CBSE Class 12 Physics Atoms MCQs Set B

Question. When an a-particle of mass m moving with velocity v bombards on a heavy nucleus of charge Ze, its distance of closest approach from the nucleus depends on m as
(a) 1/m²
(b) m
(c) 1/m
(d) 1/√m

Answer : C

Question. The electron in the hydrogen atom jumps from excited state (n = 3) to its ground state (n = 1) and the photons thus emitted irradiate a photosensitive material. If the work function of the material is 5.1 eV, the stopping potential is estimated to be (the energy of the electron in nth state En = −13.6/ n². eV )
(a) 5.1 V
(b) 12.1 V
(c) 17.2 V
(d) 7 V

Answer : D

Question. The ground state energy of hydrogen atom is –13.6 eV. When its electron is in the first excited state, its excitation energy is
(a) 10.2 eV
(b) 0
(c) 3.4 eV
(d) 6.8 eV

Answer : A

Question. The total energy of electron in the ground state of hydrogen atom is –13.6 eV. The kinetic energy of an electron in the first excited state is
(a) 6.8 eV
(b) 13.6 eV
(c) 1.7 eV
(d) 3.4 eV

Answer : D

Question. The total energy of an electron in the first excited state of hydrogen atom is about –3.4 eV. Its kinetic energy in this state is
(a) 3.4 eV
(b) 6.8 eV
(c) –3.4 eV
(d) –6.8 eV

Answer : A

Question. To explain his theory, Bohr used
(a) conservation of linear momentum
(b) quantisation of angular momentum
(c) conservation of quantum frequency
(d) none of these

Answer : B

Question. Excitation energy of a hydrogen like ion in its excitation state is 40.8 eV. Energy needed to remove the electron from the ion in ground state is
(a) 54.4 eV
(b) 13.6 eV
(c) 40.8 eV
(d) 27.2 eV

Answer : A

Question. The ionisation potential of H-atom is 13.6 V. When it is excited from ground state by monochromatic radiations of 970.6 Å, the number of emission lines will be (according to Bohr’s theory)
(a) 10
(b) 8
(c) 6
(d) 4

Answer : C

Question. The ionisation energy of hydrogen atom is 13.6 eV,the ionisation energy of a singly ionised helium atom would be
(a) 13.6 eV
(b) 27.2 eV
(c) 6.8 eV
(d) 54.4 eV

Answer : D

Question. Given the value of Rydberg constant is 10⁷ m^–1, the wave number of the last line of the Balmer series in hydrogen spectrum will be
(a) 0.25 × 10⁷ m^–1
(b) 2.5 × 10⁷ m^–1
(c) 0.025 × 10⁴ m^–1
(d) 0.5 × 10⁷ m^–1

Answer : A

Question. Ratio of longest wavelengths corresponding to Lyman and Balmer series in hydrogen spectrum is
(a) 7/29
(b) 9/31
(c) 5/27
(d) 5/23

Answer : C

Question. The wavelength of the first line of Lyman series for hydrogen atom is equal to that of the second line of Balmer series for a hydrogen like ion. The atomic number Z of hydrogen like ion is
(a) 3
(b) 4
(c) 1
(d) 2

Answer : D

Question. Which source is associated with a line emission spectrum?
(a) Electric fire
(b) Neon street sign
(c) Red traffic light
(d) Sun

Answer : B

Question. The life span of atomic hydrogen is
(a) fraction of one second
(b) one year
(c) one hour
(d) one day

Answer : A

Question. In Hydrogen spectrum, the wavelength of H_a line is 656 nm, whereas in the spectrum of a distant galaxy, H_a line wavelength is 706 nm. Estimated speed of the galaxy with respect to earth is
(a) 2 × 10⁸ m/s
(b) 2 × 10⁷m/s
(c) 2 × 10⁶ m/s
(d) 2 × 10⁵ m/s

Answer : B

Question. When a hydrogen atom is raised from the ground state to an excited state,
(a) both K.E. and P.E. increase
(b) both K.E. and P.E. decrease
(c) the P.E. decreases and K.E. increases
(d) the P.E. increases and K.E. decreases.

Answer : D

Question. In terms of Bohr radius a0, the radius of the second Bohr orbit of a hydrogen atom is given by
(a) 4a0
(b) 8a0
(c) 2a0
(d) 2a0

Answer : A

Question. For which one of the following, Bohr model is not valid?
(a) Hydrogen atom
(b) Singly ionised helium atom (He+)
(c) Deuteron atom
(d) Singly ionised neon atom (Ne+)

Answer : D

Question. An electron in hydrogen atom makes a transition n₁ → n₂ where n1 and n2 are principal quantum numbers of the two states. Assuming Bohr’s model to be valid, the time period of the electron in the initial state is eight times that in the final state. The possible values of n1 and n2 are
(a) n₁ = 6 and n₂ = 2
(b) n₁ = 8 and n₂ = 1
(c) n₁ = 8 and n₂ = 2
(d) n₁ = 4 and n₂ = 2

Answer : D

Question. Monochromatic radiation emitted when electron on hydrogen atom jumps from first excited to the ground state irradiates a photosensitive material.
The stopping potential is measured to be 3.57 V. The threshold frequency of the material is
(a) 4 × 10¹⁵ Hz
(b) 5 × 10¹⁵ Hz
(c) 1.6 × 10¹⁵ Hz
(d) 2.5 × 10¹⁵ Hz

Answer : C

Question. An electron in the hydrogen atom jumps from excited state n to the ground state. The wavelength so emitted illuminates a photosensitive material having work function 2.75 eV. If the stopping potential of the photoelectron is 10 V, then the value of n is
(a) 2
(b) 3
(c) 4
(d) 5

Answer : C

Question. Out of the following which one is not a possible energy for a photon to be emitted by hydrogen atom according to Bohr’s atomic model?
(a) 0.65 eV
(b) 1.9 eV
(c) 11.1 eV
(d) 13.6 eV

Answer : C

Question. An alpha nucleus of energy 1/2 mv2 bombards a heavy nuclear target of charge Ze. Then the distance of closest approach for the alpha nucleus will be proportional to
(a) 1/Ze
(b) v2
(c) 1/m
(d) 1/v4

Answer : C

Question. In a Rutherford scattering experiment when a projectile of charge z₁ and mass M₁ approaches a target nucleus of charge z₂ and mass M₂, the distance of closest approach is r_o. The energy of the projectile is 
(a) directly proportional to z₁z₂ 
(b) inversely proportional to z₁ 
(c) directly proportional to mass M₁ 
(d) directly proportional to M₁ x M₂

Answer : A

Question. The ratio of wavelengths of the last line of Balmer series and the last line of Lyman series is
(a) 1
(b) 4
(c) 0.5
(d) 2

Answer : B