RMK SENIOR SECONDARY SCHOOL, THIRUVERKADU
CLASS: XII CHAPTER-12 ATOMS SUBJECT: PHYSICS
MCQ
WORKSHEET
1. The ratio between Bohr radii is
(a) 1 : 2 : 3 (b) 2 : 4 : 6 (c) 1 : 4 : 9 (d) 1 : 3 : 5
2. The ionisation energy of hydrogen atom is 13.6 eV. Following Bohr’s theory the energy
corresponding to a transition between 3rd and 4th orbits is
(a) 3.40 eV (b) 1.51 eV (c) 0.85 eV (d) 0.66 eV
3. The transition of electron from n = 4, 5, 6, ………. to n = 3 corresponds to
(a) Lyman series (b) Balmer series (c) Paschen series (d) Brackett series
4. A hydrogen atom makes a transition from n = 5 to n =1 orbit. The wavelength of photon emitted
is λ. The wavelength of photon when it makes a transition from n = 5 to n = 2 orbit is
(a) (8 / 7) λ (b) (16 / 7) λ (c) (24 / 7) λ (d) (32 / 7) λ
5. The radius of the innermost electron orbit of a hydrogen atom is 5.3×10–11 m. The radius of the
n =3 orbit is
(a) 1.01 x 10-10m (b) 1.59 X 10-10m (c) 2.12 x 10-10m (d) 4.77 X 10-10m
6. Which of the following postulates of the Bohr model led to the quantization of energy of the
hydrogen atom?
(a) The electron goes around the nucleus in circular orbits.
(b) The angular momentum of the electron can only be an integral multiple of h/2π
. (c) The magnitude of the linear momentum of the electron is quantized.
(d) Quantization of energy is itself a postulate of the Bohr model.
8. The transition from the state n = 5 to n = 1 in a hydrogen atom results in UV radiation. Infrared
radiation will be
obtained in the transition
(a) 2 → 1 (b) 3 → 2 (c) 4 → 3 (d) 6 → 2 9.
9. For ionising an excited hydrogen atom, the energy required (in eV) will be
(a) a little less than 13.6 eV (b) 13.6 eV (c) more than 13.6 eV (d) 3.4 or less
10. The De Broglie wavelength associated an electron accelerated by a potential difference of
100 V would approximately equal to be
(a) 0.1227 nm (b) 12.27 nm (c) 0.01227 nm (d) 1.227 nm
11. In an experiment on scattering of α-particles by a gold nucleus, the closest distance of n
approach is 30 fermi. If the velocity of the α-particle is doubled, the closest distance of approach
will
(a) remain unaltered (b) double
(c) reduce to half of the original value (d) reduce to (1/4)th the original value
12. The ratio of the shortest wavelength of the Balmer series to the shortest wavelength of the
Lyman series is
a) 4 : 1 b) 4 : 3 c) 4 : 9 d) 5 : 9
Directions: In the following questions, a statement of Assertion (A) is followed by statement
of Reason (R). Mark the correct choice as
(A) Both A and R are true and R is the correct explanation of A.
(B) Both A and R are true but R is NOT the correct explanation of A.
(C) A is true but R is false.
(D) A is false and R is true.
1. ASSERTION (A): The force of repulsion between atomic nucleus and a-particle varies with
distance according to inverse square law.
REASON(R): Rutherford did a-particle scattering experiment.
2. ASSERTION (A): The positively charged nucleus of an atom has a radius of almost 10 −15m.
REASON(R): In a-particle scattering experiment, the distance of closest approach for a particles
is ≃ 10−15m.
3. ASSERTION (A): Electrons in the atom are held due to coulomb forces
REASON(R): The atom is stable only because the centripetal force due to Coulomb’s law is
balanced by the centrifugal force.
4. ASSERTION (A): Total energy of revolving electron in any stationary orbit is negative
REASON(R): Energy is a scalar quantity. It can have only positive value.
5. Assertion (A): Bohr had to postulate that the electrons in stationary orbits around the nucleus
do not radiate.
Reason(R): According to classical physics all accelerating electrons radiate.
ANSWER THE FOLLOWING QUESTIONS
1. The energy levels of an atom are as shown below. Which of them will result in the transition
of a photon of wavelength 275 nm?
.
2. An electron and a proton are accelerated through the same potential. Which one of the two
has (i) greater value of de-Broglie wavelength associated with and (ii) less momentum?
Justify your answer.
3. Explain Rutherford’s model of atom. Give its limitations.
4. State Bohr’s quantum condition and frequency condition.
5. Using Rutherford model of the atom, derive the expression for the total energy of the
electron in hydrogen atom. What is the significance of total negative energy possessed by
the electron?
6. Using Bohr’s postulates derive an expression for radius of nth orbit of hydrogen atom.
7. Derive an expression for total energy of the electron in the nth orbit of an atom.
8. (a) Draw a schematic arrangement of Geiger Marsden experiment showing the scattering of
alpha particles by a thin foil of gold. Why is that most of the alpha particles go straight
through thr foil and only a small fraction gets scattered at large angles?
(b) Draw the trajectory of the alpha particle in the coulomb field of the nucleus. Define
distance of Closest approach and impact parameter.
9. Find the angular momentum of an electron revolving in the second orbit in Bohr's
hydrogen atom.
10. A hydrogen atom is in its third excited state. (a) How many spectral lines can be emitted by
it before coming to the ground state? Show these transitions in the energy level diagram.
(b) In which of the above transitions will the spectral line of shortest wavelength be emitted?
11. Draw a labelled diagram for α-particle scattering experiment. Give Rutherford's observations
and discuss the significance of this experiment. Obtain the expression which helps us to get
an idea of the size of the nucleus, using these observations.
12. The total energy of electron in the first excited state of hydrogen atom is -3.4eV.
(a) What is kinetic energy of electron in this state?
(b) What is potential energy of electron in this state?
CASE STUDY QUESTIONS
1. The trajectory traced by an α-particle depends on the impact parameter, b of collision. The
impact parameter is the perpendicular distance of the initial velocity vector of the α-particle from
the centre of the nucleus. A given beam of α-particles has a distribution of impact parameters b, so
that the beam is scattered in various directions with different probabilities. (In a beam, all particles
have nearly same kinetic energy.) It is seen that an α-particle close to the nucleus (small impact
parameter) suffers large scattering. In case of head-on collision, the impact parameter is minimum
and the α-particle rebounds back (𝜃 ≅ π). For a large impact parameter, the α-particle goes nearly
undeviated and has a small deflection (𝜃 ≅ 0). A)
(i) If an α particle collides head on with the nucleus, what is impact parameter?
(a) zero (b) infinite (c) 10-10 m (d) 1010 m
(ii) The size of the atom is proportional to
(a) A (b) 𝐴 1/ 3 (c) 𝐴 2 /3 (d) 𝐴 −1/ 3
(iii) An alpha particle of energy 5 MeV is scattered through 1800 by a fixed uranium nucleus. The
distance of closest approach is of the order of
(a) 10-10 m (b) 10-10 cm (c) 10-12 cm (d) 10-15 m
(iv) According to classical theory of Rutherford’s model, the path of electron will be
(a) Parabolic (b) Hyperbolic (c) Circular (d) Elliptical
