The GATE Grind

GATE 2023 CY – Question 30

Physical Chemistry · Spectroscopy: Atomic spectroscopy; Russell-Saunders coupling; Term symbols and spectral details; origin of selection rules. Rotational, Vibrational, Electronic and Raman spectroscopy of diatomic and simple polyatomic molecules. Line broadening and line widths; simple properties of Gaussian and Lorentzian line shapes. Molecular spectroscopy: Absorbance, Beer- Lambert’s law, Einstein’s coefficient, Jablonski diagram. Relationship of transition moment integral with molar extinction coefficient and oscillator strength. Basic principles of Nuclear Magnetic Resonance: Gyromagnetic ratio; Chemical shift, nuclear coupling. · 1 mark · Multiple choice

Assertion (S): The total angular momentum for light atoms (low atomic number) is obtained by Russell-Saunders coupling, whereas jj-coupling is used for heavy atoms (high atomic number). Reasoning (R): The spin-orbit interactions are weak in light atoms (low atomic number) and strong in heavy atoms (high atomic number). The correct option is

  1. S and R are true; and R is the correct reason for S
  2. S and R are true; but R is NOT the correct reason for S
  3. S is true but R is false
  4. S is false but R is true

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Show answer and explanation

Correct answer: (A) S and R are true; and R is the correct reason for S

Explanation

Weak spin–orbit coupling allows individual orbital momenta to combine into L and spins into S before coupling J. In heavy atoms, strong spin–orbit interaction couples each li and si into ji first. Both assertion and reason are true, and the reason explains the different coupling limits. **Answer: A**.