p-Block Elements

1. Chapter at a Glance

The p-block contains groups 13 to 18, with outer configuration ns²np¹⁻⁶. It includes metals, metalloids, non-metals and the noble gases, so its chemistry is the most varied in the periodic table. NEET covers group 13 (boron family) and group 14 (carbon family) from Class 11, and groups 15 to 18 from Class 12. Expect about two questions, usually on trends, the inert pair effect, structures of oxoacids and interhalogens, and properties of important compounds.

2. General Trends

• Across the block, non-metallic character increases; down each group, metallic character increases.
• Inert pair effect: down groups 13–16, the ns² electrons become reluctant to take part in bonding (poor shielding by d and f electrons). So the lower oxidation state becomes more stable: Tl⁺ is more stable than Tl³⁺, Pb²⁺ more than Pb⁴⁺, Bi³⁺ more than Bi⁵⁺.
• The first member of each group differs from the rest: small size, high electronegativity, no d orbitals (maximum covalency 4), and a strong tendency to form pπ–pπ multiple bonds (C=C, N≡N, O=O).
• Heavier members use d orbitals to expand their octet (PCl₅, SF₆, IF₇) and form dπ–pπ bonds.

3. Group 13: Boron Family

• Elements: B, Al, Ga, In, Tl. Common oxidation states +3, and +1 becoming more stable down the group.
• Boron is a non-metal; the others are metals. Gallium has an unusually low melting point (303 K) and stays liquid over a wide range.
• Atomic radius of Ga is slightly less than Al, because of poor shielding by the 3d electrons.
• Trihalides (BF₃, AlCl₃) are electron-deficient Lewis acids. Lewis acid strength of boron halides: BF₃ < BCl₃ < BBr₃ < BI₃ (back-bonding is strongest in BF₃).
• Borax, Na₂[B₄O₅(OH)₄]·8H₂O: its aqueous solution is alkaline. On heating it gives a glassy bead used in the borax-bead test (coloured metaborates).
• Boric acid, B(OH)₃: a weak monobasic Lewis acid; it accepts OH⁻ from water rather than donating H⁺. It has a layered, hydrogen-bonded structure.
• Diborane, B₂H₆: has two 3-centre–2-electron "banana" bonds (B–H–B bridges) and four terminal B–H bonds. Each B is sp³ hybridised.
• Aluminium is amphoteric: it dissolves in both acids and alkalis.

4. Group 14: Carbon Family

• Elements: C, Si, Ge, Sn, Pb. Oxidation states +4 and +2; +2 becomes more stable down the group (Pb²⁺).
• Catenation (self-linking) is highest for carbon: C ≫ Si > Ge ≈ Sn. Lead shows almost none.
• Allotropes of carbon: diamond (sp³, 3-D network, hardest, non-conductor), graphite (sp², layers, good conductor, lubricant), fullerenes (C₆₀, sp², football shape with 20 six- and 12 five-membered rings).
• CO is neutral and poisonous: it binds haemoglobin about 300 times more strongly than O₂. CO₂ is acidic, linear and non-polar.
• SiO₂ is a covalent 3-D network of SiO₄ tetrahedra, unlike the discrete CO₂ molecule.
• Silicones are polymers with –R₂SiO– repeating units: water-repellent, heat-resistant, used as sealants.
• Silicates are built from SiO₄⁴⁻ tetrahedra; zeolites are aluminosilicates used as catalysts (ZSM-5 converts alcohol into petrol).
• CCl₄ is not hydrolysed, but SiCl₄ is (Si has vacant 3d orbitals to accept water's lone pair).

5. Group 15: Nitrogen Family

• Elements: N, P, As, Sb, Bi. Configuration ns²np³ (extra stable half-filled p). Oxidation states −3 to +5; +3 becomes more stable than +5 down the group.
• N₂ is very unreactive because of its strong triple bond (941 kJ mol⁻¹). Nitrogen cannot form pentahalides (no d orbitals).
• Hydrides EH₃: basicity and thermal stability decrease NH₃ > PH₃ > AsH₃ > SbH₃ > BiH₃; reducing character increases. Bond angle decreases from 107° (NH₃) to about 91° (BiH₃). Boiling point: NH₃ is higher than PH₃ because of hydrogen bonding.
• Ammonia is made by the Haber process (Fe catalyst, ~700 K, ~200 atm). It is a Lewis base and forms complexes such as [Cu(NH₃)₄]²⁺ (deep blue).
• Nitric acid is made by the Ostwald process (catalytic oxidation of NH₃ over Pt/Rh). It is a strong oxidising agent. Concentrated HNO₃ makes Al, Fe and Cr passive.
• Phosphorus allotropes: white P (P₄ tetrahedron, very reactive, glows in the dark, stored under water), red P (polymeric, less reactive), black P (most stable).
• PH₃ is made by heating white P with NaOH (a disproportionation reaction).
• PCl₅ is trigonal bipyramidal in the gas; in the solid it exists as [PCl₄]⁺[PCl₆]⁻.

