CBSE Class 12 Physics Atoms and Nuclei Assignment Set C

Question. The size of the atom is proportional to
(a) A
(b) A1/3
(c) A2/3
(d) A–1/3

Answer: B

Question. The binding energy of a H-atom, considering an electron moving around a fixed nuclei (proton), is B = – me⁴ / 8n² ∈²₀ h² (m = electron mass).
If one decides to work in a frame of reference where the electron is at rest, the proton would be moving arround it. By similar arguments, the binding energy would be B = – me⁴ / 8n² ∈²₀ h² (M = proton mass)
This last expression is not correct because
(a) n would not be integral
(b) Bohr-quantisation applies only to electron
(c) the frame in which the electron is at rest is not inertial
(d) the motion of the proton would not be in circular orbits, even approximately

Answer: C

Question. The simple Bohr model cannot be directly applied to calculate the energy levels of an atom with many electrons. This is because
(a) of the electrons not being subject to a central force
(b) of the electrons colliding with each other
(c) of screening effects
(d) the force between the nucleus and an electron will no longer be given by Coulomb’s law

Answer: A

Question. The ratio of the speed of the electrons in the ground state of hydrogen to the speed of light in vacuum is
(a) 1/2
(b) 2/237
(c) 1/137
(d) 1/237

Answer: C

Question. For the ground state, the electron in the H-atom has an angular momentum = h, according to the simple Bohr model. Angular momentum is a vector and hence there will be infinitely many orbits with the vector pointing in all possible directions. In actuality, this is not true,
(a) because Bohr model gives incorrect values of angular momentum.
(b) because only one of these would have a minimum energy.
(c) angular momentum must be in the direction of spin of electron.
(d) because electrons go around only in horizontal orbits.

Answer: A

Question. O₂ molecule consists of two oxygen atoms. In the molecule, nuclear force between the nuclei of the two atoms
(a) is not important because nuclear forces are short-ranged.
(b) is as important as electrostatic force for binding the two atoms.
(c) cancels the repulsive electrostatic force between the nuclei.
(d) is not important because oxygen nucleus have equal number of neutrons and protons.

Answer: A

Question. In the following transitions of the hydrogen atom, the one which gives an absorption line of highest frequency is
(a) n = 1 to n = 2
(b) n = 3 to n = 8
(c) n = 2 to n = 1
(d) n = 8 to n = 3

Answer: A

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

Answer: B

Question. Taking the Bohr radius as a0 = 53 pm, the radius of Li⁺⁺ ion in its ground state, on the basis of Bohr’s model, will be about
(a) 53 pm
(b) 27 pm
(c) 18 pm
(d) 13 pm

Answer: C

Question. The ratio of energies of the hydrogen atom in its first to second excited state is
(a) 1 : 4
(b) 4 : 1
(c) – 4 : – 9
(d) – (1/4) : – (1/9)

Answer: D

Question. If an electron in a hydrogen atom jumps from the 3rd orbit to the 2nd orbit, it emits a photon of wavelength λ. When it jumps from the 4th orbit to the 3rd orbit, the corresponding wavelength of the photon will be
(a) (16/25)λ
(b) (9/16)λ
(c) (20/7)λ
(d) (20/13)λ

Answer: C

Question. Hydrogen H, deuterium D, singly-ionised helium He+ and doubly-ionised lithium Li⁺⁺ all have one electron around the nucleus. Consider n = 2 to n = 1 transition. The wavelengths of the emitted radiations are λ₁, λ₂, λ₃, and λ₄ m m m m respectively. Then approximately
(a) λ₁ = 2λ₂ = 2√λ₃ = 3√λ₄
(b) λ₁ = λ₂ = 2λ₃ = 3λ₄
(c) λ₁ = 2λ₂ = 4λ₃ = 9λ₄
(d) 4λ₁ = 2λ₂ = 2λ₃ = λ₄

Answer: C

Question. The Bohr model for the spectra of a H-atom
(a) will not be applicable to hydrogen in the molecular from.
(b) will not be applicable as it is for a He-atom.
(c) is valid only at room temperature.
(d) predicts continuous as well as discrete spectral lines.

Answer: A, B

Question. Let E_n =1/8∈²₀ me4/n²h² be the energy of the nth level of H-atom. If all the H-atoms are in the ground state and radiation of frequency (E₂ – E₁)/h falls on it,
(a) it will not be absorbed at all.
(b) some of atoms will move to the first excited state.
(c) all atoms will be excited to the n = 2 state.
(d) no atoms will make a transition to the n = 3 state.

Answer: B, D

Question. The wavelength of the first line of Lyman series in hydrogen is 1216 Å. The wavelength of the second line of the same series will be
(a) 912 Å
(b) 1026 Å
(c) 3648 Å
(d) 6566 Å

Answer: B

Question. Two H atoms in the ground state collide inelastically. The maximum amount by which their combined kinetic energy is reduced is
(a) 10.20 eV
(b) 20.40 eV
(c) 13.6 eV
(d) 27.2 eV

Answer: A

Question. When an electron in an atom goes from a lower to a higher orbit, its
(a) kinetic energy (KE) increases, potential energy (PE) decreases
(b) KE increases, PE increases
(c) KE decreases, PE increases
(d) KE decreases, PE decreases

Answer: C

Question. According to Bohr’s theory, the energy of radiation in the transition from the third excited state to the first excited state for a hydrogen atom is
(a) 0.85 eV
(b) 13.6 eV
(c) 2.55 eV
(d) 3.4 eV

Answer: C

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. A set of atoms in an excited state decays.
(a) in general to any of the states with lower energy.
(b) into a lower state only when excited by an external electric field.
(c) all together simultaneously into a lower state.
(d) to emit photons only when they collide.

Answer: A

Fill in the Blanks

Question. The angle of scattering θ for zero value of impact parameter b is _________________.
Answer: 180°

Question. The frequency spectrum of radiation emitted as per Rutherford’s model of atom is _____________.
Answer: continuous

Question. The force responsible for scattering of alpha particle with target nucleus is _______________.
Answer: electrostatic force

Question. According to de Broglie a stationary orbit is that which contains an _______________ number of de Broglie waves associated with the revolting electron.
Answer: integral

Question. _______________ is a physical quantity whose dimensions are the same as that of Plank’s constant.
Answer: Angular momentum

Question. _______________ series of hydrogen spectrum lies in the visible region electromagnetic spectrum.
Answer: Balmer

Question. ______________ is the ionisation potential of hydrogen atom.
Answer: 13.6 eV

Question. Total energy of electron in a stationary orbit is _________________, which means the electron is bound to the nucleus and is not free to leave it
Answer: negative

Question. The value of Rydberg constant is _________________.
Answer: 1.09 × 10⁷ m^–1

Question. When an electron jumps from 2nd stationary orbit of hydrogen atom to 1st stationary orbit, the energy emitted is _________________.
Answer: 10.2 eV