# Class 12 Physics Chapter 13 Important Questions Nuclei

Science is a complex and challenging subject, as it involves so many principles and concepts that are difficult to memorize. Those student who opt for science have to face many challenges and work hard to get good marks in the exam. In this lesson, students will learn about Nuclei. The best solution of the problem is to practice as many Physics Class 12 Chapter 13 Important Questions as possible to clear the doubts.

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

Q 2:- How many electrons, protons and neutrons are there in a nucleus of atomic number 11 and mass number 24?
Ans:- 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 given nucleus = 0.

Q 3:- What holds nucleons together in a nucleus?
Ans:- Strong attractive force between neighbouring nucleons inside a nucleus holds them together.

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

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

Q 6:- Define the term 'activity of a radionuclide. Write the SI unit.
Ans:- The rate of decays (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.

Q 7:- What is the origin of β--particles, the nucleus or the orbiting electrons?
Ans:- β--particles have 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.

Q 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?
Ans:- During a β-decay, the mass number A (or the total nucleon number) of radionuclide does not change.

Read also: Nuclei Class 12 Physics Notes Chapter 13

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

Q 10:- Write the nuclear reactions for the following:

(a) α-decay of 84Po214,

(b) β- decay of 15P32, and

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

(a) 84Po21482Pb210 + 2He4

(b) 15P3216S31 + -1e0 + \bar{v}

(c) 6C115B11 + -1e0 + v

Q 11:- What are (a) exoergic, and (b) endoergic nuclear processes?
Ans:- (a) An exoergic 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.

Q 12:- What are thermal neutrons?
Ans:- 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.

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

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

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

Q 16:- What is the function of a moderator in a nuclear reactor?
Ans:- 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.

Q 17:- What is a thermonuclear reaction?
Ans:- 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.

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

Q 19:- All protons in an atom remain packed inside a small-sized nucleus in spite of the electrostatic repulsive force between them, why?
Ans:- 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.

Q 20:- What do you mean by charge independent nature of the nuclear force?
Ans:- 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.

Q 21:- It is said that nuclear forces are saturated forces. What do you mean by it?
Ans:- A nucleon inside a nucleus experiences attractive force only due to its closest neighbouring 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 saturated force.

Q 22:- Why is the mass of a nucleus always less than the sum of the masses of its constituents, neutrons and protons?
Ans:- 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 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.

Q 23:- Why is the ionising power of a-rays much greater than that of β-rays or γ-rays?
Ans:- 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 ionisation.

Q 24:- Why is the penetrating power of gamma rays very large?
Ans:- 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 photon 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.

Q 25:- What do you mean by the statement "Radioactivity is a statistical process"?
Ans:- 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 predic the probability of decay.

Q 26:- A radionuclide disintegrates by β-decay. Can you guess, whether it will emit a β-particle or a β+-particle?
Ans:- Whenever N/Z (neutron to proton ratio) ratio in a nucleus is higher than required for stability, a neutron is converted into a proton inside the nucleus leading to beta minus (β-) decay. On the other hand, if N/Z ratio of a nucleus is lower than required for stability, a proton is converted into a neutron inside the nucleus leading to beta plus (β+) decay.

Q 27:- Draw a plot of potential energy of a pair of nucleons as a function of their separation. What is the significance of negative potential energy in the graph shown?
Ans:- The plot is shown in Fig. The negative potential energy signifies that the force between the nucleons is attractive in nature and nucleons are bound together inside the nucleus, i.e, the nucleus is stable.

Q 28:- Why is heavy water used as a moderator in a nuclear reactor?
Ans:- Heavy water (D2O) is used as a moderator in a nuclear reactor because mass of deuteron nuclide (mass number = 2) is comparable to mass of fast moving neutron and during their elastic collision/scattering, neutron easily transfers energy to deuteron. Thus, neutrons are slowed down easily, i.e. the moderating action of heavy water is quite fast. Moreover, heavy water is nonradioactive and chemically inert.

