p-Block Elements (Class 11)

Group 13 (Boron family): B, Al, Ga, In, Tl. Outer config: $ns^2\,np^1$.

• B is metalloid
• Al, Ga, In, Tl are metals

Electronic Configurations:

General Trends:

Inert Pair Effect: Tendency of $ns^2$ electrons to remain inert (not participate in bonding) in heavier elements due to poor shielding by $d$ and $f$ electrons. Tl shows stable $+1$ rather than $+3$.

Anomalous Properties of Boron:

• Smallest size, highest IE in group
• Non-metal (others metals)
• Only covalent compounds (no $B^{3+}$ ion in compounds)
• Maximum covalency = $4$ (no $d$-orbitals)
• Highly stable boron-hydrogen compounds (boranes)

Differences B vs Al:

Lewis Acid Strength of BX₃: Surprisingly, despite F being most EN: $BF_3 < BCl_3 < BBr_3 < BI_3$. This is because back-bonding ($p\pi-p\pi$) from halogen to B partially fills B's empty $p$ orbital — strongest with F (best size match), weakest with I.

Aluminium:

• Most abundant metal in Earth's crust ($\sim 8.3\%$)
• Bauxite ($Al_2O_3 \cdot 2H_2O$) is the main ore
• Hall-Héroult process: Electrolysis of molten $Al_2O_3$ in cryolite ($Na_3AlF_6$)
• Amphoteric: dissolves in both acids and alkalis
• Forms protective oxide layer (resistant to corrosion)

Borax (Na₂B₄O₇·10H₂O):

Structure (in solid): $[B_4O_5(OH)_4]^{2-}$ ion with $2$ Na⁺.

Two of the four B atoms are $sp^2$ hybridized (planar, trigonal); two are $sp^3$ (tetrahedral). Anion has formula $[B_4O_5(OH)_4]^{2-}$.

Borax bead test: When heated, borax loses water → swells → forms transparent glassy bead of $Na_2B_4O_7 + 2NaBO_2 + B_2O_3$. With colored salts gives characteristic colors.

E.g., copper salt + borax bead → blue (Cu²⁺) → red (Cu in reducing flame).

Preparation of Borax:

• From colemanite ($Ca_2B_6O_{11}$): boil with Na₂CO₃; CaCO₃ precipitates, borax remains in solution.

Boric Acid (H₃BO₃ or B(OH)₃):

Preparation:

• From borax: $Na_2B_4O_7 + 2HCl + 5H_2O \to 4H_3BO_3 + 2NaCl$
• Hydrolysis of $BX_3$ or boric oxide

Structure: Trigonal planar. H-bonded layered structure in solid (each B(OH)₃ unit H-bonded to 3 others).

Properties:

• White crystalline solid; sparingly soluble in cold water; more soluble in hot water
• Weak monoprotic Lewis acid: $B(OH)_3 + 2H_2O \to [B(OH)_4]^- + H_3O^+$
• It does NOT donate H⁺ directly; instead accepts OH⁻ from water — Lewis acid behavior
• $K_a \approx 6 \times 10^{-10}$

Heating Behavior: $$H_3BO_3 \xrightarrow{370 K} HBO_2 \xrightarrow{red\ heat} B_2O_3$$ (meta-boric acid → boric oxide)

Uses: Mild antiseptic, eye washes, manufacture of borosilicate glass, enamels.

Diborane (B₂H₆):

Preparation:

• $2BF_3 + 6NaH \xrightarrow{450K} B_2H_6 + 6NaF$
• $2NaBH_4 + I_2 \to B_2H_6 + 2NaI + H_2$

Structure: "Banana bonds" or 3-center-2-electron (3c-2e) bonds — two B-H-B bridges.

• 2 H atoms are terminal (one per B), 4 are terminal end-on, 2 are bridging.
• Actually: 4 terminal H + 2 bridging H.
• Each B is $sp^3$ hybridized.
• Both B share 2 bridging H atoms with delocalized 3c-2e bonds.
• B-B distance ≈ $1.77$ Å; bridging B-H ≈ $1.33$ Å; terminal B-H ≈ $1.19$ Å.

