Frank Solutions and MCQ for Chapter 6 Modern Physics Class 10 Physics ICSE

Exercises


1. Make the correct choices in the following:
(a) In one atom of copper (mass number 63, atomic number 29) there are
(i) 63 protons and 29 neutrons.
(ii) 63 protons and 29 electrons.
(iii) 29 protons and 29 neutrons.
(iv) 29 protons and 29 electrons.
(v) 29 protons and 63 neutrons.

(b) A radioactive nuclide 86226Ra decays by emission of two alpha particles, one beta particle and gamma rays, which of the following is the resulting nuclide X ?

A

B

C

D

E

223
X
83

218
X
83

218
X
84

217
X
85

218
X
85

(c) The element X has an atomic mass number of 238 and an atomic number of 92. It emits an alpha particle forming an element y. Y can be represented by

A

B

C

D

E

234
X
90

236
X
90

235
X
91

238
X
92

238
X
93

(d) An element P has an atomic mass of 239 and an atomic number of 92. It emits a beta particle forming an element Q. Q can be represented by

A

B

C

D

E

239
X
91

239
X
92

239
X
93

238
X
92

235
X
90

(e) Which one of the following statements about alpha and beta particles is correct?
(i) They carry equal amounts of charge.
(ii) Each alpha particle has four times the mass of a beta particle.
(iii) Only alpha particles can cause ionization.
(iv) They can both be deflected by magnetic fields when they are in motion.
(v) Alpha particles have a larger range in air than beta particles.


Answer 

(a) Mass number of copper = 63
Atomic number of copper = 29
As atomic number of element gives number of proton and electrons while mass number of element gives number of protons +number of neutrons.
So, number of protons in copper = atomic number of copper = 29.
Number of electron in copper = number of protons in copper =29.
Number of neutrons = mass number atomic number = 63 29 =34.
So, answer is (iii) as copper contains 29 protons and 29 electrons.

(b) Initially element is represented by  a particle is represented by 2He4 this means it contains 2 protons and 2 neutrons so mass of a particle is 4 unit. Thus after emitting a particle mass number of element would decrease by  45 unit and atomic number of element would decrease by 2. 
So after emitting 2  a particle 
β particle is represented by -1e° after emitting a β particle atomic number of element would increase by 1 unit and mass number of element would remain unchanged. 
So after emitting a β particle 
Gamma radiations have zero mass and no charge so after emission of gamma radiation mass and atomic number remains unaltered. 
The daughter nucleus is represented by

So, answer is B.


(c) Initially element is represented bya particle is represented by 2He4  this means it contains 2 protons and 2 neutrons so mass of a particle 4 unit. Thus after emitting a particle mass number of element would decrease by  4 unit and atomic number of element would decrease by  2. 

So after emitting a particle

So, answer is A.

(d) Initially element is represented by
β particle is represented by -1e° after emitting a β particle atomic number of element would increase by 1 unit and mass number of element would remain unchanged. 

(e) Answer is (iv) as both alpha and beta particles are deflected by magnetic 

2. Define the term radioactivity. Of the three normally occurring radioactive radiations:
(a) Which has a positive charge?
(b) Which is most penetrating?
(c) Which has no electric charge?
Answer 
The phenomenon of spontaneous disintegration of an unstable nucleus of naturally occurring isotope accompanied by emission of active radiations α particles,β particles and Y radiations is called radioactivity. 
(a) a particles have positive charge.
(b) Y radiation is most penetrating.
(c) Y radiation has no electric charge. 

3. Which of the following statements are correct?

(a) Electrons have
(i) No mass,
(ii) A mass less than that of a proton,
(iii) A mass equal to that of a helium atom,
(iv) Mass greater than that of a neutron.
Answer
(a) Electron has (ii) a mass less than that of a proton.

(b) Neutrons have
(i) A double positive charge,
(ii) A single positive charge,
(iii) A negative charge,
(iv) No electric charge.
Answer 
(b) Neutrons have (iv) no electric charge.

4. Draw a diagram of a simple atom showing the nucleus and electrons. In this atom:
(a) What type of charge will there be on the nucleus?
(b) What is the value of this charge?
Answer 

The central sphere is nucleus where small spheres are electron moving in circular orbit around nucleus. 
(a) There is positive charge on nucleus.
(b) Charge on proton is equal to +1.6× 10-19 C and neutron is neutral so value of charge on nucleus is p × 1.6 ×10-19 C where p is the number of proton in nucleus. 

