# SELINA Solutions for Class 10 Physics Chapter 12 - Radioactivity

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## Chapter 12 - Radioactivity Exercise Ex. 12A

Question 1

Name the three constituent of an atom and state their mass and charge of each. How are they distributed in an atom?

Solution 1

Three constituent of an atom are:

Electrons: mass is 9.1 X10-31 kg, charge is -1.6 X 10-19C

Neutron: mass is 1.6749 X10-27 kg, charge is zero.

Protons: mass is 1.6726 X 10-27 kg, charge is +1.6 X 10-19 C

Question 1 (MCQ)

Solution 1 (MCQ)

Hint: In a single radioactive decay, and  particles are never emitted simultaneously. There will be either an -emission or a -emission, which may be accompanied by  emission.

Question 2

Define the terms:

(a)Atomic number and(b) mass number.

Solution 2

Atomic number -the number of protons in the nucleus is called atomic number.

Mass number-the total number of nucleons in the nucleus is called mass number.

Question 2 (MCQ)

In - emission from a radioactive substance, an electron is ejected. This electron comes from:

1. The outermost orbit of an atom
2. The inner orbits of an atom
3. The surface of substance
4. The nucleus of an atom

Solution 2 (MCQ)

The nucleus of the atom.

Hint: Radioactivity is a nuclear phenomenon. Hence, electrons come out from the nucleus. Electron is created as a result of decay of one neutron into a proton inside the nucleus and it is not possible for the electron to stay inside the nucleus; thus, it is spontaneously emitted.

Question 3

What is nucleus of an atom? Compare its size with that of the atom. Name its constitutents. How is the number of these constituents determined by the atomic number its atomic model.

Solution 3

The nucleus at the centre of atom, whose size is of the order of 10-15 m to 10-14 m.

The size of a nucleus is 10-5 to 10-4 times the size of an atom. It consists of protons and neutrons.

If Z is the atomic number and A is the mass number of an atom, then the atom contains Z number of electrons; Z number of protons and A - Z number of neutrons.

The atom is specified by the symbol where X is the chemical symbol for the element.

Question 3 (MCQ)

1. α - particles
2. β - particles
3. X - rays
Solution 3 (MCQ)

(a) α - particles

An α - particle rapidly loses its energy as it moves through a medium and therefore its penetrating power is quite small. It can penetrate only through 3 - 8 cm in air. It can easily be stopped by a thin card sheet or a thick paper.

Question 4

State the atomic number and mass number of  and draw its atomic model.

Solution 4

Atomic number Z = 11

Mass number A = 23

Number of neutrons A - Z = 12

Question 4 (MCQ)

The radiation suffering the maximum deflection in a magnetic field is:

1. α - particles
2. β - particles
3. X - rays
Solution 4 (MCQ)

(b) β-particles

β-particles are negatively charged, so they get deflected by the electric and magnetic fields. The deflection of β-particle is more than that of a-particle since a β-particle is lighter than the α-particle. Whereas, gamma radiations are not deflected by the electric and magnetic fields since they are not charged particles.

Question 5

What are isotopes? Give one example.

Solution 5

Isotopes: the atoms of the same element which have the same atomic number Z but differ in their mass number A are called isotopes.

Example: Hydrogen has three isotopes

Question 6

What are isobars? Give one example.

Solution 6

Isobars: the atoms of different elements which have the same mass number A, but differ in their atomic number Z are called isobars.

Example:

Question 7

What is the name given to elements with same mass number and different atomic number?

Solution 7

Isobars

Question 8

Name the atoms of a substance having same atomic number, but different mass numbers. Give one example of such a substance. How do the structures of such atoms differ?

Solution 8

Atoms of a substance having same atomic number, but different mass numbers are called isotopes.

Example: Hydrogen has three isotopes

Structure of each isotope differs by the number of neutrons in its nuclei.

Question 9

Solution 9

Radioactivity:radioactivity is a nuclear phenomenon. It is the process of spontaneous emission of radiations from the nuclei of atoms during their decay.

