⚡ s-Block Elements & Hydrogen

The s-block elements (Group 1 Alkali Metals and Group 2 Alkaline Earth Metals) exhibit low ionization energies, high electropositivity, and strong reducing powers. Key exam focuses include solvation in liquid ammonia (deep blue solutions), stability of peroxides and superoxides, hydration enthalpy trends, and anomalous diagonal relationships ($Li-Mg,Be-Al$).

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1. 🌊 Alkali Metals (Group 1) Physical & Chemical Trends

1.1 Hydration Enthalpy vs. Ionic Mobility

$$\text{Hydration Enthalpy: }L{i}^{+}>N{a}^{+}>K^{+}>R{b}^{+}>C{s}^{+}$$

Because $L{i}^{+}$ has the smallest ionic radius, it attracts the largest hydration sphere:

$$\text{Hydrated Radius: }L{i}_{(aq)}^{+}>N{a}_{(aq)}^{+}>K_{(aq)}^{+}>R{b}_{(aq)}^{+}>C{s}_{(aq)}^{+}$$$$\text{Ionic Mobility / Electrical Conductance in Aqueous Solution: }{\mathbf{Cs}}_{\mathbf{(}\mathbf{aq}\mathbf{)}}^{\mathbf{+}}\mathbf{>}{\mathbf{Rb}}_{\mathbf{(}\mathbf{aq}\mathbf{)}}^{\mathbf{+}}\mathbf{>}{\mathbf{K}}_{\mathbf{(}\mathbf{aq}\mathbf{)}}^{\mathbf{+}}\mathbf{>}{\mathbf{Na}}_{\mathbf{(}\mathbf{aq}\mathbf{)}}^{\mathbf{+}}\mathbf{>}{\mathbf{Li}}_{\mathbf{(}\mathbf{aq}\mathbf{)}}^{\mathbf{+}}$$

1.2 Solutions in Liquid Ammonia ($N{H}_3$)

When alkali metals dissolve in anhydrous liquid ammonia:

$$M+(x+y)N{H}_3\to [M(N{H}_3{)}_x{]}^{+}+[e(N{H}_3{)}_y{]}^{-}$$

1. Dilute Solutions ($<3\text{ M}$):
   • Deep Blue Color: Due to excitation of ammoniated electrons ($[e(N{H}_3{)}_y{]}^{-}$).
   • Paramagnetic: Presence of unpaired ammoniated electrons.
   • High Electrical Conductivity: Due to both solvated cations and ammoniated electrons.
2. Concentrated Solutions ($>3\text{ M}$):
   • Blue color transitions to copper-bronze metallic luster.
   • Turns diamagnetic due to pairing of ammoniated electrons: $2[e(N{H}_3{)}_y{]}^{-}\rightleftharpoons e_2^{2-}(N{H}_3{)}_{2y}$.

1.3 Reaction with Oxygen: Oxide, Peroxide & Superoxide Formation

• Lithium ($Li$): Small size stabilizes small oxide ion $O^{2-}⟹L{i}_{2}O$ (Monoxide).
• Sodium ($Na$): Larger cation stabilizes peroxide ion $O_2^{2-}⟹N{a}_2{O}_2$ (Peroxide).
• Potassium, Rubidium, Cesium ($K,Rb,Cs$): Large electropositive cations stabilize large superoxide ion $O_2^{-}⟹K{O}_2,Rb{O}_2,Cs{O}_2$ (Superoxides).
  • Superoxides are paramagnetic and colored (orange/yellow) due to 1 unpaired electron in ${\pi }^{\ast }2p$ molecular orbital!

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2. 🪨 Alkaline Earth Metals (Group 2) & Diagonal Relationships

2.1 Thermal Stability of Carbonates, Nitrates, and Sulfates

$$\text{Thermal Stability}\propto \frac{\mathbf{1}}{\text{Polarizing Power of Cation (Ionic Potential }\varphi \mathbf{=}\mathbf{z}\mathbf{/}\mathbf{r}\mathbf{)}}$$$$\text{Thermal Stability Order: }BaC{O}_3>SrC{O}_3>CaC{O}_3>MgC{O}_3>BeC{O}_3$$

• $BeC{O}_3$ is so thermally unstable that it must be stored in an atmosphere of $C{O}_2$!

2.2 Diagonal Relationship of $Li$ and $Mg$

$L{i}^{+}$ ($r=0.76\text{AA}$) and $M{g}^{2+}$ ($r=0.72\text{AA}$) have almost identical ionic potential (charge/radius ratio $\varphi \approx 1.3$):

1. Both form nitrides directly with $N_2$ ($L{i}_{3}N$ and $M{g}_3{N}_2$), whereas other alkali metals do not.
2. Both carbonates decompose on heating to give oxides and $C{O}_2$ ($L{i}_{2}C{O}_3\to L{i}_{2}O+C{O}_2$).
3. Both nitrates decompose to yield $N{O}_2$ and oxide ($4LiN{O}_3\to 2L{i}_{2}O+4N{O}_2+O_2$).
4. Both $LiCl$ and $MgC{l}_2$ are deliquescent and soluble in organic solvents (alcohol/pyridine).