2.1.4 Atomic and Ionic Radius

This topic explains the concept of atomic and ionic radius, how they are defined, and the periodic trends they follow.

Definition

Atomic Radius: The average distance from the nucleus to the outermost electrons of an atom.

Ionic Radius: The radius of an ion (a charged atom or group of atoms).

Main Idea

• Across a period (left to right): Radius decreases.
• Down a group (top to bottom): Radius increases.
• Cations are always smaller than their parent atoms.
• Anions are always larger than their parent atoms.

Core Concept

• Nuclear charge — more protons pull electrons closer.
• Shielding effect — inner shells reduce the pull on outer electrons.
• Electron-electron repulsion — more electrons push each other apart.

Detailed Explanation

Atomic Radius

• The average distance of the nucleus from the outermost electrons.
• It is the measure of the size of an atom.
• Since the boundary of the electronic cloud is not very well defined, the term average is used.

Periodic Trends

• Atomic and ionic radius decrease.
• Reason: Nuclear charge increases; shielding effect remains constant.
• Result: Electrons are pulled closer to the nucleus.

• Atomic radius increases universally in the whole periodic table.
• Reason: Increasing number of shells, one in each period.
• The shielding effect increases due to intervening electrons.

• In the transition metal series, the radius increases slightly across the period.
• Reason: Electrons are configured in the inner shell d-orbitals.

Ionic Radius

• Always smaller than their parent atoms.
• Reason: Increase in effective nuclear charge and decrease in electron-electron repulsion in the valence shell due to removal of electrons.

• Always larger than their parent atoms.
• Reason: Increase in electron-electron repulsion due to added electrons.

Trends for Ionic Radius

• The trend of ionic radius is the same as that of atomic radius.
• It goes on increasing.
• Reason: Same charge on the ions throughout the group.

• This is debatable in the periodic table from left to right.
• Reason: In a period, each element does not usually form ions with the same charge.
• The charge on the ion depends on the valence shell electronic configuration, which changes from left to right.
• If we consider the same charge on each ion from left to right, the trend is the same as that of atomic radii — it decreases.

Why Atomic and Ionic Radii Matter

• Ionization energies
• Electronegativities
• Bond energies
• Bond lengths
• Oxidizing or reducing powers

Intermediate Level

Atomic Radius

• Average distance from nucleus to outermost electrons.
• Across a period: Decreases (nuclear charge increases, shielding constant).
• Down a group: Increases (more shells, shielding increases).

Ionic Radius

• Cations: Smaller than parent atoms (loss of electrons, increased effective nuclear charge, decreased repulsion).
• Anions: Larger than parent atoms (gain of electrons, increased repulsion).

Trends Summary

Importance

- Affects ionization energy, electronegativity, bond energies, bond lengths, and oxidizing/reducing powers.

Important Points

• 1. Atomic radius = average distance from nucleus to outermost electrons.
• 2. Across a period: Radius decreases.
• 3. Down a group: Radius increases.
• 4. Cations are smaller than parent atoms.
• 5. Anions are larger than parent atoms.
• 6. Ionic radius trends are similar to atomic radius trends.
• 7. Transition metals show a slight increase across the series.
• 8. Radius affects ionization energy, electronegativity, bond energies, and bond lengths.

Remember

• - Across a period: Radius decreases
• - Down a group: Radius increases
• - Cation: Smaller than parent atom
• - Anion: Larger than parent atom

Common Mistakes

• 1. Thinking atomic radius increases across a period: It decreases.
• 2. Thinking cations are larger than parent atoms: Cations are smaller.
• 3. Forgetting the exception in transition metals: Radius increases slightly across the series.
• 4. Confusing atomic and ionic radius trends: They follow similar patterns but differ for cations and anions.

Quick Revision

• Atomic radius = average distance from nucleus to outermost electrons.
• Across a period: Radius decreases.
• Down a group: Radius increases.
• Cations are smaller than parent atoms.
• Anions are larger than parent atoms.
• Ionic radius trends are similar to atomic radius trends.

60-Second Revision

• Across a period: Radius decreases
• Down a group: Radius increases
• Cation: Smaller
• Anion: Larger
• Transition metals: Slight increase across series

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