Chemistry · Structure of atom · chapter 2 of 9

Class 11 Chemistry Chapter 2: Structure of Atom case study question with answers

One case-based question of the kind CBSE-pattern school exams ask: a passage, then 3 parts worth 4 marks in all, with the answer and the marking scheme for each. Written from the NCERT chapter.

Case study · 4 marks
Meera looks at a hydrogen discharge tube through a hand spectroscope in the school laboratory. Instead of a continuous band of colours she sees four sharp lines on a dark background: red at 656 nm, blue-green at 486 nm, blue at 434 nm and violet at 410 nm. Her teacher tells her that each line comes from many hydrogen atoms making the same jump between two energy levels, and that all four jumps end at the same level.
Part 1 · 1 mark
The four lines Meera sees belong to the
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Answer: (c) Balmer series

The Balmer series (ending at n = 2) is the only series of hydrogen lines in the visible region.

Part 2 · 1 mark
The red line at 656 nm arises from the transition
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Answer: (a) n = 3 to n = 2

In a series the longest wavelength comes from the smallest jump, n₂ = n₁ + 1: 3 to 2. 6 to 2 gives the violet line; 2 to 1 is ultraviolet and 4 to 3 infrared.

Part 3 · 2 marks
From the blue-green line (486 nm), calculate the energy gap between the two levels involved, and show that Bohr's expression with n = 4 and n = 2 gives the same value. (h = 6.626 × 10⁻³⁴ J s, c = 3.0 × 10⁸ m s⁻¹, RH = 2.18 × 10⁻¹⁸ J)
Answer and marking scheme

From the line: ΔE = hc/λ = (6.626 × 10⁻³⁴ × 3.0 × 10⁸)/(486 × 10⁻⁹) = 4.09 × 10⁻¹⁹ J. From Bohr: ΔE = 2.18 × 10⁻¹⁸ (1/2² − 1/4²) = 2.18 × 10⁻¹⁸ × 3/16 = 4.09 × 10⁻¹⁹ J. The two agree.

  • ΔE = hc/λ = 4.09 × 10⁻¹⁹ J (1)
  • Bohr ΔE with n = 4 and 2 = 4.09 × 10⁻¹⁹ J (1)
Or
Explain why Meera sees sharp lines and not a continuous spectrum. What kind of spectrum is obtained when white light is passed through hydrogen gas and then through a spectroscope, and how is it related to the emission spectrum?
Answer and marking scheme

The electron's energy in a hydrogen atom is quantised; a jump between two fixed levels releases a photon of one definite energy and so one wavelength, giving a line. When white light passes through hydrogen, the atoms absorb only the wavelengths that match jumps between their levels: an absorption spectrum, a continuous band crossed by dark lines, like a photographic negative of the emission spectrum (the dark lines lie at wavelengths where hydrogen can also emit).

  • quantised levels: each transition gives one wavelength (1)
  • absorption spectrum: dark lines on a continuous background, the negative of the emission spectrum (1)
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