Bihar Board - Class 12 Physics - Chapter 13: Nuclei Short Answer Question

1. How is the radius of a nucleus related to its mass number?
Answer:- Radius of a nucleus, R = R0 [A]1/3, where R0 = 1.2 fm.

2. How many electrons, protons and neutrons are there in a nucleus of atomic number 11 and mass number 24?
Answer:- The number of protons in the nucleus = atomic number, Z = 11

and the number of neutrons, N = A - Z = 24 -11 = 13. However, the nucleus does not contain electrons. Hence, the number of electrons in a given nucleus = 0.

3. What holds nucleons together in a nucleus?
Answer:- Strong attractive force between neighboring nucleons inside a nucleus holds them together.

4. Which of the two quantities governs the stability of a nuclide, its binding energy or the binding energy per nucleon?
Answer:- The binding energy per nucleon (Ebn)of a nuclide governs its stability. Higher the value of Ebn more stable the nuclide will be.

5. Binding energies per nucleon for 1H2 and 2He4 are 1.1 MeV and 7.2 MeV respectively. Which is more stable, and why?
Answer:- 2He4 is more stable because it has a higher value of binding energy per nucleon.

6. Define the term 'activity of a radionuclide. Write the SI unit.
Answer:- The rate of decay (i.e., number of disintegration per unit time) of nuclei in a given radioactive sample is called its 'activity'. Its SI unit is 1 becquerel (1 Bq), where 1 Bq = 1 disintegration/s.

7. What is the origin of β--particles, the nucleus or the orbiting electrons?
Answer:- β--particles have the same charge and same rest mass as the electrons, but their originating place is the nucleus. A β--particle is emitted whenever a neutron is transformed into a proton inside the nucleus.

8. Out of the two characteristics, the mass number (A) and the atomic number (Z) of a nucleus, which one does not change during β-decay?
Answer:- During a β-decay, the mass number A (or the total nucleon number) of radionuclide does not change.

9. When does emission of a neutrino (v) and an antineutrino (v) take place?
Answer:- An antineutrino (v) is emitted by a radionuclide along with an electron in β- decay. However, a neutrino v is emitted along with a positron in β+ decay.

10. Write the nuclear reactions for the following:

(a) α-decay of 84Po214,

(b) β- decay of 15P32, and

(c) β+ decay of 6C11
Answer:- The respective nuclear reaction equations are given as:

(a) 84Po214 ⟶ 82Pb210 + 2He4

(b) 15P32 ⟶ 16S31 + -1e0 +

(c) 6C11 ⟶ 5B11 + -1e0 + v

11. What are (a) exoergic, and (b) endoergic nuclear processes?
Answer:- (a) An exergonic process is the nuclear process in which energy is released and which can take place spontaneously. 

(b) An endoergic process is the nuclear process in which energy is being absorbed. Thus, energy from an external source is to be supplied to cause the given nuclear process.

12. What are thermal neutrons?
Answer:- Thermal neutrons are small energy neutrons in thermal equilibrium with the surrounding matter at room temperature. These are in motion at a thermal speed of about 2 km/s and possess kinetic energy of about 0.04 eV.

13. What are transuranic elements?
Answer:- Transuranic elements are the elements of atomic number greater than that of uranium. Thus, all elements with Z > 92 are called the transuranic elements.

14. What is fissile material?
Answer:- The material which can easily undergo the nuclear fission process is called fissile material.

15. Name the absorbing material used to control the reaction rate of neutrons in a nuclear reactor.
Answer:- Cadmium rods are mostly used as the absorptive material to control the reaction rate of neutrons in a nuclear reactor. Other neutron absorbing materials are B and Hf.

16. What is the function of a moderator in a nuclear reactor?
Answer:- A moderator is used to slow down fast moving energetic neutrons so that they may cause further fissions. Action of a moderator is based on transfer of energy by the neutron to nuclei of lighter elements during elastic collisions.

17. What is a thermonuclear reaction?
Answer:- A nuclear fusion reaction going on at the core of the stars under the conditions of high temperature and pressure is called a thermonuclear reaction.

18. Name the reaction responsible for generation of energy in the Sun.
Answer:- Nuclear fusion reaction taking place at the core of the Sun is the cause of energy generation in the Sun.

19. All protons in an atom remain packed inside a small-sized nucleus in spite of the electrostatic repulsive force between them, why?
Answer:- All protons of an atom remain packed within the small space of its nucleus in spite of the electrostatic repulsive force between them on account of a strong attractive nuclear force present between them. The magnitude of attractive nuclear force is about 100 times more than the electrostatic repulsive force and is the controlling force.

20. What do you mean by the charge independent nature of the nuclear force?
Answer:- Nuclear force is present among all nucleons whether charged or uncharged ones. Thus, there are n-n, n-p and p-p interactions. So we say that the nuclear force is charge independent.

21. It is said that nuclear forces are saturated forces. What do you mean by it?
Answer:- A nucleon inside a nucleus experiences attractive force only due to its closest neighboring nucleons. Addition of more nucleons in a nucleus does not increase the nuclear force acting on a particular nucleon. It is due to this reason that nuclear force is said to be a saturated force.

22. Why is the mass of a nucleus always less than the sum of the masses of its constituents, neutrons and protons?
Answer:- The mass of a nucleus is invariably a bit less than the sum of the masses of its constituent protons and neutrons. It is on account of the fact that some work is to be done on the nucleons to bring and bind them together within the small region of the nucleus. The energy needed to do the work is obtained at the cost of mass in accordance with mass-energy equivalence. The difference between the total mass of constituent nucleons and mass of nucleus is called the mass defect. More mass defect means that nucleons are more rigidly bound within the nucleus and more stable the given nucleus is.

23. Why is the ionizing power of a-rays much greater than that of β-rays or γ-rays?
Answer:- Alpha rays consist of heavy particles (He) having high kinetic energy as well as large momentum. As a result, on collision with neutral atoms, they can easily eject one or more valence electrons and cause ionization.

24. Why is the penetrating power of gamma rays very large?
Answer:- The gamma rays are electromagnetic waves of extremely short wavelengths. Consequently, the energy of a γ-ray photon is very high. Rest mass of γ-ray photons is zero and it is not deflected either in an electric or a magnetic field and goes unobstructed through a medium. As a result, the penetrating power of gamma rays is very large.

25. What do you mean by the statement "Radioactivity is a statistical process"?
Answer:- Radioactive decay is a statistical process because we cannot precisely predict the timing of a particular radioactivity exhibited by a particular nucleus. The nucleus may decay immediately or it may last a long long time before decaying. We can only predict the probability of decay.