Properties:

• Colorless gas; toxic; spontaneously inflames in air
• $B_2H_6 + 3O_2 \to B_2O_3 + 3H_2O + \text{heat}$
• Hydrolysis: $B_2H_6 + 6H_2O \to 2B(OH)_3 + 6H_2$
• Hydroboration of alkenes (Brown's reaction; used in synthesis of alcohols)
• Reacts with NH₃ → Borazine ($B_3N_3H_6$, "inorganic benzene")

Higher Boranes: $B_5H_9, B_{10}H_{14}$ etc. Show similar 3c-2e bonding patterns.

Group 14: C, Si, Ge, Sn, Pb. Outer config: $ns^2\,np^2$.

• C, Si: non-metals
• Ge: metalloid
• Sn, Pb: metals

Trends:

Catenation: Property of an element to form chains/rings with itself. Most pronounced in carbon due to:

• Small size (strong C-C bond, $\sim 348$ kJ/mol)
• $sp^3, sp^2, sp$ hybridization
• Tetravalency

Other elements show decreasing catenation; Si forms only short chains (silicates have Si-O-Si).

Carbon-Halogen Bonds:

• CCl₄ is stable in water (no hydrolysis); SiCl₄ readily hydrolyzes (Si has $d$-orbitals)
• $SnCl_4, PbCl_4$ hydrolyze easily

Allotropes of Carbon:

Diamond Structure: Each C bonded to 4 others tetrahedrally. C-C bond length $1.54$ Å. Very high lattice energy.

Graphite Structure: Layers of hexagonal rings; each C bonded to 3 others ($sp^2$); C-C in layer $1.42$ Å; layers $3.35$ Å apart (van der Waals); 4th electron delocalized → conduction.

Fullerene: C₆₀ molecule shaped like soccer ball; $20$ hexagons + $12$ pentagons; $sp^2$ but slightly pyramidalized (curvature).

Oxides of Carbon:

Carbon Monoxide (CO):

• Bonding: $:C\equiv O:$ — triple bond with both lone pairs
• Excellent ligand (forms metal carbonyls); $\sigma$-donor from C, $\pi$-acceptor
• Strong reducing agent

Carbon Dioxide (CO₂):

• $O=C=O$ linear, $sp$ hybridized C
• Forms carbonic acid: $CO_2 + H_2O \rightleftharpoons H_2CO_3$
• Solid form ("dry ice") sublimes; used as refrigerant
• Green house gas; rising levels causing global warming

Silicon Compounds:

Silica ($SiO_2$):

• 3D giant covalent network with $sp^3$ Si bonded to 4 O atoms (tetrahedrally)
• Quartz, sand, glass forms
• High MP ($\sim 1710°$C); insoluble; chemically inert
• Reacts with HF: $SiO_2 + 4HF \to SiF_4 + 2H_2O$
• Reacts with NaOH: $SiO_2 + 2NaOH \to Na_2SiO_3 + H_2O$

Silicates: Compounds containing $SiO_4^{4-}$ tetrahedra. Building unit; can polymerize.

Silicones: Synthetic polymers with general formula $R_2SiO$, alternating Si-O-Si backbone with R = alkyl/aryl groups.

Preparation: $R_2SiCl_2 + 2H_2O \to R_2Si(OH)_2 + 2HCl$ Condensation: $nR_2Si(OH)_2 \to (R_2SiO)_n + nH_2O$

Properties:

• Stable to heat and chemicals
• Water-repellent
• Excellent electrical insulators
• Used in lubricants, water-repellent coatings, surgical implants, sealants

Important Silicon Compounds:

• SiC (Silicon Carbide / Carborundum): Very hard ($\sim 9.5$ on Mohs); abrasive
• Si₃N₄: Refractory ceramic
• Silanes (SiH₄ etc.): Analogous to alkanes but reactive (less stable than alkanes)

Zeolites: Crystalline aluminosilicates with porous structure. Used as:

• Molecular sieves (separate molecules by size)
• Catalysts in petroleum cracking
• Water softeners (ion exchange)