The p-Block Elements

1. Group 15 to 18 Trends

The p-block is diverse, containing metals, non-metals, and metalloids. Key concepts include Inert Pair Effect, Catenation, and anomalous behavior of the first element in each group.

2. Important Compounds

• Nitrogen: Ammonia (Haber's process) and Nitric Acid (Ostwald's process).
• Sulphur: Sulphuric Acid (Contact process) and its oxoacids.
• Halogens: Interhalogen compounds (e.g., $ICl_3$) and their strong oxidizing nature.

3. Group 13 and 14 Highlights

• Boron is a non-metal; $BF_3$ is a Lewis acid (incomplete octet). Diborane $B_2H_6$ has two 3-centre–2-electron (banana) bonds.
• Boric acid, $H_3BO_3$, is a weak monobasic Lewis acid: it accepts $OH^-$ rather than releasing $H^+$.
• Carbon shows strong catenation and pπ–pπ multiple bonding; silicon does not, which is why $CO_2$ is a gas and $SiO_2$ a network solid.
• Allotropes of carbon include diamond, graphite and the fullerenes. Graphite conducts electricity because of delocalised π electrons.

4. Group 15

• $NH_3 > PH_3 > AsH_3 > SbH_3 > BiH_3$ in basicity and thermal stability; reducing character increases down the group.
• White phosphorus ($P_4$) is reactive and stored under water; red phosphorus is polymeric and much less reactive.
• Nitrogen's anomalous behaviour: small size, no d orbitals, and strong pπ–pπ bonding (N≡N).

5. Group 16

• Oxygen and sulphur allotropes: $O_2$/$O_3$; rhombic and monoclinic sulphur. Rhombic sulphur is stable below 369 K.
• Acidic strength of hydrides increases down the group: $H_2O < H_2S < H_2Se < H_2Te$ (bond strength decreases).
• Contact process: $2SO_2 + O_2 \rightleftharpoons 2SO_3$ over $V_2O_5$; $SO_3$ is absorbed in conc. $H_2SO_4$ to form oleum ($H_2S_2O_7$).

6. Group 17 and 18

• Oxidising power: $F_2 > Cl_2 > Br_2 > I_2$. Bond dissociation enthalpy order: $Cl_2 > Br_2 > F_2 > I_2$ (F–F is weakened by lone-pair repulsion).
• Acid strength of hydrogen halides in water: HI > HBr > HCl > HF.
• Oxoacids of chlorine: acid strength $HClO_4 > HClO_3 > HClO_2 > HClO$.
• Noble gases: Xe forms $XeF_2$ (linear), $XeF_4$ (square planar), $XeF_6$ (distorted octahedral) and $XeO_3$.

Practice questions

1. Boric acid is best described as:

   1. A strong tribasic acid
   2. A weak monobasic Lewis acid
   3. A Brønsted base
   4. An amphoteric oxide
   Answer

   B. B(OH)₃ accepts OH⁻ from water to form [B(OH)₄]⁻, releasing one H⁺ indirectly.

2. The strongest acid in aqueous solution among the hydrogen halides is:

   1. HF
   2. HCl
   3. HBr
   4. HI
   Answer

   D. The H–I bond is the weakest, so HI ionises most readily.

3. Which halogen has the lowest bond dissociation enthalpy?

   1. F₂
   2. Cl₂
   3. Br₂
   4. I₂
   Answer

   D. The order is Cl₂ > Br₂ > F₂ > I₂; I₂ is lowest.

4. The shape of XeF₂ is:

   1. Bent
   2. Linear
   3. T-shaped
   4. Trigonal planar
   Answer

   B. Xe has 2 bond pairs and 3 lone pairs (sp³d); the lone pairs occupy equatorial positions, leaving a linear molecule.

5. Which hydride of group 15 is the most basic?

   1. NH₃
   2. PH₃
   3. AsH₃
   4. BiH₃
   Answer

   A. Nitrogen's small size concentrates its lone pair, making NH₃ the strongest base.