The s-Block Elements

1. Group 1: Alkali Metals

Li, Na, K, Rb, Cs, Fr; configuration $ns^1$. They have the largest atomic radii and lowest ionisation enthalpies in their periods, so they are highly reactive, strong reducing agents, and always +1 in compounds.

• Flame colours: Li crimson red, Na golden yellow, K violet (lilac), Rb red-violet, Cs blue.
• Reducing power in water: Li is the strongest (most negative $E^\circ$) because its very high hydration enthalpy outweighs its high ionisation enthalpy.
• With oxygen: Li forms the oxide $Li_2O$, Na the peroxide $Na_2O_2$, and K, Rb, Cs the superoxides $MO_2$.
• In liquid ammonia they give blue, conducting, paramagnetic solutions of ammoniated electrons.
• Caesium and potassium lose electrons easily when light falls on them, which is why they are used in photoelectric cells.

Anomalous Behaviour of Lithium

Li is very small with high polarising power. It forms the nitride $Li_3N$, its carbonate decomposes on heating, LiCl is covalent and deliquescent, and it resembles magnesium (diagonal relationship).

2. Group 2: Alkaline Earth Metals

Be, Mg, Ca, Sr, Ba, Ra; configuration $ns^2$; oxidation state +2. They are harder and less reactive than group 1 metals.

• Flame colours: Ca brick red, Sr crimson, Ba apple green. Be and Mg give no flame colour (electrons too tightly held).
• Solubility of hydroxides increases down the group; solubility of sulphates decreases ($BeSO_4$ and $MgSO_4$ soluble, $BaSO_4$ almost insoluble).
• Thermal stability of carbonates increases down the group.
• Be is anomalous: it forms covalent compounds, $BeO$ and $Be(OH)_2$ are amphoteric, and $BeCl_2$ is a chain polymer in the solid state.

3. Important Compounds

• Sodium carbonate ($Na_2CO_3 \cdot 10H_2O$, washing soda) — Solvay (ammonia-soda) process. The Solvay process cannot make $K_2CO_3$, because $KHCO_3$ is too soluble to precipitate.
• Sodium hydroxide — electrolysis of brine in the Castner–Kellner cell.
• Sodium hydrogencarbonate (baking soda).
• Plaster of Paris, $CaSO_4 \cdot \frac{1}{2}H_2O$, made by heating gypsum ($CaSO_4 \cdot 2H_2O$) at 393 K. It sets into gypsum when mixed with water.
• Quicklime CaO, slaked lime $Ca(OH)_2$; $CO_2$ turns lime water milky ($CaCO_3$), and excess $CO_2$ clears it again (soluble $Ca(HCO_3)_2$).
• Cement: mainly calcium silicates and aluminates; gypsum is added to slow setting.

4. Biological Importance

$Na^+$ and $K^+$ maintain nerve signalling and cell membrane potential (the sodium–potassium pump). $Mg^{2+}$ is in chlorophyll; $Ca^{2+}$ is essential for bones, teeth, muscle contraction and blood clotting.

Practice questions

1. Which alkali metal is the strongest reducing agent in aqueous solution?

   1. Li
   2. Na
   3. K
   4. Cs
   Answer

   A. Lithium's very large hydration enthalpy gives it the most negative standard reduction potential.

2. On burning in excess air, potassium mainly forms:

   1. Oxide
   2. Peroxide
   3. Superoxide
   4. Nitride
   Answer

   C. Larger alkali metal cations stabilise the larger superoxide ion, giving KO₂.

3. Which sulphate is least soluble in water?

   1. BeSO₄
   2. MgSO₄
   3. CaSO₄
   4. BaSO₄
   Answer

   D. Sulphate solubility falls down group 2 because hydration enthalpy falls faster than lattice enthalpy.

4. The chemical formula of Plaster of Paris is:

   1. CaSO₄ · 2H₂O
   2. CaSO₄ · ½H₂O
   3. CaSO₄
   4. CaCO₃
   Answer

   B. Heating gypsum to 393 K gives the hemihydrate.

5. Which element imparts a golden yellow colour to a flame?

   1. Li
   2. Na
   3. K
   4. Ca
   Answer

   B. Sodium's characteristic emission (the D-line near 589 nm) is golden yellow.

6. Potassium carbonate cannot be prepared by the Solvay process because:

   1. K₂CO₃ is insoluble
   2. KHCO₃ is too soluble to precipitate
   3. Potassium reacts with ammonia
   4. K is too reactive
   Answer

   B. The Solvay process relies on NaHCO₃ precipitating; KHCO₃ stays in solution.