Chapter 25
Atoms
Solutions
SECTION - A
Objective Type Questions (One option is correct)
Rutherford' s Model
1. Thickness of the foil of gold used in -particle scattering experiment is
(1) 2.1 × 10–7 m (2) 3.5 × 10–5 m (3) 2.1 × 10–9 m (4) 3.5 × 10–6 m
Sol. Answer (1)
2. The scattering -particles were observed through a rotatable detector consisting of a screen of
(1) Copper sulphide (2) Zinc sulphide (3) Graphite (4) Gold
Sol. Answer (2)
3. In Rutherford’s experiment, scattering of more than 1° was observed in
(1) 14% of the incident -particles (2) About 0.14% of the incident -particles
(3) About 1.4% of the incident -particles (4) About 0.014% of the incident -particles
Sol. Answer (2)
4. In scattering experiment, -particles were deflected by
(1) Repulsive force of electrons (2) Repulsive force of gold nucleus
(3) Attractive force of electrons (4) Attractive force of gold nucleus
Sol. Answer (2)
5. Energy of the beam of -particles used by Geiger and Marsden in scattering experiment is
(1) 2.2 MeV (2) 4.2 MeV (3) 5.1 MeV (4) 5.5 MeV
Sol. Answer (4)
6. Source of -particles used in scattering experiment was
214
(1) 82Bi
216 (2) 81Bi
216 (3) 81Bi
214 (4) 83 Bi
Sol. Answer (4)
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,Solutions of Assignment Atoms 101
7. In scattering experiment, find the distance of closest approach, if a 6 MeV -particle is used
(1) 3.2 × 10–16 m (2) 2 × 10–14 m (3) 4.6 × 10–15 m (4) 3.2 × 10–15 m
Sol. Answer (2)
At distance of closest approach
PE = Kinitial
Where Kinitial is initial kinetic energy.
⎛ 1 ⎞ 2e 2 13
⎜⎝ 4 ⎟⎠ d 6 1.6 10 J
0
9 109 (1.6 10 19 )2 2
d m
6 1.6 10 13
= 4.8 × 10–16 m
8. An -particle colliding with one of the electrons in a gold atom looses
1
(1) Most of its momentum (2) About rd of its momentum
3
(3) Little of its energy (4) Most of its energy
Sol. Answer (3)
Mass of an electron is negligible as compared to -particle.
Bohr ' s Model
9. The angular momentum of an electron in a hydrogen atom is proportional to (where n is principle quantum
number)
(1) n (2) n2 (3) n3 (4) n
Sol. Answer (1)
nh
mvr =
2
10. When an electron in hydrogen atom is taken from fourth excited state to ground state
(1) Both kinetic energy and potential energy increases
(2) Both kinetic energy and potential energy decreases
(3) Kinetic energy will increase while potential energy will decrease
(4) Kinetic energy will decrease while potential energy will increase.
Sol. Answer (3)
1
Both U & K , potential energy is negative whereas kinetic energy is positive.
n2
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, 102 Atoms Solutions of Assignment
11. What should be the angular momentum of an electron in Bohr’s hydrogen atom whose energy is –0.544 eV?
h 3h 5h 2h
(1) (2) (3) (4)
2 2 2
Sol. Answer (3)
13.6
E = –0.544 eV = eV
25
n=5
5h
L=
2
4E0 E0
12. The energies of three conservative energy levels L3, L2 and L1 of hydrogen atom are E0, and
9 4
respectively. A photon of wavelength is emitted for a transition L3 to L1. What will be the wavelength of
emission for transition L2 to L1?
