In this guide
The p-block used to be the heaviest memory chapter in NEET chemistry: oxoacids, preparations, structures and dozens of reactions. The 2026 NMC syllabus has cut it back. It lists only a general introduction for groups 13 to 18: electronic configuration, trends in physical and chemical properties across periods and down groups, and the unique behaviour of the first element in each group.
That changes how to study it. Learn the trends and, more importantly, the reasons behind them: size, effective nuclear charge, the inert pair effect, and the absence of d orbitals in the second period. A question that asks you to arrange hydrides by boiling point, or to explain why nitrogen cannot form NCl₅, is testing reasons, and reasons carry across all six groups.
The p-block at a glance
The general valence configuration is ns² np¹⁻⁶ (helium, 1s², is the exception in group 18). The highest oxidation state of a group equals the number of valence electrons, which is the group number minus 10. Fluorine and oxygen are the exceptions: fluorine shows only −1, and oxygen rarely goes above +2.
| Group | Configuration | Elements | Highest state | Other common states |
|---|---|---|---|---|
| 13 | ns² np¹ | B, Al, Ga, In, Tl | +3 | +1 (stable for Tl) |
| 14 | ns² np² | C, Si, Ge, Sn, Pb | +4 | +2 (stable for Pb) |
| 15 | ns² np³ | N, P, As, Sb, Bi | +5 | −3, +3 (stable for Bi) |
| 16 | ns² np⁴ | O, S, Se, Te, Po | +6 | −2, +4 |
| 17 | ns² np⁵ | F, Cl, Br, I, At | +7 | −1 (F shows only −1) |
| 18 | ns² np⁶ | He, Ne, Ar, Kr, Xe, Rn | +8 (Xe) | Xe shows +2, +4, +6 |
Non-metallic character is highest at the top right and falls down each group. Group 13 goes from non-metal boron to metals; group 14 from non-metal carbon to metals tin and lead; group 15 from nitrogen gas to metallic bismuth.
The inert pair effect
Down groups 13, 14 and 15, the heavier elements increasingly prefer an oxidation state two less than the group maximum. The ns² pair becomes reluctant to take part in bonding, because the d and f electrons below it shield it poorly and it is held tightly.
| Group | Stability trend | Consequence |
|---|---|---|
| 13 | +1 state: Ga < In < Tl | Tl⁺ is more stable than Tl³⁺; Tl³⁺ compounds are oxidising |
| 14 | +2 state: Ge < Sn < Pb | Pb²⁺ is stable, so PbO₂ and Pb⁴⁺ compounds are strong oxidising agents; Sn²⁺ is a reducing agent |
| 15 | +3 state: stability increases down the group | Bi(V) compounds are strong oxidising agents; Bi(III) is the stable state |
Irregular trends that NEET likes
Group 13 atomic radius: Ga (135 pm) is smaller than Al (143 pm). Gallium's ten 3d electrons shield the nucleus poorly, so its outer electrons feel a larger effective nuclear charge.
Group 13 ionisation enthalpy: it does not fall smoothly. B 801, Al 577, Ga 579, In 558 and Tl 589 kJ mol⁻¹. The d and f electrons again explain why Ga and Tl are higher than expected.
Catenation (self-linking into chains) is strongest for carbon and falls down group 14: C ≫ Si > Ge ≈ Sn, with lead showing none. The C–C bond is strong; the bonds between heavier atoms are weaker.
Group 15 hydrides (EH₃):
| Property | Trend | Reason |
|---|---|---|
| Basic character | NH₃ > PH₃ > AsH₃ > SbH₃ > BiH₃ | The lone pair spreads over a larger atom and is less available |
| Thermal stability | Decreases down the group | E–H bond weakens as the atom grows |
| Reducing character | Increases down the group, BiH₃ strongest | Weaker E–H bond |
| Bond angle | NH₃ 107.8° > PH₃ 93.6° > AsH₃ 91.8° > SbH₃ 91.3° > BiH₃ ~90° | Bond pairs move away from the larger central atom |
| Boiling point | PH₃ < AsH₃ < NH₃ < SbH₃ < BiH₃ | NH₃ is raised by hydrogen bonding; after that, size wins |
Group 16 hydrides (H₂E): acidic character rises (H₂O < H₂S < H₂Se < H₂Te) and thermal stability falls down the group. Boiling points: H₂S < H₂Se < H₂Te < H₂O, with water on top because of hydrogen bonding.
Group 17 (halogens):
- Most negative electron gain enthalpy: Cl > F > Br > I. Fluorine is so small that an incoming electron is repelled by the electrons already there.
- Bond dissociation enthalpy: Cl₂ > Br₂ > F₂ > I₂. The F–F bond is weak because lone pairs on the two small atoms repel each other.
- Oxidising power: F₂ > Cl₂ > Br₂ > I₂. Fluorine is the strongest oxidising agent.
- Acid strength of HX: HF < HCl < HBr < HI, because the H–X bond weakens down the group. Boiling points: HCl < HBr < HI < HF, with HF raised by hydrogen bonding.
Group 18: noble gases have the highest ionisation enthalpies in their periods and positive electron gain enthalpies. The first true noble gas compound was made in 1962, when Neil Bartlett reasoned that since O₂ reacts with PtF₆ and xenon's first ionisation enthalpy is almost the same as oxygen's, xenon should too.
