UPSC Civil Services (Mains), Chemistry optional · 2026 · Paper I

Q5(j). Actinide contraction

Question

What is actinide contraction? Explain.

The steady decrease in the size of the M³⁺ (and M⁴⁺) ions across the actinide series as Z increases: An³⁺ falls from 104 pm (Pa) to 95 pm (Cf), CN 6.

Along the M³⁺ ions each step adds one proton and one 5f electron. The diffuse 5f orbitals shield poorly, cancelling much less than the added nuclear charge.
The effective nuclear charge on the outer electrons therefore rises from element to element and pulls the atom or ion in; relativistic effects add to it.
An³⁺ (CN 6) Pa 104, U 103, Np 101, Pu 100, Am 98, Cm 97, Bk 96, Cf 95 pm. An³⁺ ions are about 5 pm larger than their Ln³⁺ congeners (U³⁺ 103, Nd³⁺ 98 pm), and the fall Pa→Cf is slightly steeper than Pr→Dy (99 to 91 pm).
basicity falls and complex formation rises across the series; neighbouring heavy actinides are alike and are separated by ion exchange.

The full model answer

Written out in full, as it should go in the answer booklet. The solution page with its diagrams follows it.

Along the M³⁺ ions each step adds one proton and one 5f electron. The diffuse 5f orbitals shield poorly, cancelling much less than the added nuclear charge.
The effective nuclear charge on the outer electrons therefore rises from element to element and pulls the atom or ion in; relativistic effects add to it.
The fall is steady: An³⁺ (CN 6) Pa 104, U 103, Np 101, Pu 100, Am 98, Cm 97, Bk 96, Cf 95 pm. An³⁺ ions are about 5 pm larger than their Ln³⁺ congeners (U³⁺ 103, Nd³⁺ 98 pm), and the fall Pa→Cf is slightly steeper than Pr→Dy (99 to 91 pm).
Consequences: basicity falls and complex formation rises across the series; neighbouring heavy actinides are alike and are separated by ion exchange.

Practising this answer? Write it in a Mains test and get it checked by VKC. Civil Services Mains test series · Telegram