Atoms

9 NEET-UG practice questions on Atoms, part of Physics. The first 9 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

1. Question 1 · Easy

   In Rutherford's experiment, most α-particles passed through the gold foil with little or no deflection. What did this show?

   1. AMost of the atom is empty space
   2. BElectrons are located in the nucleus
   3. CThe atom's mass is spread uniformly
   4. DPositive charge is spread through the whole atom

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   Answer: A · Most of the atom is empty space

   Very few α-particles were scattered through large angles, so the positive charge and most of the mass occupy a tiny nucleus. Most α-particles pass through because the atom is mostly empty space.

   • B: The experiment gives no such result, and electrons lie outside the nucleus.
   • C: Most of the mass is concentrated in the nucleus.
   • D: A spread-out positive charge could not give large-angle scattering.

   Remember this

   Little deflection shows empty space; rare large-angle scattering shows a small, massive, positive nucleus.

2. Question 2 · Medium

   Taking the energy of the nth level of hydrogen as -13.6/n² eV, what energy does a hydrogen atom emit in the transition from n = 4 to n = 2?

   1. A3.4 eV
   2. B0.85 eV
   3. C2.55 eV
   4. D12.75 eV

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   Answer: C · 2.55 eV

   E₄ = -13.6/16 = -0.85 eV and E₂ = -13.6/4 = -3.4 eV. The energy emitted is -0.85 - (-3.4) = 2.55 eV.

   • A: This is only the magnitude of the energy of n = 2.
   • B: This is only the magnitude of the energy of n = 4.
   • D: This is the transition n = 4 to n = 1.

   Remember this

   Emitted photon energy is the difference between the levels, 13.6(1/n_f² - 1/n_i²) eV for hydrogen.

3. Question 3 · Medium

   Match List-I with List-II. Take 1/R = 91 nm, where R is the Rydberg constant. List-I (series) A. Lyman B. Balmer C. Paschen D. Brackett List-II (series limit) I. 91 nm II. 1456 nm III. 364 nm IV. 819 nm

   1. AA-I, B-III, C-II, D-IV
   2. BA-I, B-III, C-IV, D-II
   3. CA-III, B-I, C-IV, D-II
   4. DA-I, B-IV, C-III, D-II

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   Answer: B · A-I, B-III, C-IV, D-II

   At the series limit, n_i tends to infinity, so 1/λ = R/n_f², which gives λ = 91 n_f² nm. With n_f = 1, 2, 3, 4 this gives 91, 364, 819 and 1456 nm.

   • A: Paschen and Brackett limits have been interchanged.
   • C: Lyman and Balmer limits have been interchanged.
   • D: Balmer and Paschen limits have been interchanged.

   Remember this

   Series limit wavelength is n_f²/R, with n_f = 1 for Lyman, 2 for Balmer, 3 for Paschen, 4 for Brackett.

4. Question 4 · Hard

   A sample of ground-state hydrogen absorbs photons of energy 12.75 eV. With E_n = -13.6/n² eV, how many spectral lines can the sample then emit, and what is the photon energy of the longest-wavelength line?

   1. A3 lines; 0.66 eV
   2. B6 lines; 12.75 eV
   3. C6 lines; 0.66 eV
   4. D6 lines; 0.85 eV

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   Answer: C · 6 lines; 0.66 eV

   The final level has energy -13.6 + 12.75 = -0.85 eV, which is n = 4. A large sample can emit n(n - 1)/2 = 6 lines. The longest wavelength has the least energy, from n = 4 to 3: 13.6(1/9 - 1/16) = 13.6 × 7/144, about 0.66 eV.

   • A: A single atom can emit at most 3 lines, but a sample gives all 6.
   • B: 12.75 eV is the shortest-wavelength line, not the longest.
   • D: 0.85 eV is the ionisation energy from n = 4, not a line.

   Remember this

   A sample excited to level n gives n(n - 1)/2 lines; the longest wavelength comes from the two closest levels.

5. Question 5 · Medium

   In the Bohr model of hydrogen, what is the ratio of the time period of the electron's revolution in the orbit n = 3 to that in the orbit n = 1?

   1. A9
   2. B3
   3. C27
   4. D81

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   Answer: C · 27

   The radius grows as n² and the speed falls as 1/n. So T = 2πr/v varies as n³, and the ratio is 3³ = 27.

   • A: This is the ratio of the radii alone.
   • B: This is the inverse ratio of the speeds alone.
   • D: r was squared by mistake, giving n⁴.

   Remember this

   In Bohr's model r varies as n², v as 1/n, T as n³, and |E| as 1/n².

6. Question 6 · Medium

   An alpha particle heads straight at a fixed tin nucleus, as drawn, and turns back at distance d. What is d? Take e²/4πε₀ = 1.44 MeV fm.

   

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   1. A8 fm
   2. B32 fm
   3. C144 fm
   4. D16 fm

   Answer and explanation in LearnFree account

7. Question 7 · Medium

   Levels of a hypothetical atom are drawn here with four downward transitions. Which transition emits a photon of wavelength 400 nm? Take hc = 1240 eV nm.

   

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   1. AP
   2. BR
   3. CQ
   4. DS

   Answer and explanation in LearnFree account

8. Question 8 · Easy

   Match List-I with List-II. List-I (arrow on the hydrogen level diagram, adjacent levels) A. Arrow A B. Arrow B C. Arrow C D. Arrow D List-II (photon energy) I. 10.2 eV II. 1.89 eV III. 0.66 eV IV. 0.31 eV

   

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   1. AA-II, B-III, C-I, D-IV
   2. BA-II, B-IV, C-I, D-III
   3. CA-I, B-IV, C-II, D-III
   4. DA-III, B-IV, C-I, D-II

   Answer and explanation in LearnFree account

9. Question 9 · Hard

   The arrow marks a transition in He⁺. Which transition in the hydrogen atom emits a photon of exactly the same energy? Take Eₙ = −13.6 Z²/n² eV.

   

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   1. An = 2 to n = 1
   2. Bn = 4 to n = 1
   3. Cn = 4 to n = 2
   4. Dn = 3 to n = 1

   Answer and explanation in LearnFree account