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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.

In one line: Poor shielding makes the nucleus feel stronger, so size contracts.

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Understand it

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.

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.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

Explain the size trend from Ti to Cu in the 3d series.

  1. 1Nuclear charge increases across the series, and 3d-electrons shield poorly, so effective nuclear charge increases.
  2. 2Hence atomic radii tend to decrease, though the change is not perfectly regular.

Method you can reuse

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

Visual tip

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

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

Why are Zr and Hf chemically similar?

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

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.

Don't mix this up

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.

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.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

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.
  1. 1

    Enough for 1 mark

    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

    Add for 2 marks

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

  3. 3

    Add for 3 marks

    In the 3d series, electrons are added to the inner 3d subshell while nuclear charge increases.

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