Question 10

Solution 10

There will be no change in the nature of radioactivity. This is because radioactivity is a nuclear phenomenon.

Question 11

2. Name the radiations which are deflected by the electric field.
3. Name the radiation which is most penetrating.
4. Name the radiation which travels with the speed of light.
5. Name the radiation which has the highest ionizing power.
6. Name the radiation consisting of the same kind of particles as the cathode rays.

Solution 11

(a) Three types of radiations: Alpha, beta and gamma.

Question 12

(b)Name the radiation which has the lowest ionizing power.

(c)Name the radiation which has the lowest penetrating power.

(d)Give the charge and mass of particles composing the radiation in part (c).

(e)When the particle referred to in part (c) becomes neutral, they are found to be the atoms of rare gas. Name this rare gas and draw a model of its neutral atom.

(f)From which part of the atom do these radiations come?

Solution 12

(a) Gamma radiations have zero mass.

(b) Gamma radiations have the lowest ionizing power.

(c) Alpha particles have lowest penetrating power.

(d) Alpha particle has positive charge equal to 3.2 x 10-19C and rest mass equal to 4 times the mass of proton i.e. 6.68 x 10-27 kg.

(e) The gas is Helium.

(f)These radiations come from nucleus of the atom.

Question 13

The diagram in figure shows a radioactive source S placed in a thick lead walled container. The radiations given off are allowed to pass through a magnetic field. The magnetic field (shown as x) acts perpendicular to the plane of paper inwards. Arrows shows the paths of the radiation A, B and C.

(a)Name the radiations labelled A, B and C.

(b)Explain clearly how you used the diagram to arrive at the answer in part(a).

Solution 13

Radiations labeled A, B and C are respectively.

Radiation labeled A is gamma radiation because they have no charge and hence under action of magnetic field they go undeflected.

Radiation B is alpha radiation because its mass is large and it would be deflected less in comparison to beta radiation. The direction of deflection is given by Fleming's left hand rule. Also directions of deflection of alpha and beta radiations are opposite as they have opposite charge.

Question 14

Fig. shows a mixed source R of alpha and beta particles in a thick lead walled container. The particles pass through a magnetic field in a direction perpendicular to the plane of paper inwards as shown by x.

(a) Show in the diagram how the particles get affected.

(b) Name the law used in part (a)

[Hint: Alpha particles will deflect to the left while beta particles to the right]

Solution 14

(a)

(b) Fleming's left hand rule

Question 15

Fig. shows a radioactive source S in a thick lead walled container having a narrow opening. The radiations pass through an electric field between the plates A and B.

(a)Complete the diagram to show the paths of radiations.

(b)Why is the source S kept in a thick lead walled container with a narrow opening?

(c) Name the radiation which is unaffected by the electrostatic field.

(d) Which radiation is defleced the most. Given reason.

(e) Which among the three radiations causes the least biological damage?

Solution 15

(a)

(b)The radioactive substances are kept in thick lead containers with a very narrow opening, so as to stop radiations coming out from other directions because they may cause biological damage.

(e) Alpha particle.

Question 16

Explain why alpha and beta particles are deflected in an electric or a magnetic field, but gamma rays are not deflected in such a field.

Solution 16

This is because alpha and beta particles are charged particles, but gamma rays are neutral particles.

Question 17

Is it possible to deflect -radiations in a way similar to  and -particles, using the electric or magnetic field? Give reasons.

Solution 17

No, it is not possible to deflect gamma radiation in a way similar to alpha and beta particles, using the electric or magnetic field because they are neutral and hence do not deflected under the action of electric or magnetic field.

Question 18

State following four properties each of  radiations: (a) Nature, (b) Charge, (c) Mass and (d) Effect of electric field.

