CBSE Class 12 Physics VBQs Atoms and Nuclei

Question. What is the ratio of the nuclear densities of two nuclei having mass number in the ratio of 1:4?
Answer : Nuclear density is independent of mass number so ratio will be 1:1

Question. Which has greater ionizing power:- alpha or beta particle?
Answer : Alpha Particle.

Question. In Bohr’s theory of model of a Hydrogen atom, name the physical quantity which equals to an integral multiple of h/2Π
Answer : Angular momentum

Question. What is the relation between ‘n’ & radius ‘r’ of the orbit of electron in a Hydrogen atom according to Bohr’s theory?
Answer : r α n2

Question. Among the alpha, beta & gamma radiations, which are the one affected by a magnetic field?
Answer : alpha & beta

Question. Why is the classical (Rutherford) model for an atom—of electron orbiting around the nucleus—not able to explain the atomic structure?
Answer. As the revolving electron loses energy continuously, it must spiral inwards and eventually fall into the nucleus. So it was not able to explain the atomic structure.

Question. What is the maximum number of spectral lines emitted by a hydrogen atom when it is in the third excited state?
Answer. For third excited state, n₂ = 4, and n₁ = 3, 2, 1 Hence there are 3 spectral lines.

Question. If λ1 and λ2 are the wavelengths of first line of Lyman and Paschen series Find the ratio λ1: λ2
Answer. 7:108

Question. In Rutherford scattering experiment what is the scattering angle corresponding to zero impact parameter
Answer. 180 degrees

Two Marks questions:

Question. Why nuclear force is not possible in laboratory?
Answer : It require very high temperature to carry on so not possible.

Question. What is the difference between an electron and a beta particle?
Answer :  An electron coming from nucleus is called beta particle.

Question. Define half-life and mean life of a radioactive substance and its relation.
Answer : Time during which one half of the initial nuclei disintegrates, ratio of combined age of total number of atom present in the given sample. Mean life =144* half life

Question. Explain the emission of gamma ray with example.
Answer :  Emission of gamma ray by the de excitation of nucleus, cobalt 

Question. Write two properties of alpha particles.
Answer :  They are reflected by electric and magnetic field, absorbed by matter, velocity one by 10 th of velocity of light, ionizes gases. 

THREE MARKS QUESTION:

Question. Define the terms:- Half-life period and Decay constant of a radioactive sample. Derive the relation between these terms.
Answer :  Half-life Period : The half-life period of an element is defined as the time in which the number of radioactive nuclei decay to half of its initial value. The decay constant of a radioactive element is defined as the reciprocal of time in which the number of undecided nuclei of that radioactive element falls to be1/e times of its initial value.

Question. Explain the observations and significance of Rutherford Alpha particle scattering experiment.
Answer :  Most of the alpha particle passes undefleted, a few deflected at different angles. 

Question. State Bohr’s postulate for explaining the spectrum of hydrogen atom.
Answer : Electrons moves in a fixed orbit, angular momentum is quantized, electrons are revolving in stationary orbits, energy emitted is the difference between the two levels of orbits.

Question. State Displacement law for radioactive decay.
Answer : Rate of nuclear disintegration is directly proportional to the no. of decayed particles

MULTIPLE CHOICE QUESTIONS

Question. The ratio between Bohr radii is
(a) 1 : 2 : 3
(b) 2 : 4 : 6
(c) 1 : 4 : 9
(d) 1 : 3 : 5
Answer. C

Question. In terms of Rydberg constant R, the wave number of the first Balmer line is
(a) R
(b) 3R
(c) 5R/36
(d) 8R/9
Answer. C

Question. 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
Answer. D

Question. The transition of electron from n = 4, 5, 6, ………. to n = 3 corresponds to
(a) Lyman series
(b) Balmer series
(c) Paschen series
(d) Brackettseries
Answer. C

Question. Which of the following spectral series in hydrogen atom gives spectral line of 4860 A?
(a) Lyman
(b) Balmer
(c) Paschen
(d) Brackett
Answer. B

Question. Which of the following postulates of the Bohr model led to the quantization of energy of 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.
Answer. B

Question. 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
Answer. C

Question. 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
Answer. D

CASE BASED 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).

