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UPSC Civil Services (Mains)

Chemistry optional

  • 2 papers
  • 250 marks each
  • 3 h per paper
  • 5 of 8 questions, Q1 and Q5 compulsory

Paper I covers physical and inorganic chemistry.

UPSC Civil Services (Mains), Chemistry optional · 2023 · Paper I

Q6(e). Framework electrons and structures of three borane anions

Question

Predict the number of framework bonding electrons and the structure of the following boranes: (i) B₇H₇²⁻; (ii) B₁₁H₁₃²⁻; (iii) B₂H₇⁻.

Answer in brief

(i) 16 electrons (8 pairs), closo, pentagonal bipyramid; (ii) 26 electrons (13 pairs), nido, icosahedron with one vertex missing; (iii) 10 electrons (5 pairs), arachno, [H₃B–H–BH₃]⁻.

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Topic notes: main group chemistry: boranes, borazine, phosphazenes, silicates, silicones, interhalogens, xenon and sulfur–nitrogen compounds, with every Civil Services, IFoS and Geo-Scientist PYQ →

CoversWade's rulesboranescloso nido arachnopolyhedral skeletal electron pairthree-centre two-electron bond

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