Solution 18
 Property -particle -particle -particle Nature Stream of positively charged particles, i.e. helium nuclei. Stream of negatively charged particles, i.e. energetic electrons. Highly energetic electromagnetic radiation. Charge Positive charge (Two times that of a proton) = + 3.2 x 10-19 C (or +2e) Negative charge = - 1.6 x 10-19 C (or -e) No charge Mass Four times the mass of proton i.e., 6.68 x 10-27 kg Equal to the mass of electron, i.e. 9.1 x 10-31 kg No mass (Rest mass is zero) Effect of electric field Less deflected More deflected than alpha particles but in direction opposite to those of particles Unaffected

Question 19

Arrange the α, β, or γ raditions in ascending order of their (i) ionising powers, and (ii) penetrating powers.

Solution 19

Ionizing power of alpha radiation is maximum i.e., 10000 times of gamma radiation while beta particles have lesser ionizing power i.e., 100 times of gamma radiation and gamma radiation have least ionizing power.

Penetration power is least for alpha particle and maximum for gamma radiation.

Question 20

State the speed of each of - radiations.

Solution 20

Speed of  radiation is nearly 107 m/s.

Speed of  radiation is about 90% of the speed of light or 2.7 x 108 m/s.

Speed of  radiation is 3 x 108 m/s in vacuum.

Question 21

(a) What is the composition of -radiations?

(b) Can a hydrogen H11 nucleus emit an alpha particle?

(c) Which one has the least penetrating power?

Solution 21

(a) Alpha radiations are composed two protons and two neutrons.

(b)Hydrogen has less A and Z as compared to Helium, it annot emit alpha particle.

(c) Beta particles are fast moving electrons.

Gamma radiations are photons or electromagnetic waves like X rays.

Alpha radiations have the least penetrating power.

Question 22

How -radiations are produced? Mention two common properties of gamma radiations and visible light.

Solution 22

Gamma radiation are produced when a nucleus is in a state of excitation (i.e., it has an excess of energy). This extra energy is released in the form of gamma radiation.

Gamma radiations like light are not deflected by the electric and magnetic field.

Gamma radiations have the same speed as that of light.

Question 23

An -particle captures (i) one electron, (ii) two electrons. In each case, What does it change to?

Solution 23

It will become singly ionized helium.

Question 24

'Radioactivity is a nuclear phenomenon'. Comment on this statement.

Solution 24

Any physical changes (such as change in pressure and temperature) or chemical changes (such as excessive heating, freezing, action of strong electric and magnetic fields, chemical treatment, oxidation etc.) do not alter the rate of decay of the radioactive substance. This clearly shows that the phenomenon of radioactivity cannot be due to the orbital electrons which could easily be affected by such changes. The radioactivity should therefore be the property of the nucleus. Thus radioactivity is a nuclear phenomenon.

Question 25

What kind of change takes place in a nucleus when a -particle is emitted? Express it by an equation. State whether (a) atomic number and (b) mass number are conserved in a radioactive -decay?

Solution 25

On emitting a  particle, the number of nucleons in the nucleus (i.e. protons and neutrons) remains same, but the number of neutrons is decreased by one and the number of protons is increased by one.

If a radioactive nucleus P with mass number A and atomic number Z emits a beta particle to form a daughter nucleus Q with mass number A and atomic number Z+1, then the change can be represented as follows:

(a) Atomic number 'Z' is not conserved. It is increased by 1.

(b) Mass number A is conserved.

Question 26

A certain radioactive nucleus emits a particle that leaves its mass unchanged, but increased its atomic number by one. Identify the particle and write its symbol.

Solution 26

Beta particle

Its symbolis .

Question 27

What happens to the (i) atomic number, (ii) mass number of an element when (a) -particle (b) -particle and(c) -radiation are emitted?

Solution 27

(a)Atomic number decreases by 2.

(b)Atomic number increases by 1.

(c)Atomic number does not change.

Question 28

What happens to the position of an element in the periodic table when it emits (a) an alpha particle, (b) -particle and (c)-radiation? Give reasons for your answer.

Solution 28

(a) After emitting an alpha particle the daughter element occupies two places to the left of the parent element in the periodic table.

Reason: If a parent nucleus X becomes a new daughter nucleus Y as a result of -decay, then the -decay can be represented as:

Thus, the resulting nucleus has an atomic number equal to (Z-2). Hence, it shifts two places to the left of the parent element in the periodic table.

