CBSE Class 12 Physics Atoms and Nuclei Assignment Set B

Question. A radioactive material has half-life of 10 days.What fraction of the material would remain after 30 days ? 
(a) 0.5
(b) 0.25
(c) 0.125
(d) 0.33

Answer :  C

Question. The operation of a nuclear reactor is said to be critical, if the multiplication factor (K) has a value
(a) 1
(b) 1.5 
(c) 2.1
(d) 2.5

Answer :  A

Question. 238U has 92 protons and 238 nucleons. It decays by emitting an Alpha particle and becomes
(a) 234U92
(b) Th 234 90 
(c) 235U92
(d) Np 23793

Answer :  B

Question. If Alpha, Beta and Gamma rays carry same momentum, which has the longest wavelength
(a) Alpha rays 
(b) Beta rays
(c) Gamma rays
(d) None, all have some wavelength

Answer :  D

Question. What is the amount of energy released by deuterium and tritium fusion? 
(a) 60.6 eV
(b) 123.6 eV
(c) 17.6 eV
(d) 28.3 eV

Answer :  C

Question. Calculate power output of 235 92 U reactor, if it takes 30 days to use up 2 kg of fuel, and if each fission gives 185 MeV of useable energy.
Avogadro's number = 6 × 10²³ /mol? 
(a) 56.3 MW
(b) 60.3 MW
(c) 58.3 MW
(d) 54.3 MW

Answer :  C

Question. Starting with a sample of pure ⁶⁶Cu,7/8 of it decays into Zn in 15 minutes. The corresponding half life is 
(a) 15 minutes
(b) 10 minutes
(c) 7 .1/2  minutes
(d) 5 minutes

Answer :  D

Question. A radioactive material decays by simultaneous emission of two particles with respective halflives 1620 and 810 years. The time, in years, after which one-fourth of the material reamins is
(a) 1080
(b) 2430 
(c) 3240
(d) 4860

Answer :  A

Question. If the binding energy per nucleon of a nuclide is high then 
(a) It should be abundantly available in nature
(b) It will decay instantly
(c) It will have a large disintegration constant
(d) It will have a small half-life.

Answer :  A

Question. Activity of a radioactive sample decreases to (l/3)rd of its original value in 3 days. Then, in 9 days its activity will become 
(a) (1/27) of the original value
(b) (1/9) of the original value
(c) (1/18) of the original value
(d) (1/3) of the original value

Answer :  A

Question. The half life of a radioactive substance against a -decay is 1.2 × 10⁷ s. What is the decay rate for 4.0 × 1015 atoms of the substance 
(a) 4.6 × 10 ¹² atoms/s
(b) 2.3 × 10 ¹¹ atoms/s
(c) 4.6 × 10 ¹⁰ atoms/s
(d) 2.3 × 10 ⁸ atoms/s

Answer :  D

Question. Actinium 231, ²³¹Ac₈₉, emit in succession two b-particles, four alphas, one b and one alpha plus several g rays. What is the resultant isotope:
(a) ²²¹ Au₇₉
(b) ²¹¹ Au ₇₉ 
(c) ²²¹ Pb ₈₂
(d) ²¹¹ Pb ₈₂

Answer :  D

Question. An archaeologist analyses the wood in a prehistoric structure and finds that C¹⁴ (Half life = 5700 years) to C¹² is only one-fourth of that found in the cells of buried plants. The age of the wood is about 
(a) 5700 years
(b) 2850 years
(c) 11,400 years
(d) 22,800 years

Answer :  C

Question. The fossil bone has a 14C : 12C ratio, which is [1/16] of that in a living animal bone. If the halflife of 14C is 5730 years, then the age of the fossil bone is 
(a) 11460 years
(b) 17190 years
(c) 22920 years
(d) 45840 years

Answer :  C

Question. Which one of the following pairs of nuclei are isotones?
(a) 34Se74, 31Ga71
(b) 38Sr84, 38Sr86
(c) 42Mo92, 40Zr92
(d) 20Ca40, 16S32

Answer  :  A

Question. A nucleus represented by the symbol Z AX has
(a) Z neutrons and A – Z protons
(b) Z protons and A – Z neutrons
(c) Z protons and A neutrons
(d) A protons and Z – A neutrons 

Answer  :  B

Question. The mass number of a nucleus is
(a) always less than its atomic number
(b) always more than its atomic number
(c) sometimes equal to its atomic number
(d) sometimes less than and sometimes more than its atomic number 

Answer  :  C

Question. Atomic weight of Boron is 10.81 and it has two isotopes 5B10 and 5B11. Then the ratio of 5B10 : 5B11 in nature would be
(a) 15 : 16
(b) 10 : 11
(c) 19 : 81
(d) 81 : 19

Answer  :  C

Question. The constituents of atomic nuclei are believed to be
(a) neutrons and protons
(b) protons only
(c) electrons and protons
(d) electrons, protons and neutrons 

Answer  :  A

Question. A sample of radioactive element containing 4 × 10¹⁶ active nuclei. Half life of element is 10 days, then number of decayed nuclei after 30 days
(a) 0.5 × 10¹⁶
(b) 2 × 10¹⁶
(c) 3.5 × 10¹⁶
(d) 1 × 10¹⁶ 

Answer  :  C

Question. A deutron is bombarded on 8O16 nucleus then a-particle is emitted. The product nucleus is
(a) ₇N¹³
(b) ₅B¹⁰
(c) ₄Be⁹
(d) ₇N¹⁴ 
Answer  :  D

ONE MARK QUESTIONS

Question. State Bohr’s quantization condition of angular momentum
Answer. Bohr’s Quantisation condition: Only those orbits are permitted in which the angular momentum of the electron is an integral multiple of h/2π.