Question. 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
Answer.  A

Question. The size of the atom is proportional to
(a) A
(b) 𝐴1⁄3
(c) 𝐴2⁄3
(d) 𝐴−1⁄3
Answer. B

Question. 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
Answer. D

Question. According to classical theory of Rutherford’s model, the path of electron will be
(a) Parabolic
(b) Hyperbolic
(c) Circular
(d) Elliptical
Answer. B

2. Bohr’s theory was successfully able to explain the spectrum of hydrogen, but it had many shortcomings. For example, it was only successful for hydrogen like single electron atoms and fails in case of atoms with two or more electrons. Bohr’s theory does not explain why only circular orbits should be chosen when elliptical orbits are also possible. As electrons exhibit wave properties also, so orbits of electrons cannot be exactly defined as in Bohr’s theory. Bohr’s theory predicts only the frequency of these lines and fails to explain anything about the relative intensities of various spectral lines.

Question. As per Bohr’s model, the minimum energy required to remove an electron from the ground state of doubly ionized Li atom is
a) 20.8
b) 1.51
c) 13.6
d) 122.4
Answer. D

Question. Bohr’s atom model assumes
a) The nucleus is of infinite mass and is at rest
b) Electrons in the quantized orbit will not radiate energy
c) Mass of electron remains constant
d) All of the above
Answer. D

Question. Bohr’s theory of hydrogen atom failed to explain
a) Emission Spectra
b) Diameter of H atom
c) Ionisation energy
d) The fine structure of even hydrogen spectrum.
Answer. D

Question. According to Bohr theory, relation between quantum number n and radius of orbit r is,
a) n α r
b) r α 1/n
c) r α n²
d) r α 1/n²
Answer. C

Question. In Bohr model of hydrogen atom, which of the following is quantized?
a) Linear velocity of electron
b) Angular velocity of electron
c) Linear momentum of electron
d) Angular momentum of electron
Answer. D

Question. Ruchi’s uncle who was a kabadiwalah was getting weak day by day. His nails were getting blue, he stated losing his hair. This happened immediately after he purchased a big container of heavy mass from Delhi University Chemistry Department. Doctors advised him hospitalization and suspected he has been exposed to radiation. His uncle didn’t know much about radiations but Ruchi immediately convinced her uncle to get admitted and start treatment.
(i) What according to you are the values utilized by Rama to convince her uncle to get admitted in hospital
(ii) Name the radioactive radiations emitted from a radioactive element
Answer:
(i) Value displayed - awareness, critical thinking, decision making
(i) X ray and Gamma rays.

Question. Medha’s grandfather was reading article is newspaper. He read that after so many years of atomic bombing is Hiroshima or Nagasaki, Japan National census indicated that children born even now are genetically deformed. His grandfather was not able to understand the reason behind it. He asked his Granddaughter Medha who is studying in class XII science. Medha sat with her grandfather and showed him pictures from some books and explained the harmful effects of radiations.(i) What are the values/ skills utilized by kajal to make her grandfather understand the reason of genetically deformity?
(ii) Name the nuclear reactions that occurred is atom bomb.
Answer:
(i) Sympathy, compassion
(ii) Nuclear- fission reactions

Question. Muthuswami a resident of Kundakulam was all set to leave everything and shift to another place in view of the decision of Govt. to start nuclear thermal power plant at Kundakulam. His granddaughter Prachi, a science student, was really upset on the ignorant decision of her grandfather. She could finally convince him not to shift, since adequate safety measures to avoid any nuclear mishap have already been taken by the Govt. before starting nuclear thermal plants.• What is the value displayed by Prachi in convincing her grandfather
• What is the principle behind working of nuclear reactor
• What are the main components of nuclear reactor
• Why is heavy water used as moderator?
Answer:
(i) Awareness, social responsibility
(ii) Controlled chain reaction
(iii) Nuclear Fuel, Moderator, Control rods, Coolant, Shielding
(iv) Neutrons produced during fission get slowed if they collide with a nucleus of same mass. As ordinary water contains hydrogen atoms so it can be used as a moderator. But it absorb neutron at a fast rate. To overcome this difficulty, Heavy waters is used as a moderator which has negligible cross sections for neutron absorption