Ncert Exercises & Solutions

The s-block elements are characterised by their larger atomic sizes, lower ionisation enthalpies, invariable +1 oxidation state and solubilities of their oxosalts. In the light of these features describe the nature of their oxides, halides and oxosalts.

Due to low ionisation energy and large atomic size, alkali metals form cation readily and so their compounds are ionic.

Oxides

Due to +1 oxidation state, alkali metals form normal oxides of general formula M₂O.

Only Li forms normal oxide Li₂O when heated in air. Other form peroxide and superoxide. Oxides of alkali metals are

The basic character of oxide increases gradually from LiO to CsO due to increased ionic character.

Halides

Except lithium halides all other alkali metal halides are ionic. Due to high polarising power of Li⁺. Lithium halide is covalent in nature. Due to +1 oxidation states alkali metal halides have general formula MX. Low ionisation enthalpy allows dormation of ionic halides.

Oxo salts

All alkali metals form solid carbonates of general formula MCO₃. Carbonates are stable except Li₂CO₃ due to high polarising capacity of Li⁺ which is unstable and decomposed.

All the alkali metals (except Li) form solid bicarbonates MHCO₃. All alkali metals form nitrates having formula MNO₃. They are colourless, water soluble, electrovalent compounds.

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