p-Block Elements (Groups 13 to 18)

1. Group 13 Elements: The Boron Family

• General Electronic Configuration: $ns^2 np^1$. Group oxidation state is +3; stability of +1 increases down the group due to the inert pair effect:

  $$\text{Al}^{3+} > \text{Ga}^{3+} > \text{In}^{3+} > \text{Tl}^{3+} \quad \text{and} \quad \text{Tl}^+ > \text{In}^+ > \text{Ga}^+ > \text{Al}^+$$

  $\text{Tl}^{3+}$ salts act as strong oxidising agents, reducing readily to stable $\text{Tl}^+$.
• Atomic Radius Anomaly: Due to poor shielding by the ten intervening $3d$ electrons in Gallium, effective nuclear charge increases, making Gallium smaller than Aluminium:

  $$r(\text{Ga}) = 135\text{ pm} < r(\text{Al}) = 143\text{ pm}$$

• Lewis Acid Character of Trihalides: Group 13 trihalides are electron-deficient (6 valence electrons around the central atom). Lewis acid strength decreases with increasing central atom size:

  $$\text{BF}_3 < \text{BCl}_3 < \text{BBr}_3 < \text{BI}_3$$

  Explanation: Back-bonding ($2p_\pi-2p_\pi$) between Boron and Fluorine partially relieves electron deficiency in $\text{BF}_3$. Overlap becomes progressively less effective with larger halogens ($2p-3p$ in $\text{BCl}_3$, $2p-4p$ in $\text{BBr}_3$, $2p-5p$ in $\text{BI}_3$).
• Diborane ($\text{B}_2\text{H}_6$):
  • Preparation: $4\text{BF}_3 + 3\text{LiAlH}_4 \xrightarrow{\text{ether}} 2\text{B}_2\text{H}_6 + 3\text{LiF} + 3\text{AlF}_3$.
  • Industrial Route: $2\text{BF}_3 + 6\text{NaH} \xrightarrow{450\text{ K}} \text{B}_2\text{H}_6 + 6\text{NaF}$.
  • Inorganic Benzene (Borazine): Heating the diammoniate adduct yields borazine:

    $$3\text{B}_2\text{H}_6 + 6\text{NH}_3 \longrightarrow 3[\text{BH}_2(\text{NH}_3)_2]^+[\text{BH}_4]^- \xrightarrow{\Delta} 2\text{B}_3\text{N}_3\text{H}_6 + 12\text{H}_2$$

• Orthoboric Acid ($\text{H}_3\text{BO}_3$): Layer structure built of planar $\text{BO}_3$ units connected through hydrogen bonds. Heating behavior:

  $$\text{H}_3\text{BO}_3 \xrightarrow{370\text{ K}} \text{HBO}_2 \text{ (metaboric acid)} \xrightarrow{\text{red heat}} \text{B}_2\text{O}_3 \text{ (boric anhydride)}$$

• Borax Bead Test: Borax on strong heating swells and melts into a glassy bead of sodium metaborate and boric anhydride:

  $$\text{Na}_2\text{B}_4\text{O}_7 \cdot 10\text{H}_2\text{O} \xrightarrow{\Delta} \text{Na}_2\text{B}_4\text{O}_7 \xrightarrow{\Delta} 2\text{NaBO}_2 + \text{B}_2\text{O}_3$$

  With transition metal oxides, metaborates with characteristic bead colours form (e.g., $\text{CoO} + \text{B}_2\text{O}_3 \rightarrow \text{Co(BO}_2)_2$ [blue bead]).

2. Group 14 Elements: The Carbon Family

• General Electronic Configuration: $ns^2 np^2$. Common oxidation states are +4 and +2. Due to the inert pair effect, the stability of +2 increases:

  $$\text{Ge} < \text{Sn} < \text{Pb}$$

  Compounds of $\text{Pb}^{2+}$ are stable, while $\text{Pb}^{4+}$ compounds ($\text{PbO}_2, \text{PbCl}_4$) are powerful oxidising agents. $\text{Sn}^{2+}$ acts as a reducing agent ($\text{Sn}^{2+} \rightarrow \text{Sn}^{4+} + 2e^-$).
• Catenation Tendency: Carbon has an exceptional ability to catenate due to high $\text{C-C}$ bond energy ($348\text{ kJ mol}^{-1}$):

  $$\text{C} \gg \text{Si} > \text{Ge} \approx \text{Sn} \quad (\text{Pb does not show catenation})$$