Q 29:- A general impression exists that mass-energy interconversion takes place only in nuclear reaction and never in chemical reaction. This is strictly speaking incorrect. Explain.
Ans:- The energy released or absorbed in a chemical reaction is on account of difference in chemical binding energies of atoms and molecules on the two sides of a chemical reaction. The chemical binding energy gives a mass defect to the total mass of an atom or molecule. Hence, we can very well say that mass-energy interconversion takes place in chemical reactions too. However, mass defects and energies involved in chemical reactions are almost a million times smaller than those in a nuclear reaction, hence the general impression is that mass-energy interconversion does not take place in chemical reactions.

Q 30:- A fusion reaction is more energetic than a fission reaction. Comment.
Ans:- Yes, for the same mass of matter (say, 1 kg of hydrogen and 1 kg of uranium-235) the energy released in fusion reaction is about 8 times the energy released in fission reaction. Hence, the statement is correct.

Q 31:- Why nuclear fusion reaction has not been performed in a laboratory so far?
Ans:- In a nuclear fusion reaction, two lighter nuclei fuse together at a high temperature of = 108 K and extremely high pressures. Such high temperatures cannot be maintained in laboratories even for a fraction of a second, hence nuclear fusion reaction has not been performed in a laboratory so far.

Q 32:- State with reason, why light nuclei usually undergo nuclear fusion.
Ans:- Binding energy per nucleon (Ebn) of light nuclei is generally less and nucleons are less tightly bound in the nucleus. When two light nuclei fuse to form a single larger nucleus, nucleons are more tightly bound and value of Ebn increases. Hence, the product nuclide is more stable. It is due to this reason that light nuclei usually undergo nuclear fusion.

Q 33:- Why does the binding energy per nucleon decrease with increase in mass number for heavy nuclei like 238U?
Ans:- Binding energy per nucleon Ebn decreases with increase in mass number A > 170. So, value of Ebn is less for heavy nuclei like 238U. Such heavier nuclei contain a large size and mass and a large number of protons. These protons exert a strong coulombian repulsive force and as a result, the binding of nucleons becomes less tight and value of Ebn falls.

Q 34:- If the nucleons bound in a nucleus are separated from each other, the sum of their masses is greater than the mass of the nucleus. Where does this mass difference come from? Explain briefly.
Ans:- When we want to divide a composite nucleus into its constituent nucleons, we have to supply energy, equal to the binding energy of the nucleus. In accordance with mass-energy relationship, the supplied energy it transformed into mass. Thus, mass difference comes from conversion of energy into mass.

Q 35:- Two nuclei have mass numbers in the ratio 2:5. What is the ratio of their nuclear densities?
Ans:- The nuclear density is independent of mass number A of the nucleus. Hence, both the given nuclei will have same value of nuclear density.

Q 36:- A free neutron is unstable but a free proton is stable. Why?
Ans:- A free neutron is unstable because it can spontaneously decay into a proton, an electron and an antineutrino. This reaction has a positive Q-value and it takes place easily. However, a proton cannot spontaneously decay into a neutron, positron and neutrino because proton is slightly lighter than a neutron and Q-value of the reaction is negative. Therefore, a free proton is stable.

Q 37:- Neutrons produced in fission can be slowed down even by using ordinary water. Then, why is heavy water used for this purpose?
Ans:- Although fast neutrons produced in nuclear fission can be easily slowed down by using water (H2O) as a moderator but there is a considerable chance that water may absorb a neutron and transform into heavy water. It will mean loss of neutrons. To avoid this possibility, generally, heavy water is used as moderator in a reactor to slow down fast moving neutrons.

Q 38:- If both the number of protons and the number of neutrons are conserved in each nuclear reaction, in what way is mass converted into energy (or vice versa) in a nuclear reaction?
Ans:- In each nuclear reaction, the proton number and neutron number are conserved. So, the total rest mass of neutrons and protons is same on either side of reaction equation. However, binding energy on one side need not be same as on the other side of equation. It is the difference in binding energies which appears as energy released or absorbed in a nuclear reaction. Since binding energy contributes to mass, hence we say that the difference in the total mass of nuclei on the two sides gets converted into energy or vice versa.