(b) After emitting a -particle, the daughter element occupies one place to the right of the parent element in the periodic table.

Reason: If a parent nucleus X becomes a new daughter nucleus Y as a result of -decay, then the -decay can be represented as:

Thus, the resulting nucleus has an atomic number equal to (Z+1). Hence, it shifts one place to the right of the parent element in the periodic table.

(c) After emitting -radiation, the element occupies the same position in the periodic table.

Reason: If a parent nucleus X becomes a new daughter nucleus Y as a result of -decay, then the -decay can be represented as:

Thus, the resulting nucleus has atomic number equal to Z. Hence, it occupies the same position as the parent element in the periodic table.

Question 29

What changes occurs in the nucleus of radioactive elements when it emits (a) an alpha particle (b) beta particle and (c) gamma radiations? Give one example in each case in support of your answer.

Solution 29

The following changes occur when an atom emits

An alpha particle: atomic number decreases by 2 and mass number decreases by 4.

Example:

A beta particle: atomic number increases by one, but mass number does not change.

Example:

Gamma particle: it does not change anything in the nucleus, the energy of the nucleus decreases.

Example:

Question 30

(a)An atomic nucleus A is composed of 84 protons and 128 neutrons. The nucleus A emits an -particle and is transformed into a nucleus B. What is the composition of B?

(b)The nucleus B emits a -particle and is transformed into a nucleus C. What is the composition of C?

(c)What is the mass number of nucleus A?

(d)Does the composition of nucleus C change if it emits a -radiation?

Solution 30

(a)The composition of B - 82 protons and 126 neutrons.

(b)The composition of C – 83 protons and 125 neutrons.

(c)The mass number of nucleus A = no. of protons +no. of neurons = 84+128=212.

(d)Their will be no change in the composition of nucleus C.

Question 31

A certain nucleus A (mass number 238 and atomic number 92) is radioactive and becomes a nucleus B (mass number 234 and atomic number 90) by the loss of one particle.

(a)What particle was emitted?

(c)State the change in the form of a reaction.

Solution 31

(a)The alpha particle was emitted.

(b)This is because the atomic number has decreased by 2 and mass number has decreased by 4.

(c)

Question 32

State whether the following nuclear disintegrations are allowed or not (star indicate an exited state). Give reason if it is not allowed.

Solution 32

(a)This is allowed.

(b)This is not allowed because mass number is not conserved.

Question 33

1. What are the numbers 24 and 11 called?
2. Write the equation represent β-decay.
3. What general name is given to the product nucleus with respect to
Solution 33

An atom is specified by the symbol where X is the chemical symbol for the element.

Z is the atomic number and A is the mass number of an atom, then the atom contains Z number of electrons.

1. 24 is the mass number and 11 is the atomic number.
2. Isobar
Question 34

A nucleus of stable phosphorus has 15 protons and 16 neutrons.

1. What is its atomic number and mass number.
2. The nucleus of radio phosphorous has one neutron more than the stable nucleus. What will be its atomic number and mass number?
3. What will be the atomic number and mass number of new nucleus formed by decay of a β-particle by the radio phosphorus in part(b)?
Solution 34

If Z is the atomic number and A is the mass number of an atom, then the atom contains Z number of electrons; Z number of protons and A - Z number of neutrons.

The atom is specified by the symbol  where X is the chemical symbol for the element.

1. Atomic number is 15 and mass number is 31.
2. Atomic number is 15 and mass number is 32.
3. Atomic number is 16 and mass number is 32.
Question 35

An element P disintegrates by -emission and the new element suffers two further disintegrations, both by -emission, to form an element Q. Explain the fact that P and Q are the isotopes.

Solution 35

The atomic number of P decreases by 2 and mass no. decreases by 4 due to the emission of one alpha particle and then increases by 1 due to the emission of each beta particle, so the atomic number of Q formed after the emission of one alpha and two beta particles is same as that of P. Hence P and Q are the isotopes.

Question 36

A nucleus emits 2 α particles and 1 β particle to form a nucleus. Find the atomic number and mass number of X.