16 27 19
(1) (2) (3) (4)
31 7 20
Sol. Answer (2)
13. Ground state energy of H-atom is –13.6 eV. The energy needed to ionise H-atom from its second excited state
is
(1) 1.51 eV (2) 3.4 eV (3) 13.6 eV (4) 12.1 eV
Sol. Answer (1)
For second excited state, n = 3
13.6
E = eV = –1.51 eV
9
14. The product of angular speed and tangential speed of electron in nth orbit of hydrogen atom is
(1) Directly proportional to n2 (2) Directly proportional to n3
(3) Inversely proportional to n4 (4) Independent of n
Sol. Answer (3)
v n2
vnn =
rn
1
v n n
n n2
2
1
v n n
n4
15. The speed of an electron in the 4th orbit of hydrogen atom is
c c c
(1) c (2) (3) (4)
137 2192 548
Sol. Answer (4)
v0 c c
V= = =
n 137 4 548
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Atoms
Solutions
SECTION - A
Objective Type Questions (One option is correct)
Rutherford' s Model
1. Thickness of the foil of gold used in -particle scattering experiment is
(1) 2.1 × 10–7 m (2) 3.5 × 10–5 m (3) 2.1 × 10–9 m (4) 3.5 × 10–6 m
Sol. Answer (1)
2. The scattering -particles were observed through a rotatable detector consisting of a screen of
(1) Copper sulphide (2) Zinc sulphide (3) Graphite (4) Gold
Sol. Answer (2)
3. In Rutherford’s experiment, scattering of more than 1° was observed in
(1) 14% of the incident -particles (2) About 0.14% of the incident -particles
(3) About 1.4% of the incident -particles (4) About 0.014% of the incident -particles
Sol. Answer (2)
4. In scattering experiment, -particles were deflected by
(1) Repulsive force of electrons (2) Repulsive force of gold nucleus
(3) Attractive force of electrons (4) Attractive force of gold nucleus
Sol. Answer (2)
5. Energy of the beam of -particles used by Geiger and Marsden in scattering experiment is
(1) 2.2 MeV (2) 4.2 MeV (3) 5.1 MeV (4) 5.5 MeV
Sol. Answer (4)
6. Source of -particles used in scattering experiment was
214
(1) 82Bi
216 (2) 81Bi
216 (3) 81Bi
214 (4) 83 Bi
Sol. Answer (4)
Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph.011-47623456
,Solutions of Assignment Atoms 101
7. In scattering experiment, find the distance of closest approach, if a 6 MeV -particle is used
(1) 3.2 × 10–16 m (2) 2 × 10–14 m (3) 4.6 × 10–15 m (4) 3.2 × 10–15 m
Sol. Answer (2)
At distance of closest approach
PE = Kinitial
Where Kinitial is initial kinetic energy.
⎛ 1 ⎞ 2e 2 13
⎜⎝ 4 ⎟⎠ d 6 1.6 10 J
0
9 109 (1.6 10 19 )2 2
d m
6 1.6 10 13
= 4.8 × 10–16 m
8. An -particle colliding with one of the electrons in a gold atom looses
1
(1) Most of its momentum (2) About rd of its momentum
3
(3) Little of its energy (4) Most of its energy
Sol. Answer (3)
Mass of an electron is negligible as compared to -particle.
Bohr ' s Model
9. The angular momentum of an electron in a hydrogen atom is proportional to (where n is principle quantum
number)
(1) n (2) n2 (3) n3 (4) n
Sol. Answer (1)
nh
mvr =
2
10. When an electron in hydrogen atom is taken from fourth excited state to ground state
(1) Both kinetic energy and potential energy increases
(2) Both kinetic energy and potential energy decreases
(3) Kinetic energy will increase while potential energy will decrease
(4) Kinetic energy will decrease while potential energy will increase.
Sol. Answer (3)
1
Both U & K , potential energy is negative whereas kinetic energy is positive.
n2
Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph.011-47623456
, 102 Atoms Solutions of Assignment
11. What should be the angular momentum of an electron in Bohr’s hydrogen atom whose energy is –0.544 eV?
h 3h 5h 2h
(1) (2) (3) (4)
2 2 2
Sol. Answer (3)
13.6
E = –0.544 eV = eV
25
n=5
5h
L=
2
4E0 E0
12. The energies of three conservative energy levels L3, L2 and L1 of hydrogen atom are E0, and
9 4
respectively. A photon of wavelength is emitted for a transition L3 to L1. What will be the wavelength of
emission for transition L2 to L1?
16 27 19
(1) (2) (3) (4)
31 7 20
Sol. Answer (2)
13. Ground state energy of H-atom is –13.6 eV. The energy needed to ionise H-atom from its second excited state
is
(1) 1.51 eV (2) 3.4 eV (3) 13.6 eV (4) 12.1 eV
Sol. Answer (1)
For second excited state, n = 3
13.6
E = eV = –1.51 eV
9
14. The product of angular speed and tangential speed of electron in nth orbit of hydrogen atom is
(1) Directly proportional to n2 (2) Directly proportional to n3
(3) Inversely proportional to n4 (4) Independent of n
Sol. Answer (3)
v n2
vnn =
rn
1
v n n
n n2
2
1
v n n
n4
15. The speed of an electron in the 4th orbit of hydrogen atom is
c c c
(1) c (2) (3) (4)
137 2192 548
Sol. Answer (4)
v0 c c
V= = =
n 137 4 548
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