s-Block Elements Class 11 Notes

Position of the s-Block Elements

• Group 1 (alkali metals): Li, Na, K, Rb, Cs, Fr.
• Group 2 (alkaline earth metals): Be, Mg, Ca, Sr, Ba, Ra.
• They are strongly metallic, electropositive and form ionic compounds predominantly.

Alkali Metals — General Characteristics

• One ns¹ electron; low ionisation enthalpy that decreases down the group (Cs has the least).
• Large atomic and ionic radii that increase down the group; the alkali metal ion is the smallest per electron shell, so it is heavily hydrated in water.
• Soft, low melting points because metallic bonding is weak with only one electron per atom.
• Flame colour: Li crimson, Na golden yellow, K lilac, Rb and Cs violet — from easily excited outer electrons.
• Cs is the most reactive; Li is the least.

Chemical Properties of Alkali Metals

• 2M + 2H₂O → 2MOH + H₂: reactivity with water increases down the group; Na reacts vigorously, K and Rb catch fire, Cs is explosive.
• They react with halogens to form ionic halides MX.
• They dissolve in liquid ammonia giving deep blue solutions that contain solvated electrons and are powerful reducing agents.
• They form ionic hydrides (LiH, NaH) with hydrogen — an exception to the usual molecular hydrides.

Alkaline Earth Metals — General Characteristics

• Two ns² electrons; ionisation enthalpies higher than the alkali metals and decreasing down the group.
• M²⁺ ions are smaller and more heavily hydrated than the corresponding alkali metal ions; hydration enthalpy decreases from Be²⁺ to Ba²⁺.
• Harder, denser and higher melting than alkali metals because two electrons take part in metallic bonding.
• Less reactive than alkali metals; Mg and Be are relatively slow with water, Ca onwards react readily.
• Flame colour: Ca brick-red, Sr crimson, Ba apple-green; Be and Mg give no characteristic flame colour.

Anomalous Behaviour of Lithium and Beryllium

• Li (like Mg): forms a nitride Li₃N, carbonate that decomposes on heating, and a less soluble carbonate and hydroxide; both are harder than other group members.
• Be (like Al): forms covalent compounds, amphoteric oxide BeO, and stable complexes.
• Li and Be compounds are predominantly covalent, unlike the ionic compounds of the rest of their groups.

Important Compounds — Sodium and Calcium

• Caustic soda NaOH: strong base, made by electrolysis of brine; soap, paper, textiles and bauxite extraction.
• Quicklime CaO (from limestone calcination) + water → slaked lime Ca(OH)₂.
• Plaster of Paris is hemihydrate 2CaSO₄·H₂O, made by heating gypsum CaSO₄·2H₂O at ~120 °C.

Plaster of Paris and Cement

• Cement is mostly a mixture of calcium silicates and aluminates from limestone + clay.
• The key oxide components are CaO (~50%), SiO₂ and Al₂O₃.
• Cement sets when mixed with water: hydrates interlock and give strength over days; gypsum is added to regulate the setting time.

Solved Examples

Example: Which alkali metal has the lowest ionisation enthalpy and why?

Solution: Caesium (Cs). Down the group the atomic radius grows and inner electrons shield the outer electron better, so the single ns¹ electron is pulled away more easily — ionisation enthalpy falls from Li to Cs.

Example: Why is Li⁺ the least mobile alkali metal ion in aqueous solution?

Solution: Li⁺ has the smallest ionic radius and the highest charge density, so it is hydrated the most. The large hydrated ion moves slowly, giving Li⁺ the lowest ionic mobility of the alkali metals.