anomalous behaviour of lithium is due to:
- exceptionally small size of its atom and ion.
- its high polarizing power (ie, high charge/radius ratio).
Differences between lithium and other alkali metals#
| Lithium | Alkali Metals |
|---|---|
| Li is much harder. | Alkali metals are soft. |
| Li has higher melting and boiling points. | Alkali metals have low melting and boiling points. |
| Li is the least reactive among alkali metals. | Alkali metals are highly reactive. |
| Li is the strongest reducing agent. | Alkali metals are strong reducing agents. |
| Li forms mainly monoxide, Li2O. | Alkali metals form peroxides and superoxides. |
| Li reacts directly with N2 to form Li3N. | Alkali metals are unreactive towards N2. |
| LiCl is deliquescent and crystallizes as hydrate, LiCl.2H20. | Other alkali metal chlorides do not form hydrates. |
| Lithium hydrogen carbonate is not obtained in solid form. | Alkali metals form solid hydrogen carbonate. |
| Lithium does not form ethynide on reaction with ethyne. | Alkali metals form ethynide on reaction with ethyne. |
| Lithium nitrate on heating gives Li2O. 4LiNO3 → 2Li2O + 4NO2 + O2 | Other alkali metal nitrates decompose to give corresponding nitrite. 2NaNO3 → 2NaNO2 + O2 |
| LiF and Li2O are much less soluble in water. | Alkali metal halides and oxides are highly soluble in water. |
Similarities between lithium and magnesium#
- Lithium shows similarities with magnesium due to their similar atomic and ionic radii.
- Both lithium and magnesium are harder and lighter than other elements in their respective groups.
- Lithium and magnesium react slowly with water.
- Their oxides and hydroxides are much less soluble in water.
- Their hydroxides decompose on heating. Other hydroxides of elements of group 1 and group 2 do not decompose on heating.
\[LiOH + Heat → Li_2O + H_2O ↑\]
\[Mg(OH)_2 + Heat → MgO + H_2O ↑\]
- Both lithium and magnesium form nitride by direct combination with N2.
\[6Li + N_2 → 2Li_3N\]
\[3Mg + N_2 → Mg_3N_2\]
- The oxides of Li2O and MgO do not combine with excess oxygen to give superoxide.
- The carbonates of lithium and magnesium decompose easily on heating to form oxides and CO2.
\[Li_2CO_3 + Heat → Li_2O + CO_2\]
\[MgCO_3 + Heat → MgO + CO_2\]
- Lithium and magnesium do not form solid hydrogen carbonates.
- Both LiCl and MgCl2 are soluble in ethanol.
- Both LiCl and MgCl2 are deliquescent and crystallize from aqueous solution as hydrates (Example: LiCl.2H2O, MgCl2.8H2O).