JEE Mains Chemistry · The p-Block Elements

Group 13: Periodic Trends and the Inert Pair Effect

Boron, aluminium, gallium, indium and thallium (ns² np¹) grow down the group with one break at gallium, and the heavier ones keep their s² pair, so thallium prefers +1 to +3.

All The p-Block Elements notes JEE Mains Chemistry strategy

Why this matters

Twenty-five PYQs, all multiple choice, and three from 2026. Nine test the trends in atomic and ionic radius, ionisation enthalpy, electronegativity and density; seven ask about boron's hard B₁₂ lattice, gallium's long liquid range and the uses of the elements; nine turn on the inert pair effect, from the stability of Tl⁺ to TlI₃, GaAlCl₄ and the most basic oxide.

Concept 1 of 3: Group 13 trends in radius, ionisation enthalpy and electronegativity

Down a group the atom normally grows and holds its electrons less tightly. Group 13 breaks this pattern at gallium. Gallium comes just after the ten 3d elements, and d electrons shield the nucleus poorly, so gallium's outer electrons feel a larger nuclear charge. Gallium is therefore slightly SMALLER than aluminium, and its ionisation enthalpy is slightly higher. The same poor shielding by 4f electrons makes thallium's ionisation enthalpy rise again.

Definition

Atomic radius: $\mathrm{B < Ga < Al < In < Tl}$ . Gallium is smaller than aluminium.
$\mathrm{M^{3+}}$ ionic radius rises steadily: $\mathrm{B^{3+} < Al^{3+} < Ga^{3+} < In^{3+} < Tl^{3+}}$ . Only the atomic radius has the break.
First ionisation enthalpy is irregular: highest for B, lowest for In, order $\mathrm{In < Al < Ga < Tl < B}$ . The fall from B to Al is large; from Al to Ga there is almost none.
Electronegativity falls from B to Al, then rises slightly: $\mathrm{Al < Ga < In < Tl < B}$ .
Density rises down the group, from B to Tl.
The trichlorides and tri-iodides are covalent; a small, highly charged $\mathrm{M^{3+}}$ polarises a large anion.

Element	Atomic radius (pm)	M³⁺ radius (pm)	First ionisation enthalpy (kJ/mol)	Electronegativity
B	85	27	801	2.0
Al	143	53.5	577	1.5
Ga	135	62.0	579	1.6 Smaller than Al and with a slightly higher ionisation enthalpy: poor shielding by 3d electrons.
In	167	80.0	558	1.7
Tl	170	88.5	589	1.8

NCERT values. Read each column on its own: the atomic radius dips at Ga, the M³⁺ radius does not, and the ionisation enthalpy is lowest at In, not Tl.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2025 · 3 Apr 2025 · Q118Moderate

Example 1 · The p-Block Elements · Group 13: Periodic Trends and the Inert Pair Effect

The correct orders among the following are Atomic radius :

B < Al < Ga < In < Tl

Electronegativity:

Al < Ga < In < Tl < B

Density :

Tl < In < Ga < Al < B

1st Ionisation Energy:

In < Al < Ga < Tl < B

Choose the correct answer from the options given below :

The atomic radius is not a smooth rise

The order

\mathrm{B < Al < Ga < In < Tl}

looks natural but is wrong. Gallium is smaller than aluminium, so the correct order is

\mathrm{B < Ga < Al < In < Tl}

. The

\mathrm{M^{3+}}

radius, by contrast, does rise steadily.

Thallium does not have the lowest ionisation enthalpy

Indium has the lowest first ionisation enthalpy in group 13, 558 kJ/mol. Thallium's is higher, 589 kJ/mol, because its 4f and 5d electrons shield poorly.

Electronegativity does not simply fall down group 13

It falls from boron (2.0) to aluminium (1.5), then rises a little through gallium, indium and thallium. A statement that it decreases down the whole group is false.

Drill 8 more on group 13 trends in radius, ionisation enthalpy and electronegativity

Concept 2 of 3: Group 13 melting points, boron's lattice and gallium's liquid range

Boron is a non-metal. Its atoms form B₁₂ icosahedra linked into a giant covalent network, so boron is very hard and melts far above the others. The rest are metals. Gallium is the odd one: it melts at 303 K, just above room temperature, yet boils near 2676 K. No other element stays liquid over so wide a range, which is why gallium fills thermometers for high temperatures.

Definition

Boron: black, very hard, icosahedral $\mathrm{B_{12}}$ units in a strong covalent lattice; highest melting and boiling points in the group.
Boron isotopes: $\mathrm{^{10}B}$ about 19% and $\mathrm{^{11}B}$ about 81%; the abundant one has 6 neutrons.
Amorphous boron burns in air to $\mathrm{B_2O_3}$ , with boron at +3.
Melting points: $\mathrm{B > Al > Tl > In > Ga}$ . Gallium's low value comes from its unusual structure of $\mathrm{Ga_2}$ pairs.
Gallium thermometers measure high temperatures. Gallium freezes at 303 K, so it cannot read temperatures below that.
Uses: boron fibres in bullet-proof vests and light composites for aircraft; $\mathrm{^{10}B}$ absorbs neutrons in nuclear control rods; aluminium in alloys, wires and packaging.