Solution 36

Question 37

Complete the following sentences:

(a)The mass number (A) of an element is not changed when it emits __________.

(b)The atomic number of a radioactive element is not changed when it emits ________.

(c)During the emission of a beta particle, the ________ number remains same.

Solution 37

(a)The mass number (A) of an element is not changed when it emits beta and gamma radiations.

(b)The atomic number of a radioactive element is not changed when it emits gamma radiations.

(c)During the emission of a beta particle, the mass number remains same.

Question 38

Complete the following nuclear changes:

Solution 38

(a)

(b)

(c)

(d)

(e)

Question 39

What are radio isotopes? Give one example of a radio isotope. State one use of radio isotopes.

Solution 39

Radio isotopes: The isotopes of some elements with atomic number Z

Example: carbon (Z=6, A=14).

Radio isotopes are used in medical and scientific and industrial fields. Radio isotopes such as are used as fuel for atomic energy reactors.

Question 40

Why are the alpha particles not used in radio therapy?

Solution 40

Because they cannot penetrate the human skin.

Question 41

Why do we usually use isotopes emitting gamma radiations as radioactive tracers in medical science?

Solution 41

Gamma radiations have very high penetration power and can easily pass through the human body. Therefore they are used as radioactive tracers in medical science.

Question 42

When does the nucleus of an atom become radioactive?

Solution 42

When the number of neutrons exceeds much than the number of protons in a nuclei, it become unstable or radioactive.

Question 43

Which of the following is the radio isotope in each pair?

Solution 43

(a), (b), (c)

The reason is that the number of neutrons exceeds the number of protons.

Question 44

State the medical use of radioactivity.

Solution 44

Many diseases such as leukemia, cancer, etc., are cured by radiation therapy. Radiations from cobalt -60 are used to treat cancer by killing the cells in the malignant tumor of the patient.

Question 45

Arrange the α, β, and γ radiation in ascending order of their biological damage. Give reason.

Solution 45

a  <  β  <  γ

An α-particle rapidly loses its energy as it moves through a medium and therefore its penetrating power is quite small. It can penetrate only through 3 - 8 cm in air. It can easily be stopped by a thin card sheet or a thick paper.

The penetrating power of β-particles is more than that of the α-particles. They can pass through nearly 5 m in air, through thin card sheet, and even through thin aluminium foil, but a 5 mm thick aluminium sheet can stop them.

Whereas, the penetrating power of γ-rays is high. It is about 104 times that of α-particles and 102 times that of β-particles. They can pass through 500 m in air or through 30 cm thick sheet of iron. Thick sheet of lead is required to stop them.

Question 46

Name two main sources of nuclear radiations. How are these radiations harmful?

Solution 46

Two main sources of nuclear radiations are:

1.Radioactive fallout from nuclear plants and other sources.

2.Disposal of nuclear waste.

These radiations are harmful because when these radiations falls on the human body, they kill the human living tissues and cause radiation burns.

Question 47

State two safety measures to be taken while establishing a nuclear power plant?

Solution 47

The following safety measures must be taken in a nuclear power plant:

1.The nuclear reactor must be shielded with lead and steel walls so as to stop radiations from escaping out to the environment during its normal operation.

2.The nuclear reactor must be housed in an airtightbuilding of strong concrete structure which can withstand earthquakes, fires and explosion.

3.There must be back up cooling system for the reactor core, so that in case of failure of one system, the other cooling system could take its place and the core is saved from overheating and melting.

Question 48

What is meant by nuclear waste? State one way for the safe disposal of nuclear waste?

Solution 48

The radioactive material after its use is known as nuclear waste.

It must be buried in the specially constructed deep underground stores made quite far from the populated area.

Question 49

State three safety precautions that you would take while handling the radioactive substances.

Solution 49

Three safety precautions that we would take while handling the radioactive substances are:

3. Keep the radioactive substances in thick lead containers with a very narrow opening, so as to stop radiations coming out from other directions.

Question 50

Why should a radioactive substance not be touched by hands?