• Allotropes of Carbon:
  • Diamond: Rigid 3D network, each carbon is $sp^3$ hybridised ($\text{C-C} = 154\text{ pm}$). Hardest natural abrasive; electrical insulator.
  • Graphite: Layered hexagonal planar lattice, $sp^2$ hybridised ($\text{C-C} = 141.5\text{ pm}$, interlayer gap $= 340\text{ pm}$). Delocalised $\pi$-electrons allow electrical conduction along sheets; soft, slippery solid lubricant.
  • Fullerenes ($\text{C}_{60}$): Soccer-ball-shaped cage (Buckminsterfullerene) with 60 vertices ($sp^2$). Contains 20 six-membered and 12 five-membered rings. Purest allotrope because it has no dangling bonds.
• Silicon Compounds:
  • Silicones: Organosilicon polymers with repeating $[R_2\text{SiO}]_n$ units. Water-repellent, thermally stable, and used as biocompatible prosthetics and greases.
  • Silicates: Basic structural unit is the tetrahedral $[\text{SiO}_4]^{4-}$ ion.
  • Zeolites: Microporous aluminosilicates; ZSM-5 converts alcohols directly into gasoline.

3. Group 15 Elements: The Nitrogen Family

• General Electronic Configuration: $ns^2 np^3$. Stable half-filled $p$-subshell gives high first ionization enthalpy.
• Anomalous Properties of Nitrogen: Dinitrogen exists as a diatomic gas with a triple bond ($N \equiv N$); heavier elements form single-bonded tetrahedral molecules ($P_4$). $\text{N-N}$ single bond ($160\text{ kJ mol}^{-1}$) is weaker than $\text{P-P}$ ($201\text{ kJ mol}^{-1}$) due to inter-electronic repulsion of non-bonding lone pairs in the compact $2p$ shell.
• Hydride Properties ($\text{NH}_3$ to $\text{BiH}_3$):
  • Thermal Stability: $\text{NH}_3 > \text{PH}_3 > \text{AsH}_3 > \text{SbH}_3 > \text{BiH}_3$ (matches decreasing $\text{E-H}$ bond enthalpy).
  • Reducing Power: $\text{BiH}_3 > \text{SbH}_3 > \text{AsH}_3 > \text{PH}_3 > \text{NH}_3$ ($\text{BiH}_3$ is the strongest reductant).
  • Basicity: $\text{NH}_3 > \text{PH}_3 > \text{AsH}_3 > \text{SbH}_3 > \text{BiH}_3$ (decreasing electron density on the central atom).
  • Boiling Point: $\text{BiH}_3 > \text{SbH}_3 > \text{NH}_3 > \text{AsH}_3 > \text{PH}_3$ (intermolecular H-bonding elevates $\text{NH}_3$).

Oxoacids of Phosphorus

4. Group 16 Elements: The Chalcogens

• General Electronic Configuration: $ns^2 np^4$. Allotropes: Oxygen exists as diatomic $\text{O}_2$ gas and angular $\text{O}_3$; Sulphur exists as puckered crown-shaped $\text{S}_8$ rings ($\alpha$-rhombic below 369 K, $\beta$-monoclinic above 369 K). At 1000 K, $\text{S}_2$ is paramagnetic with 2 unpaired electrons in $\pi^*$ orbitals.
• Hydride Acidic Strength & Boiling Points:
  • Acidic strength: $\text{H}_2\text{O} < \text{H}_2\text{S} < \text{H}_2\text{Se} < \text{H}_2\text{Te}$ ($\text{H-E}$ bond dissociation enthalpy decreases).
  • Boiling point: $\text{H}_2\text{S} (213\text{ K}) < \text{H}_2\text{Se} (232\text{ K}) < \text{H}_2\text{Te} (269\text{ K}) < \text{H}_2\text{O} (373\text{ K})$.
• Ozone ($\text{O}_3$): Prepared by passing a silent electrical discharge through dry oxygen ($3\text{O}_2 \rightarrow 2\text{O}_3$, endothermic). Decomposes spontaneously into $\text{O}_2 + [\text{O}]$ ($\Delta G < 0$ due to $-\Delta H$ and $+\Delta S$), acting as an oxidising agent (oxidises $\text{PbS}$ to $\text{PbSO}_4$, and $\text{I}^-$ to $\text{I}_2$).
• Sulphur Oxoacids & Oleum:
  • Sulphuric Acid ($\text{H}_2\text{SO}_4$): Contact Process ($2\text{SO}_2 + \text{O}_2 \xrightarrow{\text{V}_2\text{O}_5} 2\text{SO}_3$). Absorbing $\text{SO}_3$ into $\text{H}_2\text{SO}_4$ forms Oleum ($\text{H}_2\text{S}_2\text{O}_7$).
  • Peroxodisulphuric Acid ($\text{H}_2\text{S}_2\text{O}_8$ - Marshall's acid): Contains an $\text{-O-O-}$ peroxy linkage with two $\text{SO}_3\text{H}$ units.
  • Peroxomonosulphuric Acid ($\text{H}_2\text{SO}_5$ - Caro's acid): Contains one peroxy linkage; sulfur is in the +6 oxidation state.

