Distinctive Behaviour of Lithium

Although lithium heads group 1, it behaves noticeably differently from the rest of its own family -- a pattern the first member of every group in the periodic table tends to show. Lithium's distinctive (anomalous) behaviour traces to four compounding factors:

• its exceptionally small size (of both the atom and the Li⁺ ion),
• the resulting high polarising power of Li⁺,
• its high hydration energy, and
• the non-availability of d-orbitals in its valence shell.

Lithium versus the rest of group 1 (Table 5.5). Lithium is comparatively hard, with a high melting and boiling point, whereas the rest of the family is soft with low melting/boiling points. Lithium is the least reactive alkali metal (it forms only the simple, normal oxide Li₂O with oxygen -- forming a peroxide is very difficult for it, and its higher oxides are unstable), while the other members are markedly more reactive. Lithium reacts directly with nitrogen gas to give the nitride: 6Li + N₂ → 2Li₃N -- the other alkali metals do not react with nitrogen at all. Lithium reacts only slowly with bromine, where the others react violently. Lithium reacts directly with carbon to give the ionic carbide lithium carbide: 2Li + 2C → Li₂C₂; the other metals do not react with carbon directly, though they can react with carbon compounds such as acetylene to form acetylides, e.g. 2Na + C₂H₂ → Na₂C₂ + H₂. Finally, lithium nitrate decomposes on heating to give the oxide, while the nitrates of the other alkali metals decompose to give nitrites instead.

Similarity to magnesium and the diagonal relationship. Curiously, several of lithium's "unusual" properties make it resemble not sodium (its own group-mate) but magnesium, the second element of the next group (Table 5.6): both are harder than their group neighbours; both react only slowly with water, and their oxides/hydroxides are sparingly soluble with hydroxides that decompose on heating; both form a nitride directly with N₂ (Li₃N, Mg₃N₂); neither forms a superoxide, giving only the normal oxide (Li₂O, MgO); the carbonates of both decompose on heating to the oxide + CO₂; neither forms a solid bicarbonate; and both LiCl and MgCl₂ are soluble in ethanol, deliquescent, and crystallise from aqueous solution as hydrates (LiCl·2H₂O, MgCl₂·8H₂O). …