Question. Which series of the hydrogen spectrum has wavelength in the visible range
Answer. Balmer

Question. When an electron jumps from fourth excited state to the ground state identify the number of spectral lines produced in H spectrum
Answer. Number of spectral lines =10

Question. What is the energy required for transition from ground state to the highest permitted state
Answer. 13.6eV

Question. What is the significance of negative energy of electron in the orbit
Answer. Electrons are bound to the nucleus.

MULTIPLE CHOICE QUESTIONS

ASSERTION –REASON TYPE QUESTIONS
Answer: A Both are correct and reason is correct explanation of assertion.
Answer: B Both are correct but reason is not the correct explanation of assertion.
Answer: C Reason is wrong.
Answer: D Both are wrong.

Question. Assertion: According to Bohr’s atomic model the ratio of angular momenta of an electron in first excited state and in ground state is 2:1.
Reason: In a Bohr’s atom the angular momentum of the electron is directly proportional to the principal quantum number.
Answer. A

Question. Assertion: The force of repulsion between atomic nucleus and a-particle varies with distance according to inverse square law.
Reason: Rutherford did a-particle scattering experiment.
Answer. B

Question.Assertion: The positively charged nucleus of an atom has a radius of almost 10−15m.
Reason: In a-particle scattering experiment, the distance of closest approach for particles is ≃ 10−15m.
Answer. A

Question. Assertion: Electrons in the atom are held due to coulomb forces
Reason: The atom is stable only because the centripetal force due to Coulomb’s law is balanced by the centrifugal force.
Answer. C

Question. Assertion: For the scattering of a-particles at a large angles, only the nucleus of the atom is responsible.
Reason: Nucleus is very heavy in comparison to α particle.
Answer. A

CASE BASED QUESTIONS

1. Due to many reasons, Rutherford model of atom had many limitations. Rutherford model failed to explain the stability of an atom. As the electron's energy decreases gradually, it collapses into the nucleus Hence the charge of the atom will disrupt making the atom unstable. In Rutherford model, an electron can revolve in orbit of all possible radii hence it should emit a continuous spectrum but atoms like hydrogen always emit a discrete line spectrum. From the developing ideas of quantum hypothesis, Bohr made modifications to Rutherford's model. By studying the Rutherford's model for months he presented three postulates by combining classical and early quantum concepts. Bohr gave the conditions of momentum and energy associated with different orbits. By following his postulates, we were able to find total energy on an electron, the radius of the orbit, velocity of an electron revolving around the nucleus.

Question. In Bohr Model, what is the atomic radius of 3rd orbit?
a) 𝑟₀/9
b) 𝑟₀
c) 9𝑟₀
d) 3𝑟₀
Answer. C

Question. Radii of the permitted orbits increase in …………… ratio.
a) 1 : 2 : 3
b) 2: 4 : 6
c) 1 : 4 : 9
d) 1: 4 : 6
Answer. C

Question. As per Bohr model, the radius of stationary orbit are related to principal quantum number n as
a) Inversely proportional to square of n
b) Inversely proportional to n
c) Directly proportional to n
d) Directly proportional to square of n
Answer. D

Question. If the orbital radius of electron in the hydrogen is 4.7x10^-11m. Kinetic energy of electron
a) 15.3 eV
b) -15.3 eV
c) 13.6 eV
d) -13.6 eV
Answer. A

Question. What is the angular momentum of an electron in the third orbit of an atom?
a. x 10^-34 Js
b. 3.15 x 10^-35 Js
c. 3 x 10^-34 Js
d. 1.15 x 10^-28 Js
Answer. A
 

2. Rutherford's model of atom was modified by Bohr using classical quantum concepts. The greatness of Bohr's theory is that it not only successfully explained the already known series of layman, Balmer and Paschen but also predicted two new series in the infrared region which was later discovered by Brackett and Pfund. From Bohr's theory, energy level diagram was established in which different energies of different stationary states of an atom are represented by parallel horizontal lines, drawn according to some suitable energy scale. An electron can have definite values of energy while revolving in different orbits. This is called energy quantisation. Atoms may acquire sufficient energy to raise the electron in higher energy level. It is known as excitation energy and the accelerating potential which excites the electron to higher energy level is known as excitation potential. In various lines, when photons are emitted, it means electrons jump from higher energy to lower energy state. Those spectral lines are called emission lines. But when an atom absorbs a photon, it has energy equal to the energy needed by the electron in a lower. State to make the transition to higher state. This is called absorption spectra.

Question. For scattering α-particles, Rutherford suggested that
a) Mass of atom and its positive charge were concentrated at centre of atom
b) Only mass of atom is concentrated at centre of the atom
c) Only positive charge of atom is concentrated at centre of atom
d) Mass of atom is uniformly distributed throughout it’s volume
Answer. A

Question. In Pfund series, the ratio of maximum to minimum wavelength of emitted spectral lines is
a) 𝜆𝑚𝑎𝑥/𝜆𝑚𝑖𝑥 = 4/3
b) 𝜆𝑚𝑎𝑥/𝜆𝑚𝑖𝑥 = 9/5
c) 𝜆𝑚𝑎𝑥/𝜆𝑚𝑖𝑥 = 2/3
d) 𝜆𝑚𝑎𝑥/𝜆𝑚𝑖𝑥 = 36/11
Answer. D

Question. A set of atoms in excited state decays
a) In general to any of the state with lower energy
b) Into a lower state only when excited by an external field
c) All together simultaneously into lower state
d) To emit photons only when they collide
Answer. A

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

Question. The ground state energy of the hydrogen atom is E0. The Kinetic energy of the electron in 3rd excited level is
(a) – 𝐸˳/16
(b) – 𝐸˳/9
(c) 𝐸˳/16
(d) 𝐸˳/9
Answer. A