# p-Block Elements

> Groups 13-18 chemistry — inert pair effect, hydrides, oxoacids, halogens and noble gases from NCERT Class 11-12 for NEET-UG Chemistry.

- Canonical URL: https://prepelephant.com/topics/neet-ug/chemistry/p-block-elements-ncert
- Exam / course: NEET-UG · Subject: Chemistry
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-02
- Last updated: 2026-10-02
- How to cite: "p-Block Elements", PrepElephant, https://prepelephant.com/topics/neet-ug/chemistry/p-block-elements-ncert

## Direct answer

Boron heads the p block as its only non-metal group head, and from there to helium the p-block story is the first-element anomaly and the inert pair effect — the reluctance of ns^2 electrons to bond, which makes Tl+ more stable than Tl3+ and Pb2+ than Pb4+. Down any group, metallic character grows, oxide acidity falls, and bond enthalpies misbehave: fluorine's F–F bond is weaker than chlorine's because the small atoms repel. Halogen acidity runs HF < HCl < HBr < HI, oxidising power F2 > Cl2 > Br2 > I2, and the noble gases finally react with fluorine — XeF2 linear, XeF4 square planar, XeF6 distorted octahedral.

## What you must remember

- Boric acid H3BO3 is a weak monobasic Lewis acid — it accepts OH− rather than donating H+.
- Diborane B2H6 holds itself together with banana-shaped three-centre two-electron bonds.
- CO binds haemoglobin 200–300 times more strongly than oxygen, which is the chemistry of carbon monoxide poisoning.
- PCl5 is trigonal bipyramidal as a gas but ionic in the solid, as [PCl4]+[PCl6]−; white phosphorus is stored under water.
- Fluorine's bond enthalpy (158 kJ/mol) is lower than chlorine's (242 kJ/mol) — lone-pair repulsion between the small atoms.
- Acid strength order: HF < HCl < HBr < HI; oxidising power: F2 > Cl2 > Br2 > I2 (fluorine oxidises water to oxygen).
- Interhalogen compounds (ClF3, ICl) form by direct combination and are more reactive than the parent halogens (except with F2).
- Bartlett's Xe+PtF6− opened noble gas chemistry; XeO3 is explosive, and He is used in diving mixtures to avoid nitrogen narcosis.

## How the first-element anomaly and inert pair effect run the block

Take group 13 and walk down it with two questions in mind. Why is boron so different from aluminium? Boron's tiny size, high electronegativity and absence of d orbitals make its chemistry covalent — its halides are strong Lewis acids and its hydride is electron-deficient — while aluminium forms hydrated Al3+ salts. Now ask the second question: why does the +1 state grow down the group? The inert pair effect — poor shielding by intervening d and f electrons holds the ns^2 pair back from bonding — so gallium still prefers +3 but thallium's Tl+ is the stable state, a complete reversal by the bottom of the group. The same reasoning runs right across the block: Pb2+ beats Pb4+ in group 14 (why PbO2 is a strong oxidant), Bi3+ beats Bi5+ in group 15. Once you internalise these two threads, the group-by-group facts stop being random: the first element misbehaves because it is small, and the last elements misbehave because their lone pair is inert.

## The structures the exam always asks

Structure questions about xenon fluorides are near-guaranteed: XeF4 is square planar (sp3d2 with two lone pairs) while XeF2 is linear (sp3d with three lone pairs), and options swap them casually. Second, the bleach mechanism pair: chlorine bleaches by oxidation and is permanent, sulphur dioxide bleaches by reduction and is reversible — damp litmus returning its colour in air is SO2's signature. Third, the F2 bond enthalpy ordering (Cl2 > Br2 > F2 > I2) contradicts the "smaller is stronger" instinct and is asked precisely because of that. Fourth, know the industrial routes: Contact process for H2SO4, Ostwald for HNO3 over a platinum–rhodium gauze. Finally, ammonia's boiling point exceeds phosphine's because of hydrogen bonding — the p-block hydride questions always circle back to it.

## Frequently asked questions

### Why is the bond enthalpy of F2 less than that of Cl2?

The fluorine atoms are so close that lone-pair repulsion weakens the bond, whereas chlorine's lone pairs sit farther apart in larger orbitals.

### What are the shapes of XeF2, XeF4 and XeF6?

Linear, square planar and distorted octahedral respectively, from five, six and seven electron pairs with three, two and one lone pairs.

### Why is boric acid a Lewis acid?

It completes its sextet by accepting a hydroxide's electron pair, so it releases H+ only indirectly through water — hence monobasic despite three OH groups.

### Why is bleaching by sulphur dioxide temporary?

SO2 bleaches by reduction; atmospheric oxygen slowly reoxidises the bleached substance, restoring the colour, unlike chlorine's oxidative permanent bleach.

### Why does nitrogen show anomalous behaviour in group 15?

Small size, high electronegativity, absence of d orbitals and the ability to form multiple (pπ-pπ) bonds set N2's triple bond apart from the rest of the group.