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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 · 2021 · Mains Paper I

Q1(i). Balancing nuclear reactions

Question

Complete the following nuclear reactions by giving the value of X: (i) ¹⁴₇N + ⁴₂He → X + ¹₁H; (ii) X + ⁴₂He → ³⁰₁₅P + ¹₀n; (iii) ²³₁₁Na + X → ²³₁₂Mg + ¹₀n; (iv) ¹¹³₄₈Cd + X → ¹¹⁴₄₈Cd + energy; (v) X → ⁰₋₁e + ²³²₉₁Pa.

Answer in brief

X = (i) ¹⁷₈O; (ii) ²⁷₁₃Al; (iii) ¹₁H; (iv) ¹₀n; (v) ²³²₉₀Th.

Key points

  • Conserve mass number A and atomic number Z independently on the two sides.
  • (i) A = 14 + 4 − 1 = 17 and Z = 7 + 2 − 1 = 8: oxygen-17. (ii) A = 30 + 1 − 4 = 27 and Z = 15 − 2 = 13: aluminium-27.
  • (iii) A = 1, Z = 1: a proton. (iv) A = 1, Z = 0: neutron capture, normally followed by γ emission.
  • (v) A = 232 and Z = 91 − 1 = 90: thorium-232. This is the formal balance; ordinary ²³²Th decay is α decay.

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(i), 2021 Mains Paper I: solution pages 9 and 10
Solution pages 9 and 10 of 29
Read the whole 2021 solution2021 question paper

Topic notes: nuclear chemistry: radioactive decay, binding energy, radiocarbon dating and the GM counter, with every Civil Services, IFoS and Geo-Scientist PYQ →

Coversnuclear reactionsbalancing nuclear equationsmass numberatomic numberneutron captureartificial transmutation

All parts of Question 1