5. What are the names of the three types of rays given off by a radioactive substance? Define the properties you know they possess. Why are the largest atoms radioactive?
Answer 
A radioactive source emits three types of radiations. These are α radiations, β radiations and γ-radiations.
α radiation
  1. a particles are Helium nucleus 2He4  . 
  2. a particles are positively charged. 
  3. The mass of each a particle is 4 a.m.u. 
  4. Ionization power of a particle is maximum. It is about 100 times more than that of β particles. 
  5. a particle has velocity less than the velocity of light i.e 1×107 ms-1.
  6. The penetrating power of a particle is least. 
  7. a particles are less deflected by electric and magnetic fields.

β particles
  1. β particles are like electrons -1β°.
  2. β particles are negatively charged. 
  3. The mass of each p particle is negligible. 
  4. velocity of β particle is comparable to the velocity of light.
  5. Ionization power of β particle is about 100 times more than that of γ radiations. 
  6. Penetrating power of β particles is less but 100 times more than that of a particle.
  7. β particles are more deflected by electric and magnetic field but in opposite direction. 

γ radiations
  1. γ radiations are electromagnetic radiations. 
  2. γ radiations have no charge.
  3. γ radiations have no mass.
  4. Velocity of γ radiation is equal to the velocity of light.
  5. Ionization power is least. 
  6. γ radiations have maximum penetration power. They are 100 times more penetrating than β particles.
  7. γ radiations are unaffected by electric and magnetic field. Heavy nucleus undergo radioactivity because they are unstable. 


6. One isotopes of uranium has a mass number of 235 and atomic number 92.
(a) What is the number of electrons in a neutral atom of this isotope?
(b) How many protons are there in the nucleus of this isotope?
(c) For another isotope of Uranium state which one of the numbers (238 or 92) changes?
(d) What is the number of protons in 238U?
Answer 
(a) Mass number of uranium = 235
Atomic number of uranium = 92
As atomic number of element gives number of proton and electrons while mass number of element gives number of protons + number of neutrons.
So, number of protons in uranium = atomic number of uranium = 92.

(b) Number of electron in uranium = number of protons in uranium =92.

(c)The atoms of same elements having the same atomic number Z but different mass number A are called isotopes. So, for another isotope of uranium mass number 235 changes.
(d)Number of protons in isotopes is same and as U238 is isotope of 92U235. So, number of protons in U235 is also 92.

7. Radioactive substances were found to give off three types of rays. Name them. How do they
(a) React to the magnetic field?
(b) React to the electric field?
(c) Act when different thickness of lead sheets is placed in their path?
Answer 
A radioactive source emits three types of radiations. These are α radiations, β radiations and γ radiations.
  1. α particles are heavy in mass and have positive charge so they are less deflected by magnetic field. β particles are lighter in mass and have negative charge so they are deflected more by magnetic field in opposite direction to a particle's deflection. γ radiations are charge less and mass less so they are not deflected by magnetic field.
  2. a particles are heavy in mass and have positive charge so they are less deflected by electric field towards negative plate. a particles are lighter in mass and have negative charge so they are deflected more by electric field towards positive plate. γ radiations are charge less and mass less so they are not deflected by electric field.
  3. The penetrating power of a particle is least. They can't penetrate lead sheet very much. Penetrating power of β particles is less but 100 times more than that of a particle. So, it will penetrate lead sheet more than a particles. γ radiations have maximum penetration power. They are 100 times more penetrating than β particles. So, γ radiations would penetrate in lead sheet by largest amount. 

8. Chlorine has an atomic number 17 in the form of isotope, one of atomic weight 35 and the other atomic weight 37. What do you understand by an isotope? Name two other elements which have isotopes.
Answer 
The atoms of same elements having the same atomic number Z but different mass number a are called isotopes. Hydrogen have 3 isotopes protium 1H1, deuterium 1H2, tritium 1H3.

