p-Block Elements (Class 12): Groups 15 and 16 Without the Memorising
Most of what looks like an arbitrary list of facts follows from three trends — size, electronegativity and the inert pair effect. Learn the trends and the facts stop needing separate storage.
Why the first member always misbehaves
Nitrogen and oxygen differ from the rest of their groups for two structural reasons, not as a special case to be memorised: they are small, and they have no d-orbitals available. Small size makes pπ–pπ overlap effective, so N2 and O2 exist as multiply bonded diatomic molecules while phosphorus and sulfur form single-bonded cages and rings. No d-orbitals means nitrogen cannot expand its octet, so NCl5 does not exist while PCl5 does.
| Trend down the group | Group 15 | Group 16 |
|---|---|---|
| Atomic size | Increases N → Bi | Increases O → Po |
| Ionisation enthalpy | Decreases | Decreases |
| Stable oxidation state | +5 gives way to +3 (inert pair) | +6 gives way to +4 |
| Hydride stability | NH3 > PH3 > AsH3 > SbH3 > BiH3 | H2O > H2S > H2Se > H2Te |
| Hydride basicity | Decreases down | Decreases down |
| Hydride boiling point | NH3 anomalously high (H-bonding) | H2O anomalously high (H-bonding) |
The inert pair effect, stated usefully
Down a group the ns2 pair becomes progressively reluctant to participate in bonding, because poor shielding by intervening d and f electrons holds it more tightly. The consequence is examinable and simple: Bi(III) is stable while Bi(V) is a strong oxidising agent, and the same logic explains why PbCl2 is more stable than PbCl4. Whenever a question asks why the lower oxidation state dominates at the bottom of a group, this is the answer.
Compounds worth knowing properly
- HNO3 — a strong oxidising agent; its reaction with metals gives different nitrogen oxides depending on concentration, and this is examined regularly.
- PCl3 and PCl5 — PCl5 is trigonal bipyramidal with two longer axial bonds, which is exactly why it is more reactive than the equatorial positions suggest.
- H2SO4 — the contact process, and its behaviour as a dehydrating and oxidising agent.
- O3 — resonance-stabilised, both bonds identical, and a stronger oxidising agent than O2.
How to revise this chapter
Do not build a list of facts. Build one table of trends, then take each “exception” the textbook mentions and write in one line which trend explains it. When the exceptions are stored as consequences rather than as separate items, the volume of the chapter drops by more than half.
FAQs
Why does nitrogen not form pentahalides?
Nitrogen has no vacant d-orbitals in its valence shell, so it cannot expand its octet beyond four bonds. Phosphorus has accessible 3d orbitals and so forms PCl5 and PF5 readily.
Why is H2O a liquid while H2S is a gas?
Oxygen is small and highly electronegative, so water molecules hydrogen bond extensively. Sulfur is larger and much less electronegative, so H2S has only weak dipole interactions and boils far lower despite its greater molar mass.
Why is Bi(V) a strong oxidising agent?
Because of the inert pair effect. At the bottom of Group 15 the 6s2 pair is held tightly and the +5 state is unstable relative to +3, so Bi(V) readily accepts electrons to reach Bi(III).
How many marks does p-block usually carry?
It is consistently one of the heavier inorganic sections in the Class 12 paper, and much of it is short-answer reasoning rather than recall — which is why trend-based revision scores better than list-based revision.
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