Class 11 Chemistry Notes

p-Block Elements Class 11 Notes

Complete, exam-ready notes on the p-block elements: where they sit in the periodic table, the general trends down the groups, the inert pair effect, the boron family (Group 13) with diborane and borax, and the carbon family (Group 14) with allotropes, silicates and silicones — written for CBSE, JEE and NEET revision.

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Written byDeep Narayan· Science & Mathematics EducatorReviewed byPushpanjali

What are p-block elements in one line?

The p-block elements fill the p subshell (ns²np¹ to ns²np⁶) and span Groups 13 to 18, containing metals, metalloids and non-metals with the whole range of oxidation states.

Position of the p-Block Elements

p-block elements

ns^2np^1,\; ns^2np^2,\; \ldots,\; ns^2np^6

The last six groups (13 to 18) hold the p-block: valence shell configuration ns²np¹ to ns²np⁶. Only helium of Group 18 is actually an s-block element; the rest have a filled s plus partially filled p subshell.

Groups 13 B, 14 C, 15 N, 16 O, 17 F (halogens), 18 noble gases.
The group shows the full spread: metals (Al), metalloids (Si, Ge), and non-metals (C, N, O).
Oxidation states run from +3 to −3; the highest oxidation state equals the group number (sum of s + p electrons).

General Trends Down the Group

Atomic and ionic radii increase down the group, but increase less than in s-block because more electrons share the same shell.
Ionisation enthalpy is high for the lightest member, then tends to fall; Group 15 has an anomalous upward jump due to half-filled orbitals.
Metallic character increases down each group — B is a metalloid, Al and Tl are metals.
Electronegativity falls down the group but not smoothly; the heavy elements show the inert pair effect.
Noble gases have maximum ionisation enthalpy, making them the least reactive.

Inert Pair Effect

Inert pair effect

\text{Group 13: } M^{+3} \text{ and } M^{+1};\quad \text{Group 14: } M^{+4} \text{ and } M^{+2}

Down each group the heavier elements increasingly prefer the lower oxidation state (group number − 2) because the ns² pair is reluctant to participate in bonding. Tl⁺, Sn²⁺, Pb²⁺ and Bi³⁺ are all stable, halide chemistry included — the driving force behind the inert pair effect.

The ns² pair is held tight by the poor shielding of the intervening d and f electrons, so it is not easily lost.
PbCl₂ and TlCl are stable because the lower oxidation state is preferred.
The +1 oxidation state of Tl, +2 of Pb and Sn is more stable than the group number oxidation state.

Boron Family (Group 13)

B, Al, Ga, In, Tl; general configuration ns²np¹ and oxidation states +3 and +1.
Boron is a metalloid with a very high melting point; Al onwards are soft metals with lower melting points.
Boron has a unique ability to form electron-deficient compounds and stable covalent bonds (3-centre-2-electron bonding in diborane).
Boric acid H₃BO₃ is a weak monobasic acid that acts as a Lewis acid by accepting OH⁻ from water; it gives a green flame test with methanol.
Borax Na₂B₄O₇·10H₂O is the common sodium borate — the classic bead test gives coloured beads with transition metals.

Anomalous Boron and Diborane

Diborane B₂H₆

B_2H_6 + 6H_2O \to 2B(OH)_3 + 6H_2

Diborane is the simplest boron hydride. It has a 'banana bond' structure where two hydrogen bridges hold the boron atoms together — each bridge is a three-centre two-electron (3c-2e) bond. It is spontaneously flammable and hydrolyses in water to boric acid with hydrogen gas.

In the gas phase diborane burns with a green flame to give boric oxide and water.
It adds across alkenes — hydroboration — which is a cornerstone reaction in organic chemistry.
Boron's diagonal relationship with silicon shows in the acid character of B(OH)₃ and Si(OH)₄ and their reaction with NaOH.

