Atoms

28 NEET-UG practice questions on Atoms, part of Physics. The first 25 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

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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

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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

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10. Question 10 · Medium

    In the ground state of hydrogen, the electron moves at about 2.2 × 10⁶ m/s. At about what speed does it move in the n = 4 orbit?

    1. A8.8 × 10⁶ m/s
    2. B1.4 × 10⁵ m/s
    3. C1.1 × 10⁶ m/s
    4. D5.5 × 10⁵ m/s

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11. Question 11 · Hard

    The total energy of the electron in a hydrogen atom in a certain state is -1.51 eV. Taking E_n = -13.6/n² eV, what are the principal quantum number of this state and the potential energy of the electron in it?

    1. An = 3 and +1.51 eV
    2. Bn = 3 and -3.02 eV
    3. Cn = 3 and -1.51 eV
    4. Dn = 9 and -3.02 eV

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12. Question 12 · Easy

    Which quantity is restricted to whole-number multiples of h/2π in Bohr's model of the hydrogen atom?

    1. AThe kinetic energy of the electron
    2. BThe linear momentum of the electron
    3. CThe orbital radius of the electron
    4. DThe orbital angular momentum of the electron

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13. Question 13 · Medium

    In the hydrogen spectrum, what is the ratio of the longest wavelength of the Lyman series to the longest wavelength of the Balmer series?

    1. A5/27
    2. B4/9
    3. C27/5
    4. D3/4

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14. Question 14 · Medium

    Taking E_n = -13.6/n² eV for hydrogen and hc = 1240 eV nm, what is the shortest wavelength in the Balmer series?

    1. AAbout 656 nm
    2. BAbout 365 nm
    3. CAbout 122 nm
    4. DAbout 91 nm

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15. Question 15 · Medium

    Consider these statements about the Bohr model of the hydrogen atom: A. The electron's angular momentum in an allowed orbit is a whole-number multiple of h/2π. B. The electron radiates energy continuously while it moves in an allowed orbit. C. When the electron jumps from a higher level to a lower one, it emits a photon of frequency (E_i - E_f)/h. D. The speed of the electron is greater in orbits of larger n. Which of these are correct?

    1. AC and D only
    2. BA and C only
    3. CA, C and D only
    4. DA, B and C only

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16. Question 16 · Hard

    An electron in an excited hydrogen atom drops straight to the ground state and emits a photon of energy 13.056 eV. Taking E_n = -13.6/n² eV, how did the radius and the speed of the electron in its initial orbit compare with their ground-state values?

    1. ARadius 25 times, speed one-fifth
    2. BRadius 5 times, speed one-fifth
    3. CRadius 25 times, speed one twenty-fifth
    4. DRadius 5 times, speed one twenty-fifth

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17. Question 17 · Medium

    A hydrogen atom makes a transition from n = 3 to n = 1. Taking E_n = -13.6/n² eV and hc = 1240 eV nm, to which series does the emitted line belong, and what is its wavelength?

    1. ABalmer series, about 103 nm
    2. BBalmer series, about 656 nm
    3. CLyman series, about 122 nm
    4. DLyman series, about 103 nm

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18. Question 18 · Easy

    Bohr's model gives the radius of the innermost orbit of hydrogen as a₀. An electron raised to the third orbit (n = 3) moves on a circle of what radius?

    1. A3a₀
    2. Ba₀/9
    3. C27a₀
    4. D9a₀

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19. Question 19 · Medium

    Match List-I with List-II for the electron in the nth Bohr orbit of hydrogen. List-I (Quantity) A. Orbit radius B. Orbital speed C. Magnitude of total energy D. Orbital angular momentum List-II (Proportional to) I. n II. n² III. 1/n IV. 1/n² Choose the correct answer.

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

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20. Question 20 · Medium

    Taking Eₙ = −13.6/n² eV and hc = 1240 eV nm, what is the wavelength of the photon emitted when a hydrogen atom falls from n = 3 to n = 2?

    1. AAbout 365 nm
    2. BAbout 656 nm
    3. CAbout 122 nm
    4. DAbout 486 nm

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21. Question 21 · Medium

    The electron of a hydrogen atom is in a level of energy −0.85 eV. Taking Eₙ = −13.6/n² eV, how many times the ground-state orbit radius is the radius of its orbit?

    1. A2
    2. B64
    3. C4
    4. D16

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22. Question 22 · Hard

    A single hydrogen atom in its ground state absorbs a photon of energy 12.09 eV. Taking Eₙ = −13.6/n² eV, what is the greatest number of photons of different wavelengths this one atom can emit as it returns to the ground state?

    1. A1
    2. B6
    3. C3
    4. D2

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23. Question 23 · Easy

    Which series of lines in the hydrogen spectrum lies in the visible region?

    1. ABrackett series
    2. BPaschen series
    3. CBalmer series
    4. DLyman series

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24. Question 24 · Easy

    In the alpha-particle scattering experiment, which observation led Rutherford to conclude that the positive charge and almost all the mass of an atom are concentrated in a tiny nucleus?

    1. AMost alpha particles passed through the foil undeflected
    2. BA very small fraction of the alpha particles was deflected by more than 90°
    3. CAll alpha particles lost the same small amount of energy in the foil
    4. DThe foil gave out electrons when struck

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25. Question 25 · Medium

    In the Bohr model of hydrogen, with Eₙ = −13.6/n² eV, what are the kinetic energy and the potential energy of the electron in the n = 2 orbit?

    1. A3.4 eV and −3.4 eV
    2. B6.8 eV and −3.4 eV
    3. C1.7 eV and −5.1 eV
    4. D3.4 eV and −6.8 eV

    Answer and explanation in LearnFree account