9. (a) What is the meaning of mass number?
(b) What do you mean by atomic number?
(c) In what way are these numbers related to the isotopes of an element?
(d) Explain the use of radioactive in the field of medicine, agriculture and industry.
Answer 
(a) Mass number is the sum of protons and neutrons present in the nucleus. 
(b) Atomic number is the number of protons present in the nucleus. As number of protons is equal to the number of electrons so atomic number also gives the number of electrons in an atom. 
(c) Isotopes have same number of atomic number and different number of atomic masses.
(d) Uses of radioactivity in medical, agriculture and industry : 
Uses in medical field : 
  1. Rays from Radium produce satisfactorily improvement in skin diseases.
  2. Radiation from Co60 is used in cancer treatment.
  3. Radio I131 is used to diagnose and treat thyroid disorders. 
Uses in agriculture: 
  1. Radioactive Phosphorus P32 is used in the study of metabolism.
  2. Radioactive Sulphur S35. helps to study advantages and disadvantages of fungicides. (iii) Pests and insects can be killed by Y-radiation. 
Uses in industry: 
  1. In manufacturing paper& plastic and Metal sheets to control the thickness of the sheets. (ii) Radioisotopes can be used to estimate the amount of wear in bearings.
  2. Radioisotopes can be used to detect crack in welding, casting etc.

10. Distinguish between α, β and Ò® rays. 
Answer 

Sl. no

α particles

β particles

γ radiations

1

α particles are Helium nucleus 2He4 .

β particles are like electrons -1β0.

γ radiations are electromagnetic radiations.

2

α particles are positively charged.

β particles are negatively charged.

γ radiations have no charge.

3

The mass of each α particle is 4 a.m.u.

The mass of each β particle is negligible.

γ radiations have no mass.

4

α particles has velocity less than the velocity of light i.e. 1 ×107 ms-1.

Velocity of β particle is comparable to the velocity of light .

Velocity of γ radiation is equal to the velocity of light.

5

Ionisation power of α particle is maximum. It is about 100 times more than that of β particles.

Ionisation power of β particle is about 100 times more than that of γ particle.

Ionisation power is least.

6

The penetrating power of α particle is least.

Penetrating power of β particles is less but 100 tiems more than that of α particle.

γ radiations have maximum penetration power. They are 100 times more penetrating than β particles.

7

α particle are less deflected by electric and magnetic fields.

β particles are more deflected by electric and magnetic field but in opposite direction.

γ radiations are unaffected by electric and magnetic field.

11. What do you understand by radioactive decay? An isotope of Uranium 92U238 is radioactive and changes into Thorium 92 by the emission of an alpha particle. Write a symbolic equation for this decay process.
Answer 
The process of destruction of the original nucleus during the formation of new nuclesu due to radioactivity is called radioactive decay. 

Question is wrong as Thorium has atomic number 90 not 92. 
Symbolic equation of decay is 


12. The element 11Na24 changes to magnesium by the emission of a beta particle. Write down the symbolic equation for the beta emission.
Answer 
Symbolic equation of decay is 


14. What are the uses of radioactive isotopes in following?
(a) Medical field,
(b) Agriculture,
(c) Industries?
Answer 
Radioactive isotopes are used in medical field
  1. Rays from Radium produce satisfactorily improvement in skin diseases.
  2. Radiation from Co60 is used in cancer treatment.
  3. Radio I131 is used to diagnose and treat thyroid disorders.

Radioactive isotopes are used in agriculture
  1. Radioactive Phosphorus P32 is used in the study of metabolism.
  2. Radioactive Sulphur S35 helps to study advantages and disadvantages of fungicides.
  3. Pests and insects can be killed by Y-radiation.

Radioactive isotopes are used in industry. 
  1. In manufacturing papers, plastic and metal sheets to control the thickness of the sheets.
  2. Radioisotopes can be used to estimate the amount of wear in bearings.
  3. Radioisotopes can be used to detect crack in welding, casting etc. 

15. What do you mean by background radiations? Name its sources. Is it possible for us to keep ourselves away from it?
Answer 
The low temperature microwave radiation that arrives at the earth's surface from all directions of outer space is called background radiation.
Sources of background radiations are:
  1. Radiation from the sun.
  2. Rocks in the earth which contain traces of radioactive substances.
  3. Naturally occurring isotopes.
  4. Artificial radioisotopes.
No, it is not possible to keep ourselves away from these radiations.

16. State some of the safety precautions that we have to taken when we are exposed to some of the radiations or when we are handling some of the radioactive substances.
Answer 
Following precautions should be taken while handling the radioactive substances.
  1. The sources should only be handled by the forceps provided and never touched by hand.
  2. They should never be pointed towards a person.
  3. Food should not be taken where the sources are being used, as it may be contaminated.
  4. Never smoke near a radioactive source.
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