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UPSC Combined Geo-Scientist

Chemist

  • 3 descriptive papers
  • 200 marks each
  • 3 h per paper

Mains Paper I covers inorganic chemistry.

UPSC Combined Geo-Scientist, Mains · 2023 · Mains Paper I

Q1(c). Effective atomic numbers

Question

Calculate the effective atomic number of the following complex ions: (i) [Co(NH₃)₆]³⁺ (ii) [Pt(NH₃)₆]⁴⁺

Answer in brief

EAN([Co(NH₃)₆]³⁺) = 36; EAN([Pt(NH₃)₆]⁴⁺) = 86.

Key points

  • Use EAN = Z − oxidation state + ligand-donated electrons. Each NH₃ is a neutral two-electron donor.
  • For cobalt, Z = 27 and the oxidation state is +3: EAN = 27 − 3 + 6 × 2 = 36, the krypton count.
  • For platinum, Z = 78 and the oxidation state is +4: EAN = 78 − 4 + 6 × 2 = 86, the radon count.
  • Both centres are d⁶ and have 18 valence electrons after adding the six donor pairs; EAN includes core electrons, the 18-electron count does not.

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.

Model answer to Q1(c), 2023 Mains Paper I: solution pages 3 and 4
Solution pages 3 and 4 of 26
Read the whole 2023 solution2023 question paper

Topic notes: metal carbonyls, nitrosyls, the 18-electron and EAN rules, metal–metal bonds and clusters, with every Civil Services, IFoS and Geo-Scientist PYQ →

Coverseffective atomic numberEAN ruleSidgwick EAN18-electron rule[Co(NH3)6]3+

All parts of Question 1