Solution 50

Radioactive substance should not be touched by hands because these radiations are harmful; when radiation falls on the human body, they kill the human living tissues and cause radiation burns.

Question 51

What do you mean by background radiations? Name its sources. Is it possible for us to keep ourselves away from it?

Solution 51

Background radiation: These are the radioactive radiations to which we all are exposed even in the absence of an actual visible radioactive source.

There are two sources of background radiation:

Internal source: potassium, carbon and radium are present inside our body.

It is not possible for us to keep ourselves away from the background radiations.

## Chapter 12 - Radioactivity Exercise Ex. 12B

Question 1

What do you mean by nuclear energy? What is responsible for its release?

Solution 1

Energy released by combining of nuclei of an atom or by decay of an unstable radioactive nucleus during a nuclear reaction i.e., during fusion or fission is known as nuclear energy.

Question 1 (MCQ)

The particle used in nuclear fission for bombardment is :

1. Alpha particle
2. Proton
3. Beta particle
4. neutron
Solution 1 (MCQ)

(d) neutron

A neutron is used in nuclear fission for bombardment.

Question 1 (Num)

In fission of one uranium - 235 nucleus, the loss in mass is 0.2 a.m.u. Calculate the energy released.

Solution 1 (Num)

1 a.m.u. = 1.66 × 10-27 kg

→ 0.2 a.m.u. = 0.2 × 1.66 × 10-27 kg

Δm = 0.332 Δ 10-27 kg

Question 2

Write down the Einstein's mass-energy equivalence relation, explaining the meaning of each symbol used in it.

Solution 2

Einstein's mass-energy equivalence relation : E = Δmc2

Where E is the energy released due to the loss in the mass Δm and c is the speed of light.

Question 2 (MCQ)

The temperature required for the process of nuclear fusion is nearly:

1. 1000 K
2. 104K
3. 105K
4. 107K
Solution 2 (MCQ)

(d) 107 K

To make the fusion possible, a high temperature of approximately 107 K and high pressure is required.

Question 2 (Num)

When four hydrogen nuclei combine to form a helium nucleus in the interior of sun, the loss in mass is 0.0265 a.m.u. How much energy is released ?

Solution 2 (Num)

Given that Δm = 0.0265 a.m.u.

1 a.m.u. liberates 931.5 MeV of energy. Thus, energy liberated equivalent to 0.0265 a.m.u. is

= 0.0265 a.m.u. × 931.5 MeV

= 24.7 meV

Question 3
1. What is a.m.u ? Express 1 a.m.u. in MeV.
2. Write the approximate mass of a proton, neutron and electron in a.m.u.
Solution 3
1. The mass of atomic particles is expressed in terms of atomic mass unit (a.m.u.). 1 a.m.u. of mass is equivalent to 931 MeV of energy.
2. Mass of proton = 1.00727 a.m.u.

Mass of neutron = 1.00865 a.m.u.

Mass of electron = 0.00055 a.m.u.

Question 4

What is nuclear fission? Name the particle used for it. Write one fission reaction.

Solution 4

Nuclear fission is the process in which a heavy nucleus is splits into two light nuclei nearly of the same size by bombarding it with slow neutrons.

Question 5
1. Name two isotopes of uranium.
2. Which of the isotope mentioned in part (a) above is easily fissionable? Give reason.
3. State whether the neutron needed for fission reaction of the isotope mentioned in part (b) above, is slow or fast?
Solution 5
1. Experimentally it is found that isotope of  is more easily fissionable because the fission of  is possible by sloe neutron unlike  where fission is possible only by the fast neutrons.
2. Slow and fast both.
Question 6

Write the approximate value of the energy released in the fission of one nucleus of . What is the reason for it?

Solution 6

Nearly 190 MeV of energy is released due to fission of one nucleus of . The cause of emission of this energy is the loss in mass i.e., the sum of masses of product nuclei is less than the sum of mass of the parent nucleus and neutron.

Question 7

Complete the following nuclear fission reactions.

Solution 7

Question 8

What do you mean by the chain reaction in nuclear fission ? How is it controlled ?