Why the first element is different
B, C, N, O and F each behave unlike the rest of their group. The reasons are the same every time:
- Small size and high electronegativity and ionisation enthalpy.
- No d orbitals in the valence shell, so the maximum covalency is 4 (one s and three p orbitals).
- A strong tendency to form pπ–pπ multiple bonds with itself and its neighbours (C=C, C≡C, N≡N, O=O, C=O).
| First element | Anomaly | Heavier members |
|---|---|---|
| Boron | Non-metal; maximum covalency 4 (BF₄⁻) | Al forms [AlF₆]³⁻ |
| Carbon | Forms CO₂ as discrete gas molecules with C=O | SiO₂ is a giant network solid with Si–O single bonds |
| Carbon | Cannot form CF₆²⁻ | Si forms SiF₆²⁻ |
| Nitrogen | Exists as N≡N (very stable); cannot form NCl₅ | Phosphorus is P₄; PCl₅ exists |
| Oxygen | Diatomic gas; strong hydrogen bonding in H₂O | Sulphur is S₈, a solid; H₂S is a gas |
| Fluorine | Only −1 state; HF is hydrogen bonded | Cl, Br, I show +1, +3, +5, +7 |
Shapes you still need (through VSEPR)
| Molecule | Bond pairs + lone pairs | Hybridisation | Shape |
|---|---|---|---|
| XeF₂ | 2 + 3 | sp³d | Linear |
| XeF₄ | 4 + 2 | sp³d² | Square planar |
| XeF₆ | 6 + 1 | sp³d³ | Distorted octahedral |
| ClF₃ | 3 + 2 | sp³d | T-shaped (bent-T) |
| PCl₅ | 5 + 0 | sp³d | Trigonal bipyramidal |
| SF₄ | 4 + 1 | sp³d | See-saw |
Worked NEET-style questions
Example 1: inert pair effect
Which is the strongest oxidising agent: SnO₂, PbO₂, SiO₂ or GeO₂?
- In group 14, the +4 state becomes less stable down the group, and +2 more stable.
- Pb⁴⁺ most wants to become Pb²⁺, so it takes electrons most readily.
- Answer: PbO₂.
Example 2: boiling points of hydrides
Arrange HF, HCl, HBr and HI in increasing order of boiling point.
- From HCl to HI, molecular size and van der Waals forces increase.
- HF is an exception because of strong hydrogen bonding.
- Order: HCl < HBr < HI < HF.
Example 3: a first-element anomaly
Why does phosphorus form PCl₅ while nitrogen forms only NCl₃?
- Nitrogen's valence shell (n = 2) has only 2s and 2p orbitals, so its covalency cannot exceed 4.
- Phosphorus (n = 3) has vacant 3d orbitals and can expand its octet to five bonds.
Example 4: shape and hybridisation
Find the shape of XeF₄.
- Xe has 8 valence electrons; four bond to F, leaving 4 electrons, or 2 lone pairs.
- 6 electron pairs means sp³d² (octahedral arrangement).
- The two lone pairs sit opposite each other, so the shape is square planar.
Practice MCQs
- The element with the most negative electron gain enthalpy is: (a) F (b) Cl (c) Br (d) O
- The most basic group 15 hydride is: (a) NH₃ (b) PH₃ (c) AsH₃ (d) BiH₃
- Which species exists? (a) CF₆²⁻ (b) SiF₆²⁻ (c) NCl₅ (d) OF₄
- The order of bond dissociation enthalpy is: (a) F₂ > Cl₂ > Br₂ > I₂ (b) Cl₂ > Br₂ > F₂ > I₂ (c) I₂ > Br₂ > Cl₂ > F₂ (d) Cl₂ > F₂ > Br₂ > I₂
- Tl⁺ is more stable than Tl³⁺ because of: (a) lanthanoid contraction (b) the inert pair effect (c) hydrogen bonding (d) back bonding
- The group 13 element with the lowest first ionisation enthalpy is: (a) B (b) Al (c) Ga (d) In
- The shape of ClF₃ is: (a) trigonal planar (b) pyramidal (c) T-shaped (d) see-saw
- The weakest acid among the halogen acids is: (a) HF (b) HCl (c) HBr (d) HI
Answers
- (b) Fluorine's small size causes electron–electron repulsion.
- (a) The lone pair on the small N atom is most available.
- (b) Silicon can use d orbitals; C, N and O cannot exceed covalency 4.
- (b) F–F is weakened by lone pair repulsion.
- (b) The 6s² pair of thallium resists bonding.
- (d) In is 558 kJ mol⁻¹, just below Al at 577.
- (c) 3 bond pairs and 2 lone pairs in a trigonal bipyramidal arrangement.
- (a) The H–F bond is the strongest.
What to do next
- Make one page of trends per group, and write a one-line reason beside every trend.
- Learn the first-element anomaly table and the three reasons behind it until you can write them in a minute.
- Draw the six VSEPR shapes above, counting lone pairs every time, using chemical bonding as your reference.
- Revise periodic trends, since this chapter is those trends applied group by group, then move to d- and f-block elements.
A note on dates and numbers. Exam patterns, vacancies and schedules change from year to year. Always confirm the current details in the latest notification on the National Testing Agency website .
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