Element	Melting point (K)	Boiling point (K)	Density (g/cm³)	What to remember
B	2453	3923	2.35	Giant covalent $\mathrm{B_{12}}$ network: very hard, highest melting point
Al	933	2740	2.70	Light metal; made passive by concentrated $\mathrm{HNO_3}$ , which coats it with oxide
Ga	303	2676	5.90	Liquid from 303 K to 2676 K, the widest liquid range; used in high-temperature thermometers The lowest melting point in the group, and still a liquid in boiling water.
In	430	2353	7.31	Soft metal that melts above gallium
Tl	576	1730	11.85	The densest member of the group

Melting points fall from boron to gallium and then rise a little: B > Al > Tl > In > Ga.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2024 · 27 Jan 2024 · Q43Moderate

Example 2 · The p-Block Elements · Group 13: Periodic Trends and the Inert Pair Effect

Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R). Assertion (A): Melting point of Boron (2453 K) is unusually high in group 13 elements. Reason (R): Solid Boron has very strong crystalline lattice. In the light of the above statements, choose the most appropriate answer from the options given below;

Gallium thermometers are for HIGH temperatures

Gallium is useful in thermometers because it stays liquid up to about 2676 K. It freezes at 303 K, so a gallium thermometer cannot measure a low temperature such as the freezing point of brine.

Boron's hardness is not metallic bonding

Boron is a non-metal. Its high melting point and hardness come from a giant covalent network of

\mathrm{B_{12}}

icosahedra, not from metallic bonds.

Drill 6 more on group 13 melting points, boron's lattice and gallium's liquid range

Concept 3 of 3: The inert pair effect in group 13: Tl⁺ is more stable than Tl³⁺

To reach +3 an atom must use both its s electrons and its p electron. Down the group the s pair is held more tightly, because the d and f electrons below it shield poorly, so the heavy elements prefer to keep it and show +1. The +1 state grows more stable from aluminium to thallium. For thallium, +1 is the stable state, so

\mathrm{Tl^{3+}}

grabs two electrons to become

\mathrm{Tl^{+}}

: it is a strong oxidising agent. For aluminium, +3 is the only stable state, and

\mathrm{Al^{3+}}

is very hard to reduce.

Definition

Stability of +1: $\mathrm{Al < Ga < In < Tl}$ . For Tl, +1 is more stable than +3.
$\mathrm{Tl^{3+}}$ is a powerful oxidising agent; $\mathrm{Al^{3+}}$ is not easily reduced. $\mathrm{Al^{3+}}$ and $\mathrm{Tl^{+}}$ are both stable.
Boron has a very high sum of first three ionisation enthalpies, so it forms only covalent compounds; aluminium forms $\mathrm{Al^{3+}}$ and is strongly electropositive.
$\mathrm{TlI_3}$ is $\mathrm{Tl^{+}[I_3]^{-}}$ , like $\mathrm{CsI_3}$ : thallium is +1, because $\mathrm{Tl^{3+}}$ would oxidise iodide.
$\mathrm{GaAlCl_4}$ is $\mathrm{Ga^{+}[AlCl_4]^{-}}$ : gallium is +1 and is the cation; every Cl is bonded to Al.
Oxides $\mathrm{M_2O_3}$ : $\mathrm{B_2O_3}$ acidic, $\mathrm{Al_2O_3}$ and $\mathrm{Ga_2O_3}$ amphoteric, $\mathrm{In_2O_3}$ and $\mathrm{Tl_2O_3}$ basic.

Element	More stable oxidation state	E° for M³⁺ reduction (V)	How M³⁺ behaves
Al	+3 only	$-1.66$ ( $\mathrm{Al^{3+}/Al}$ )	Very stable; hard to reduce
Ga	+3	$-0.56$ ( $\mathrm{Ga^{3+}/Ga}$ )	Stable; +1 appears only in salts such as $\mathrm{GaAlCl_4}$
In	+3	$-0.34$ ( $\mathrm{In^{3+}/In}$ )	Stable; $\mathrm{In^{+}}$ is easily oxidised back to +3
Tl	+1	$+1.26$ ( $\mathrm{Tl^{3+}}$ reduced to $\mathrm{Tl^{+}}$ )	Strong oxidising agent The positive potential is the inert pair effect in numbers: Tl³⁺ is eager to become Tl⁺.