Carbon Family (Group 14)

C, Si, Ge, Sn, Pb; configuration ns²np² with oxidation states +4 and +2.
Carbon is non-metallic, silicon and germanium are metalloids, tin and lead are metals.
Carbon bonds catenate — forms long chains — giving the vast chemistry of organic compounds.
Allotropic forms: diamond (network covalent, hardest), graphite (layered, conducts electricity) and fullerenes (molecular cages); graphite is the thermodynamically stable form.
Inert pair effect again: the stability of +2 outweighs +4 for Sn and Pb.

Silicates, Silicones and Zeolites

Silicates

Silicon forms SiO₄⁴⁻ tetrahedra that link in chains, sheets or 3-D networks — the silicate minerals (nepheline, asbestos, mica, feldspar, quartz).

Silicones

R_2SiCl_2 \xrightarrow{\text{hydrolysis}} [-Si(R)_2-O-]_n

Silicones are polymers with Si–O backbones and organic groups on silicon. They are stable, water-resistant, and used in sealants, lubricants and waterproofing because the Si–O bond is very strong.

Zeolites

Aluminosilicate frameworks with pores that trap molecules by size — natural or synthetic sieves used in water softening, catalysis and ion exchange.

Solved Examples

Example: Why is PbCl₂ stable but PbCl₄ unstable?

Solution: The inert pair effect makes the ns² electrons of lead reluctant to bond, so the +2 state is preferred. Pb⁴⁺ is strongly oxidising and PbCl₄ tends to lose chlorine, whereas PbCl₂ is covalent and stable.

Example: Which factor decides metallic character within the p-block?

Solution: Metallic character increases down a group because the ionisation enthalpy falls and the atomic size increases, so electrons are lost more easily. Metals give way to metalloids and then non-metals as you move right across the p-block.

Revision

Key formulas at a glance

Memorise these before attempting numericals — most exam questions hinge on one of them.

p-block configuration

ns^2np^{1-6}

Highest oxidation state

\text{ox. state}_{\max} = \text{group number}

Inert pair oxidation states

^{13}M^{+3}/M^{+1},\; ^{14}M^{+4}/M^{+2}

Silicate unit

\text{SiO}_4^{4-}

Hydrolysis of diborane

B_2H_6 + 6H_2O \to 2B(OH)_3 + 6H_2

Silicone backbone

[-Si(R)_2-O-]_n

Boric acid as Lewis acid

B(OH)_3 + H_2O \rightleftharpoons [B(OH)_4]^- + H^+

Exam tips

How this chapter is asked

Where this topic appears in CBSE, JEE Main and NEET papers.

p-block valence configuration ns²np¹⁻⁶; Group 13 → 18.
Inert pair effect: heavier elements prefer lower oxidation state — Tl⁺, Pb²⁺, Sn²⁺, Bi³⁺.
Diborane uses 3-centre-2-electron banana (bridge) bonds.
Boric acid is weak monobasic but acts as a Lewis acid.
Carbon catenates; graphite is the most stable allotrope.
SiO₄⁴⁻ tetrahedra build all silicates; silicones have Si–O backbone.
Bead test with borax is used to identify transition-metal cations.

FAQ

Common questions

What are p-block elements?

Elements of Groups 13–18 whose valence electrons fill the p subshell, with configurations from ns²np¹ to ns²np⁶. They include metals, metalloids and non-metals with oxidation states from +group number down to −3.

What is the inert pair effect?

Down the group, the ns² pair of electrons becomes reluctant to take part in bonding, so heavier elements prefer an oxidation state two less than the group number — for example Tl⁺, Pb²⁺, Sn²⁺ and Bi³⁺.

Why does boron behave differently from the rest of its group?

Boron has a very small size, high ionisation enthalpy and lacks d-orbitals. It forms covalent, electron-deficient compounds like diborane (with 3-centre-2-electron bonds) and shows a diagonal relationship with silicon.

What distinguishes graphite from diamond?

Diamond is a 3-D covalent network of carbons — the hardest natural substance. Graphite has flat layers of hexagons with weak van der Waals forces between layers, so it is soft, slippery and conducts electricity.

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