C
CraftExam
high importancemedium8 min

Trends in Atomic and Ionic Radii

Across a transition series, atomic and ionic radii generally decrease at first due to increasing nuclear charge, but the decrease becomes small because added d-electrons shield each other poorly. In f-block elements, poor shielding by f-electrons causes lanthanoid contraction.

Practice This Concept

Learn the concept

Student-friendly explanation

In the 3d series, electrons are added to the inner 3d subshell while nuclear charge increases. The shielding by d-electrons is not very effective, so effective nuclear charge increases and size decreases. In lanthanoids, 4f-electrons shield very poorly, causing a gradual decrease in atomic and ionic radii from La to Lu, known as lanthanoid contraction.

How to write this in exams

  1. 1

    Start with the exact idea

    Across a transition series, atomic and ionic radii generally decrease at first due to increasing nuclear charge, but the decrease becomes small because added d-electrons shield each other poorly. In f-block elements, poor shielding by f-electrons causes lanthanoid contraction.

  2. 2

    Then show how to use it

    Identify the series, note where the added electron enters, compare shielding ability, then state the effect on effective nuclear charge and radius.

  3. 3

    Add one concrete example

    Zr and Hf have very similar radii because the lanthanoid contraction before Hf offsets the expected increase in size down the group.

  4. 4

    Avoid this incomplete answer

    A common wrong answer is saying lanthanoid contraction occurs because electrons are removed. It actually occurs across the series as nuclear charge increases and 4f shielding remains poor.

Definition

Across a transition series, atomic and ionic radii generally decrease at first due to increasing nuclear charge, but the decrease becomes small because added d-electrons shield each other poorly. In f-block elements, poor shielding by f-electrons causes lanthanoid contraction.

Example

Zr and Hf have very similar radii because the lanthanoid contraction before Hf offsets the expected increase in size down the group.

Rule to remember

Trend rule: across a transition series, effective nuclear charge generally increases because d-electrons shield poorly. Lanthanoid contraction: steady decrease in lanthanoid radii due to poor shielding by 4f-electrons.

Memory hook

Poor shielding makes the nucleus feel stronger, so size contracts.

Examples and method

Worked example

Explain the size trend from Ti to Cu in the 3d series. Nuclear charge increases across the series, and 3d-electrons shield poorly, so effective nuclear charge increases. Hence atomic radii tend to decrease, though the change is not perfectly regular.

Method to apply

Identify the series, note where the added electron enters, compare shielding ability, then state the effect on effective nuclear charge and radius.

Diagram support

A line trend showing decreasing ionic radii across lanthanoids can support the idea of contraction, but the core answer is reason-based.

How CBSE asks it

It is commonly asked as a reason for lanthanoid contraction, similarity of Zr and Hf, separation difficulty of lanthanoids, or small change in radii across transition series.

Avoid common mistakes

Common confusion

Students often say size always decreases uniformly across the d-block. The trend is gradual and irregularities occur due to electron-electron repulsions and electronic configurations.

Common wrong answer

A common wrong answer is saying lanthanoid contraction occurs because electrons are removed. It actually occurs across the series as nuclear charge increases and 4f shielding remains poor.

Exam tip

For trend answers, mention both forces: increasing nuclear charge pulls electrons inward, while poor shielding by d or f electrons explains why contraction occurs and why it affects later elements.

Quick check

Why are Zr and Hf chemically similar?

Due to lanthanoid contraction, Hf does not become much larger than Zr. Their similar radii lead to similar chemical properties.

Answer writing and exam use

1-mark answer

Across a transition series, atomic and ionic radii generally decrease at first due to increasing nuclear charge, but the decrease becomes small because added d-electrons shield each other poorly. In f-block elements, poor shielding by f-electrons causes lanthanoid contraction.

2-mark answer

Across a transition series, atomic and ionic radii generally decrease at first due to increasing nuclear charge, but the decrease becomes small because added d-electrons shield each other poorly. In f-block elements, poor shielding by f-electrons causes lanthanoid contraction. Trend rule: across a transition series, effective nuclear charge generally increases because d-electrons shield poorly. Lanthanoid contraction: steady decrease in lanthanoid radii due to poor shielding by 4f-electrons. Zr and Hf have very similar radii because the lanthanoid contraction before Hf offsets the expected increase in size down the group.

3-mark answer

In the 3d series, electrons are added to the inner 3d subshell while nuclear charge increases. The shielding by d-electrons is not very effective, so effective nuclear charge increases and size decreases. In lanthanoids, 4f-electrons shield very poorly, causing a gradual decrease in atomic and ionic radii from La to Lu, known as lanthanoid contraction. Trend rule: across a transition series, effective nuclear charge generally increases because d-electrons shield poorly. Lanthanoid contraction: steady decrease in lanthanoid radii due to poor shielding by 4f-electrons. Explain the size trend from Ti to Cu in the 3d series. Nuclear charge increases across the series, and 3d-electrons shield poorly, so effective nuclear charge increases. Hence atomic radii tend to decrease, though the change is not perfectly regular. It is commonly asked as a reason for lanthanoid contraction, similarity of Zr and Hf, separation difficulty of lanthanoids, or small change in radii across transition series. A common wrong answer is saying lanthanoid contraction occurs because electrons are removed. It actually occurs across the series as nuclear charge increases and 4f shielding remains poor.
MCQ Quiz

Practice this concept with focused MCQs

Open the concept quiz intro first, review the test details, and then start a focused MCQ set from this concept only. Instant score and answer review are live now.

10 MCQs5 MinutesInstant Results
Practice This Concept

Help improve this page

Found something confusing, incorrect, or missing?

Practice This Concept