5. Group 17 Elements: The Halogens

• General Electronic Configuration: $ns^2 np^5$. Smallest atomic radii and highest electronegativities in their respective periods.
• Halogen Anomalies & Oxidising Power:
  • Oxidising power order: $\text{F}_2 > \text{Cl}_2 > \text{Br}_2 > \text{I}_2$.
  • Driven by low $\text{F-F}$ bond dissociation energy and high hydration enthalpy of the small $\text{F}^-$ ion.
  • $\text{F}_2$ oxidises water to oxygen: $2\text{F}_2 + 2\text{H}_2\text{O} \rightarrow 4\text{H}^+ + 4\text{F}^- + \text{O}_2$.
  • Chlorine and bromine undergo disproportionation with water: $\text{Cl}_2 + \text{H}_2\text{O} \rightarrow \text{HCl} + \text{HOCl}$.
• Reactivity with Alkalies:
  • With cold, dilute $\text{NaOH}$:

    $$2\text{NaOH} + \text{Cl}_2 \longrightarrow \text{NaCl} + \text{NaOCl} + \text{H}_2\text{O}$$

  • With hot, concentrated $\text{NaOH}$:

    $$6\text{NaOH} + 3\text{Cl}_2 \longrightarrow 5\text{NaCl} + \text{NaClO}_3 + 3\text{H}_2\text{O}$$

• Interhalogen Compounds ($\text{XX}'_n$, $n=1, 3, 5, 7$):
  • Central halogen $X$ is larger and more electropositive than surrounding halogen $X'$.
  • More reactive than parent halogens (except $\text{F}_2$) because the polar $\text{X-X}'$ bond is weaker than the homo-nuclear halogen bond.
  • Geometries via VSEPR: $\text{ClF}_3$ is bent T-shaped ($sp^3d$, 2 lp); $\text{BrF}_5$ is square pyramidal ($sp^3d^2$, 1 lp); $\text{IF}_7$ is pentagonal bipyramidal ($sp^3d^3$, 0 lp).

6. Group 18 Elements: The Noble Gases

• Inertness: Closed valence shell ($ns^2 np^6$, except He $1s^2$), high ionization enthalpies, and positive electron gain enthalpies.
• The Neil Bartlett Discovery: Prepared $\text{O}_2^+[\text{PtF}_6]^-$ and realized that $IE_1(\text{O}_2) = 1175\text{ kJ mol}^{-1}$ is comparable to $IE_1(\text{Xe}) = 1170\text{ kJ mol}^{-1}$. Reacting $\text{Xe}$ with $\text{PtF}_6$ yielded the first noble gas compound: $\text{Xe}^+[\text{PtF}_6]^-$.
• Xenon Fluorides & Oxides Synthesis:

  $$\text{Xe (excess)} + \text{F}_2 \xrightarrow{673\text{ K, 1 bar}} \text{XeF}_2\quad (sp^3d,\ \text{Linear, 3 lp})$$

  $$\text{Xe} + 2\text{F}_2\ (1:5\text{ ratio}) \xrightarrow{873\text{ K, 7 bar}} \text{XeF}_4\quad (sp^3d^2,\ \text{Square Planar, 2 lp})$$

  $$\text{Xe} + 3\text{F}_2\ (1:20\text{ ratio}) \xrightarrow{573\text{ K, 60-70 bar}} \text{XeF}_6\quad (sp^3d^3,\ \text{Distorted Octahedral, 1 lp})$$

• Hydrolysis Reactions of Xenon Fluorides:
  • Complete hydrolysis of $\text{XeF}_6$: $\text{XeF}_6 + 3\text{H}_2\text{O} \rightarrow \text{XeO}_3 + 6\text{HF}$ (non-redox).
  • Disproportionation hydrolysis of $\text{XeF}_4$: $6\text{XeF}_4 + 12\text{H}_2\text{O} \rightarrow 4\text{Xe} + 2\text{XeO}_3 + 24\text{HF} + 3\text{O}_2$.
  • Partial hydrolysis of $\text{XeF}_6$: $\text{XeF}_6 + \text{H}_2\text{O} \rightarrow \text{XeOF}_4 + 2\text{HF}$, and $\text{XeF}_6 + 2\text{H}_2\text{O} \rightarrow \text{XeO}_2\text{F}_2 + 4\text{HF}$.

7. High-Yield Trend Maps across Groups 13 to 18