what is lanthanoid contraction class 12

Lanthanoid contraction is the gradual decrease in the atomic and ionic radii of lanthanoids (elements from cerium, Ce, to lutetium, Lu) as their atomic number increases.

Simple Class 12 definition

For Class 12 exams, you can write:

The steady decrease in the atomic and ionic radii of lanthanoid elements (from La to Lu) with an increase in atomic number, mainly due to poor shielding of 4f electrons, is called lanthanoid contraction.

Why does lanthanoid contraction happen?

• In the lanthanoid series, electrons are added to the inner 4f subshell.

• 4f electrons poorly shield one another from the increasing nuclear charge because f‑orbitals are very diffuse.

• As atomic number increases, nuclear charge increases, but shielding does not increase effectively, so effective nuclear charge on outer electrons rises.

• This stronger pull of the nucleus reduces atomic and ionic size step by step across the series.

A typical numerical example you may quote in answers:
On moving from Ce to Lu, atomic radii decrease only slightly (about 183 pm to 173 pm), showing a small but consistent contraction.

Key consequences (Class 12 points)

You are often asked: “What are the consequences of lanthanoid contraction?” You can list:

1. Similarity of 2nd and 3rd transition series
   • Because of lanthanoid contraction, elements like Zr–Hf, Nb–Ta, Mo–W end up with almost the same radii and very similar properties.

 * This is why second and third transition series resemble each other more than first and second series.

2. Irregularities in ionisation enthalpies
   • Due to small and irregular changes in size along the lanthanoids, trends in ionisation enthalpy are not smooth.

3. Separation of lanthanoids becomes difficult
   • Because their ionic radii and chemical properties are very similar, lanthanoids are hard to separate and require special methods (e.g., ion-exchange).

4. Change in basicity of lanthanoid hydroxides
   • As size of M³⁺ ions decreases from La³⁺ to Lu³⁺, covalent character in M–OH bonds increases.

 * So basic strength of their hydroxides decreases from La(OH)₃ (more basic) to Lu(OH)₃ (less basic).

Quick exam-ready answer (3–4 lines)

If the question is: “What is lanthanoid contraction? Write two consequences.” you can write:

• Lanthanoid contraction is the gradual decrease in atomic and ionic radii of lanthanoids (La to Lu) with increase in atomic number due to poor shielding effect of 4f electrons.

• Consequences:
  1. Elements of the 2nd and 3rd transition series (e.g., Zr–Hf, Nb–Ta) have almost identical sizes and very similar properties.

2. Separation of lanthanoids is difficult because their ionic radii and properties are very similar.

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