Why is Gallium's atomic radius smaller than Aluminium's, despite being lower in the group?

This is a classic anomaly known as the 'd-block contraction'. As we move from Aluminium to Gallium, the 3d orbitals are filled. The electrons in these 3d orbitals are very poor at shielding the outermost 4s and 4p valence electrons from the increasing nuclear charge. This ineffective shielding leads to a significantly higher effective nuclear charge ($Z_{eff}$) experienced by the valence electrons in Gallium, pulling them closer to the nucleus and resulting in a smaller atomic radius compared to Aluminium.

What is the 'inert pair effect' and how does it affect Group 13 elements?

The 'inert pair effect' describes the reluctance of the $ns^2$ valence electrons in heavier p-block elements to participate in chemical bonding. For Group 13, this means that for elements like Indium and especially Thallium, the +1 oxidation state (where only the $np^1$ electron is lost) becomes more stable than the +3 oxidation state (where both $ns^2$ and $np^1$ electrons are lost). This occurs because the $ns^2$ electrons become more tightly bound due to the poor shielding by intervening d and f electrons, making their removal energetically unfavorable.

Why do the ionization enthalpies of Group 13 elements show an irregular trend?

The irregular trend (B > Al In < Tl) is primarily due to the poor shielding effect of inner d and f electrons. For Gallium, the filled 3d orbitals poorly shield the valence electrons, increasing the effective nuclear charge and thus making it harder to remove an electron than from Aluminium. Similarly, for Thallium, the combined poor shielding from 4f and 5d electrons leads to an even higher effective nuclear charge, making its ionization enthalpy greater than Indium's.

Is Boron a metal or a non-metal? How does its nature compare to other Group 13 elements?

Boron is distinctly a non-metal, exhibiting properties like high melting point, hardness, and forming predominantly covalent compounds. In contrast, Aluminium is considered a metal (though sometimes classified as a metalloid), and Gallium, Indium, and Thallium are all soft metals. This trend illustrates the increase in metallic character as one descends Group 13, with Boron being an outlier due to its small size and high ionization enthalpy, favoring covalent bonding.

What are the common oxidation states for Group 13 elements, and how do they vary down the group?

The most common oxidation state for Group 13 elements is +3, arising from the loss of all three valence electrons ($ns^2 np^1$). Boron and Aluminium predominantly show this +3 state. However, for heavier elements like Gallium, Indium, and particularly Thallium, the +1 oxidation state becomes increasingly stable due to the 'inert pair effect'. For Thallium, the +1 state is more stable than the +3 state, while for Ga and In, +3 is generally more stable, but +1 is also observed.

← ALL TOPICS

Chemistry

NEXT TOPIC →

Boron and its Compounds

Electronic Configuration and General Properties

Q: Is Boron a metal or a non-metal? How does its nature compare to other Group 13 elements?

Boron is distinctly a non-metal, exhibiting properties like high melting point, hardness, and forming predominantly covalent compounds. In contrast, Aluminium is considered a metal (though sometimes classified as a metalloid), and Gallium, Indium, and Thallium are all soft metals. This trend illustrates the increase in metallic character as one descends Group 13, with Boron being an outlier due to its small size and high ionization enthalpy, favoring covalent bonding.

Q: What are the common oxidation states for Group 13 elements, and how do they vary down the group?

The most common oxidation state for Group 13 elements is +3, arising from the loss of all three valence electrons ($ns^2 np^1$). Boron and Aluminium predominantly show this +3 state. However, for heavier elements like Gallium, Indium, and particularly Thallium, the +1 oxidation state becomes increasingly stable due to the 'inert pair effect'. For Thallium, the +1 state is more stable than the +3 state, while for Ga and In, +3 is generally more stable, but +1 is also observed.

Chemistry

NEET UG

Version 1Updated 22 Mar 2026

Explore This Topic

Definition Deep Explanation Core Principles NEET Importance Problem Strategy Practice Problems MCQ Practice Quick Revision

The electronic configuration of an element dictates its chemical behavior and physical properties, particularly for main group elements like those in Group 13. These elements, often referred to as the Boron family, share a common valence shell electronic configuration of $ns^2 np^1$ . This configuration implies the presence of three valence electrons, which are primarily responsible for their chara…

Quick Summary

Group 13 elements, comprising Boron (B), Aluminium (Al), Gallium (Ga), Indium (In), and Thallium (Tl), share a common valence shell electronic configuration of $ns^2 np^1$ . This configuration typically leads to a +3 oxidation state.

However, the presence of filled d-orbitals in Ga and In, and filled d and f-orbitals in Tl, significantly alters their properties due to poor shielding effects. This results in anomalies in expected periodic trends.

For instance, Gallium has a smaller atomic radius than Aluminium (d-block contraction), and its ionization enthalpy is higher than Aluminium's. Similarly, Thallium exhibits an even higher ionization enthalpy than Indium.

The 'inert pair effect' becomes prominent for heavier elements, making the +1 oxidation state increasingly stable, especially for Thallium, where it is more stable than the +3 state. Metallic character increases down the group, with Boron being a non-metal and the rest being metals.

Understanding these electronic configurations and the resulting deviations from ideal periodic trends is crucial for comprehending the chemistry of Group 13 elements.

Vyyuha

Your 6-Month Blueprint, Updated Nightly

AI analyses your progress every night. Wake up to a smarter plan. Every. Single.…

Key Concepts

Inert Pair Effect

The inert pair effect is a significant phenomenon observed in heavier elements of the p-block, including…

d-block Contraction

The d-block contraction is a specific type of atomic radius contraction observed for elements immediately…

Ionization Enthalpy Anomalies

Ionization enthalpy (IE) is the energy required to remove an electron. Generally, IE decreases down a group…

Valence Configuration: —

ns^2 np^1

Elements: — B, Al, Ga, In, Tl

Atomic Radii Trend: — B < Al > Ga < In < Tl (Ga smaller than Al due to d-block contraction; Tl slightly larger than In due to lanthanide contraction offsetting size increase)

Ionization Enthalpy Trend: — B > Al < Ga > In < Tl (Ga > Al, Tl > In due to poor shielding by d/f electrons, increasing

Z_{eff}

)

Electronegativity Trend: — B > Al < Ga > In < Tl (Irregular, similar reasons as IE)

Oxidation States: — +3 (common for B, Al, Ga, In); +1 (increasingly stable for Ga, In, Tl; most stable for Tl due to inert pair effect)

Metallic Character: — B (non-metal)

\to

Al, Ga, In, Tl (metals); increases down the group.

Key Effects: — d-block contraction (Ga), Lanthanide contraction (Tl), Inert Pair Effect (In, Tl).

To remember the tricky Ionization Enthalpy trend (B > Al < Ga > In < Tl), think: Boys Always Get Into Trouble. The 'trouble' refers to the irregular jumps in IE, where Ga is higher than Al, and Tl is higher than In.

← ALL TOPICS

Chemistry

NEXT TOPIC →

Boron and its Compounds