6. Oxoacids of Phosphorus and Sulphur

P–H bonds are not ionisable, which is why H₃PO₂ is monobasic. Oxoacids with P–H bonds (H₃PO₂, H₃PO₃) are good reducing agents.

7. Group 16: Oxygen Family

• Elements: O, S, Se, Te, Po. Configuration ns²np⁴; oxygen shows −2 mainly; S, Se and Te also show +2, +4 and +6.
• Hydrides H₂E: acidic character increases H₂O < H₂S < H₂Se < H₂Te (weaker H–E bond down the group); thermal stability decreases. H₂O has an abnormally high boiling point (hydrogen bonding).
• Ozone, O₃: bent molecule (117°), resonance hybrid, powerful oxidising agent. The ozone layer absorbs harmful UV radiation; it is damaged by chlorofluorocarbons and nitrogen oxides.
• Oxides can be acidic (SO₂, SO₃), basic (Na₂O, CaO), amphoteric (Al₂O₃, ZnO) or neutral (CO, NO, N₂O).
• Sulphur allotropes: rhombic (α, stable below 369 K) and monoclinic (β). Both contain puckered S₈ rings.
• Sulphuric acid is made by the contact process: 2SO₂ + O₂ ⇌ 2SO₃ over V₂O₅ catalyst (~720 K, 2 bar); SO₃ is absorbed in H₂SO₄ to form oleum, which is then diluted. Concentrated H₂SO₄ chars sugar (dehydration).

8. Group 17: Halogens

• Elements: F, Cl, Br, I, At. Configuration ns²np⁵; very reactive non-metals. Colour deepens down the group: F₂ pale yellow, Cl₂ greenish-yellow, Br₂ red-brown, I₂ violet.
• Fluorine is the most electronegative element and the strongest oxidising agent. It shows only −1. Its bond energy is lower than Cl₂ (F–F lone-pair repulsion). Order of bond energy: Cl₂ > Br₂ > F₂ > I₂.
• Hydrogen halides: acid strength HF < HCl < HBr < HI; boiling point HCl < HBr < HI < HF (HF is hydrogen bonded).
• Chlorine: made by Deacon's process or electrolysis of brine. It bleaches by oxidation (in the presence of moisture), and the effect is permanent.
• Oxoacids of chlorine: acid strength HClO < HClO₂ < HClO₃ < HClO₄ (more O atoms stabilise the anion).
• Interhalogens (XX′, XX′₃, XX′₅, XX′₇) are more reactive than halogens (except F₂) because the X–X′ bond is weaker. Shapes: ClF₃ T-shaped, BrF₅ square pyramidal, IF₇ pentagonal bipyramidal.

9. Group 18: Noble Gases

• Elements: He, Ne, Ar, Kr, Xe, Rn. Full octet (He: 1s²), very high ionisation enthalpies, positive electron gain enthalpies.
• Only Xe (and Kr to a small extent) forms compounds, because its ionisation enthalpy is low enough. Bartlett made the first noble-gas compound, Xe⁺PtF₆⁻, after noticing that the IE of Xe is close to that of O₂.
• Uses: He in balloons and for deep-sea breathing mixtures; Ne in advertising lights; Ar as an inert atmosphere in welding and bulbs.

10. Formula and Trend Master Table

11. Master Summary Table

12. Mind Map

13. Tips and Tricks

14. Important One-liners

• Boron is the only non-metal of group 13.
• Diamond is the hardest natural substance; graphite conducts electricity.
• Nitrogen shows a maximum covalency of 4.
• White phosphorus is stored under water because it catches fire in air.
• Ozone is a stronger oxidising agent than oxygen.
• Chlorine bleaches by oxidation; SO₂ bleaches by reduction (temporary).
• Helium is used for filling weather balloons because it is light and non-inflammable.
• XeF₂ is linear and XeF₄ is square planar.

15. NEET-pattern Questions with Detailed Answers

These questions are original and based on concepts that NEET UG asks again and again.

16. Quick Revision Summary