A more positive reduction potential means the ion is more easily reduced, so a stronger oxidising agent.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2025 · 7 Apr 2025 · Q118Moderate

Example 3 · The p-Block Elements · Group 13: Periodic Trends and the Inert Pair Effect

The correct statements from the following are : (A)

{Tl}^{3 +}

is a powerful oxidising agent (B)

{Al}^{3 +}

does not get reduced easily (C) Both

{Al}^{3 +}

and

{Tl}^{3 +}

are very stable in solution (D)

{Tl}^{+}

is more stable than

{Tl}^{3 +}

(E)

{Al}^{3 +}

and

{Tl}^{+}

are highly stable Choose the correct answer from the options given below :

Not every group 13 element has a stable +1 state

The +1 state becomes important only for the heavier elements and is truly stable only for thallium. Boron and aluminium show +3; a statement that all group 13 elements show a highly stable +1 state is false.

TlI₃ is not thallium(III) iodide

Thallium in

\mathrm{TlI_3}

is +1. The compound is

\mathrm{Tl^{+}}

with the tri-iodide ion

\mathrm{I_3^{-}}

, because

\mathrm{Tl^{3+}}

is a strong enough oxidant to turn iodide into iodine.

In GaAlCl₄, gallium is +1

The salt is

\mathrm{Ga^{+}[AlCl_4]^{-}}

. All four chlorines surround aluminium; gallium is the separate cation. Giving gallium +3 would leave the formula unbalanced.

Drill 8 more on the inert pair effect in group 13: tl⁺ is more stable than tl³⁺

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Reference tables (3)

Group 13 trends in radius, ionisation enthalpy and electronegativity5 rows

Element	Atomic radius (pm)	M³⁺ radius (pm)	First ionisation enthalpy (kJ/mol)	Electronegativity
B	85	27	801	2.0
Al	143	53.5	577	1.5
Ga	135	62.0	579	1.6 Smaller than Al and with a slightly higher ionisation enthalpy: poor shielding by 3d electrons.
In	167	80.0	558	1.7
Tl	170	88.5	589	1.8

NCERT values. Read each column on its own: the atomic radius dips at Ga, the M³⁺ radius does not, and the ionisation enthalpy is lowest at In, not Tl.

Group 13 melting points, boron's lattice and gallium's liquid range5 rows

Element	Melting point (K)	Boiling point (K)	Density (g/cm³)	What to remember
B	2453	3923	2.35	Giant covalent $\mathrm{B_{12}}$ network: very hard, highest melting point
Al	933	2740	2.70	Light metal; made passive by concentrated $\mathrm{HNO_3}$ , which coats it with oxide
Ga	303	2676	5.90	Liquid from 303 K to 2676 K, the widest liquid range; used in high-temperature thermometers The lowest melting point in the group, and still a liquid in boiling water.
In	430	2353	7.31	Soft metal that melts above gallium
Tl	576	1730	11.85	The densest member of the group

Melting points fall from boron to gallium and then rise a little: B > Al > Tl > In > Ga.

The inert pair effect in group 13: Tl⁺ is more stable than Tl³⁺4 rows

Element	More stable oxidation state	E° for M³⁺ reduction (V)	How M³⁺ behaves
Al	+3 only	$-1.66$ ( $\mathrm{Al^{3+}/Al}$ )	Very stable; hard to reduce
Ga	+3	$-0.56$ ( $\mathrm{Ga^{3+}/Ga}$ )	Stable; +1 appears only in salts such as $\mathrm{GaAlCl_4}$
In	+3	$-0.34$ ( $\mathrm{In^{3+}/In}$ )	Stable; $\mathrm{In^{+}}$ is easily oxidised back to +3
Tl	+1	$+1.26$ ( $\mathrm{Tl^{3+}}$ reduced to $\mathrm{Tl^{+}}$ )	Strong oxidising agent The positive potential is the inert pair effect in numbers: Tl³⁺ is eager to become Tl⁺.

A more positive reduction potential means the ion is more easily reduced, so a stronger oxidising agent.

Watch out for (8)

The atomic radius is not a smooth rise
→ Group 13 trends in radius, ionisation enthalpy and electronegativity
Thallium does not have the lowest ionisation enthalpy
→ Group 13 trends in radius, ionisation enthalpy and electronegativity
Electronegativity does not simply fall down group 13
→ Group 13 trends in radius, ionisation enthalpy and electronegativity
Gallium thermometers are for HIGH temperatures
→ Group 13 melting points, boron's lattice and gallium's liquid range
Boron's hardness is not metallic bonding
→ Group 13 melting points, boron's lattice and gallium's liquid range
Not every group 13 element has a stable +1 state
→ The inert pair effect in group 13: Tl⁺ is more stable than Tl³⁺
TlI₃ is not thallium(III) iodide
→ The inert pair effect in group 13: Tl⁺ is more stable than Tl³⁺
In GaAlCl₄, gallium is +1
→ The inert pair effect in group 13: Tl⁺ is more stable than Tl³⁺

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