UPSC Combined Geo-Scientist, Mains · 2025 · Mains Paper I
Q1(j). First ionisation enthalpy of 5d elements
Question
Why is the first ionization enthalpy of 5d series elements higher than that of 3d and 4d series elements?
Answer in brief
Lanthanide contraction: 14 poorly shielding 4f electrons precede the 5d series, so Zeff rises sharply while size stays equal to 4d; relativistic contraction of 6s adds to it.
Key points
- From 4d to 5d, Z rises by 32 (14 of them across 4f).
- 4f orbitals are diffuse, so they shield poorly; Zeff on the 6s electron rises.
- Size gained by one more shell is cancelled: Hf ≈ Zr, Ta ≈ Nb. Same radius, larger Zeff → higher IE₁.
- Relativistic effect: fast 6s electrons contract and are held tighter (Au, Hg most).
The full model answer
As written for the answer booklet: the diagram carries the working, the lines stay short. Read here only; it is not offered as a download.
Coversfirst ionisation enthalpy5d elementslanthanide contractionrelativistic effecteffective nuclear charge
All parts of Question 1
- 1(a) Defect in silver chloride5 marks
- 1(b) Isomerism in [(NH₃)₅CoONO]Cl₂ and [(NH₃)₅CoNO₂]Cl₂5 marks
- 1(c) Trans effect: Niᴵᴵ, Ptᴵᴵ, Pdᴵᴵ5 marks
- 1(d) Phenolphthalein in dilute and in concentrated alkali5 marks
- 1(e) Solubility product of silver chromate5 marks
- 1(f) Colour change interval of diphenylamine5 marks
- 1(g) Eriochrome Black T and calcium5 marks
- 1(h) Tendency to form metal clusters5 marks
- 1(i) Complete the nuclear equations5 marks
- 1(j) First ionisation enthalpy of 5d elements5 marks