Solution 8

A chain reaction is a series of nuclear fissions whereby the neutrons produced in each fission cause additional fissions, releasing enormous amount of energy.

It is controlled by absorbing some of the neutrons emitted in the fission process by means of moderators like graphite, heavy water, etc. then the energy obtained in fission can be utilized for the constructive purposes

Question 9

State two uses of nuclear fission.

Solution 9

(i) It is used in a nuclear bomb.

(ii) It is used in a nuclear reactor where the rate of release of energy is slow and controlled which is used to generate electric power.

Question 10

Give two differences between the radioactive decay and nuclear fission.

Solution 10

 Radioactive decay Nuclear Fission It is a self process. It does not occur by itself. Neutrons are bombarded on a heavy nucleus. The nucleus emits either the a or b particles with the emission of energy in form of g rays which is not very large. A tremendous amount of energy is released when a heavy nucleus is bombarded with neutrons and the nucleus splits in two nearly equal fragments. The rate of radioactive decay cannot be controlled. The rate of nuclear fission can be controlled.

Question 11
1. What is nuclear fission? Give one example and write its nuclear reaction.
2. What other name is given to nuclear fusion? Give reason.
Solution 11

a. Nuclear fission is the process in which a heavy nucleus is splits into two light nuclei nearly of the same size by bombarding it with slow neutrons.

When uranium with Z = 92 is bombarded with neutron, it splits into two fragments namely barium (Z = 56) and krypton (Z = 36) and a large amount of energy is released which appears due to decrease in the mass.

b. Nuclear fusion is also known as thermo-nuclear reaction. This is because nuclear fusion takes place at very high temperature.

Question 12

Why is a very high temperature required for the process of nuclear fusion? State the approximate temperature required.

Solution 12

When two nuclei approach each other, due to their positive charge, the electrostatic force of repulsion between them becomes too strong that they do not fuse. Thus, nuclear fusion is not possible at ordinary temperature and ordinary pressure.

Hence to make the fusion possible, a high temperature of approximately 107 K and high pressure is required. At such a high temperature, due to thermal agitations both nuclei acquire sufficient kinetic energy so as to overcome the force of repulsion between them when they approach each other, and so they get fused.

Question 13
1. Write one nuclear fusion reaction.
2. State the approximate value of energy released in the reaction mentioned in part (a).
3. Give reason for the release of energy stated in part (b).
Solution 13

1. In all three deuterium nuclei fuse to form a helium nucleus with a release of 21·6 MeV energy.

1. When two deuterium nuclei () fuse, nucleus of helium isotope  is formed and 3·3 MeV energy is released. This helium isotope again gets fused with one deuterium nucleus to form a helium nucleus  and 18·3 MeV of energy is released in this process.
Question 14

Complete the following fusion reactions :

Solution 14

Question 15

a. Name the process, nuclear fission or nuclear fusion, in which the energy released per unit mass is more?

b. Name the process, fission or fusion which is possible at ordinary temperature.

Solution 15
1. Nuclear fusion
2. Nuclear fission
Question 16
1. State the similarity in the process of nuclear fission or fusion
2. State two difference between the process of nuclear fission or fusion.
Solution 16
1. Both fission and fusion create release of neutrons and large amount of energy.

1. Nuclear fission: A heavy nucleus splits in two nearly equal light fragments when bombarded with neutrons. It is possible at very ordinary temperature and pressureNuclear fusion: Two light nuclei combine to form a heavy nucleus at very high temperature and high pressure. Possible only at a very high temperature (≈107 K) and a very high pressure.

Question 17

Give two examples of nuclear fusion.

Solution 17

Question 18

What is the source of energy of Sun or stars?

Solution 18

The source of energy in the Sun and stars is the nucleus fusion of light nuclei such as hydrogen present in them in their inner part. This takes place at a very high temperature and high pressure due to which helium nucleus is formed with the release of high amount of energy.

Question 19

Name the following nuclear reactions:

Solution 19

(a) Nuclear fission

(b) Nuclear fusion

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