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Nuclear Physics
1. State one difference between a chemical change and a nuclear change.
Ans. A chemical change takes place due to transfer/sharing of orbital electrons of
atoms of different elements, whereas nuclear change takes place due to the
change in number of nucleons in the nucleus of an atom.
2. (a) What do you understand by the following terms ?
(i) Nucleus, (ii) Nucleons.
(b) Compare the mass and charge of nucleons of different kinds.
Ans.(a) (i) The central core of the atom is called nucleus. The nucleus contains
protons and neutrons.
(ii) Neutrons and protons are collectively called nucleons.
(b) Protons have a mass equal to 1.67 × 10–27 kg and charge equal to
+ 1.6 × l0–19C.
Neutrons have a slightly more mass than the protons but have no electric
charge.
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3.(a) What do you understand by the following terms?
(i) Atomic number, (ii) Mass number.
(b) An element X has a symbol AZ X . What do A and Z stand for? What is the
number of (i) electrons in element, (ii) neutrons in the nucleus?
Ans.(a) (i) The number of protons in an atom is called atomic number.
(ii) The number of nucleons in the atom of an element is called its mass
number.
(b) A stands for mass number and Z stands for atomic number.
The number of electrons is equal to Z.
The number of neutrons is equal to (A – Z).
4. An element is represented by
235
92
X . Calculate its (i) mass number,
(ii) atomic number, (iii) number of electrons, (iv) number of neutrons.
Ans. (i) Mass number = 235
(ii) Atomic number = 92
(iii) No. of electrons = 92
(iv) Number of neutrons = (235 – 92) = 143.
5. (a) What are isotopes ?
(b) Give two examples of isotopes.
Ans. (a) The atoms of same element, having the same atomic number, but different
mass numbers are called isotopes.
(b) (i) Chlorine has two isotopes, i.e.,
37
17
cl and
35
17
cl .
(ii) Hydrogen has three isotopes, i.e., 11 H , 21 H ; 31 H .
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6. (a) What are isobars ?
(b) Give two examples of isobars.
(c) Why do isotopes of an element have the same chemical properties ?
Ans. (a)The atoms of different elements, having different atomic number, but same
mass number are called isobars.
(b) (i)
(ii)
40
18
Ar and
24
11
40
20
Na and
Ca are isobars.
24
12
mg are isobars.
(c) Chemical properties of an element are a function of its atomic number.
As the atomic number in isotopes does not change, therefore, they have the
same chemical properties.
7.(a) One isotope of uranium has mass number 235 and atomic number 92.
(i) How many number of electrons are present in the neutral atom of uranium ?
(ii) How many protons are present in the above nucleus ?
(iii) How many neutrons are present in the above nucleus ?
(iv) Do other isotopes of uranium have the same number of neutrons ?
(v) State the number of protons in 238
92 U .
Ans. (i) 92 electrons
(ii) 92 protons
(iii) 143 neutrons
(iv) No. Different isotopes have different number of neutrons.
(v) 92 protons
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8. What do you understand by the following terms ?
(i) Radioactive elements
(ii) Radioactivity
(iii) α - particles
(iv) β - particles
(v) γ - radiations
Ans. (i) Radioactive elements are those, whose nucleus decays into nucleus of some
other element, with the ejection of alpha particles or beta particles or gamma
radiations.
(ii) The property due to which the nucleus of an element decays into the nucleus
of some other element, with the ejection of alpha particles or beta particles or
gamma radiations is called radioactivity.
(iii) α-particles are helium nucleus or doubly ionised helium atoms which have
lost two electrons.
(iv) β-particles are negatively charged particles, having a charge of –1.6 ×
l0–19C and mass 9.106 × l0–31 kg.
(v) γ-radiations are electromagnetic waves, having wavelength less than 0.01 Å.
9. A radioactive substance is oxidised. What changes would you expect to take
place in the nature of radioactivity? Give a reason for your answer.
Ans. There will be no change in the radioactivity of the oxide of the substance
compared to the original substance. It is because, oxidation is a chemical
process and does not involve the nucleus of the substance.
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10. (a)Who discovered the phenomenon of radioactivity ?
(b) Name four radioactive substances.
Ans. (a) Henry Becquerel discovered the phenomenon of radioactivity.
(b) (i) Uranium, (ii) Radium, (iii) Thorium, (iv) Polonium.
11.(a) What are becquerel rays ?
(b) State four properties of becquerel rays.
Ans. (a) The radiations (particles) given by a radioactive element are called
becquerel rays.
(b) (i) They affect photographic plate.
(ii) They ionise the gas through which they pass.
(iii) They can penetrate through matter.
(iv) They are affected by electrostatic and magnetic fields.
12. Diagram below shows a lead cube containing radioactive substance R. A
magnetic field is applied perpendicular to the path of nuclear radiations, emitted
by radioactive source when the radiations split into three beams X, Y and Z.
(a) Identify X, Y and Z.
(b) State the magnetic polarity at P and Q.
(c)Which law helped you to identify the polarity ?
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Ans.(a) (i) X ⇒ β-particles
(ii) Y ⇒ γ-radiations
(iii) Z ⇒ α-particles.
(b) (i) P is north pole (ii) Q is south pole
(c) Fleming’s left hand rule.
13. (a) What is an alpha particle ?
(b) Name the gas formed from these particles.
Ans. (a) Doubly charged helium nuclei are called alpha particles.
(b) Helium gas is formed from α-particles, when they acquire two electrons.
14. What changes occur in the nucleus of an atom if it emits :
(i) gamma radiations only? (ii) beta radiations only? (iii) alpha radiations only ?
Ans. (i) No change in mass number or atomic number. The nucleus slowly attains
normal state from its excited state.
(ii) The mass number remains the same, but atomic number increases by l a.m.u.
(iii) The mass number decreases by 4 a.m.u. and atomic number by 2 a.m.u.
15. What happens to the position of an element, relative to its original position in
periodic table when it emits (i) γ-radiation, (ii) β-particle, (iii) α-particle?
Ans. (i) No change in position in the periodic table.
(ii) It occupies one place ahead in periodic table, relative to its original position.
(iii) It occupies two places behind in the periodic table, relative to its original
position.
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16. State Rutherford and Soddy’s Laws of
(i) α-emission
(ii) β-emission
(iii) γ-emission
Ans. (i) Law of α-emission : When a radioactive nucleus ejects an α-particle, its
mass number decreases by 4 a.m.u. and atomic number by 2 a.m.u, such that
it occupies a position, two places behind in periodic table as compared to its
original position.
(ii) Law of β-emission : When the nucleus of an element ejects a β particle, its
mass number remains the same, but its atomic number increases by one a.m.u.
and hence its position is one place ahead in the periodic table, as compared to
its original position.
(iii) Law of γ-radiation : During gamma emission, the mass number and atomic
number remain unchanged and hence no new element is formed.
17. State two differences between chemical change and nuclear change.
Ans. (i) During chemical change no new elements are formed, whereas during nuclear
change new elements are formed.
(ii) During chemical change no harmful radiations are given out, whereas during
nuclear change, harmful radiations are given out.
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18. State four properties of (i) α -particles, (ii) β-particles, (iii) γ-radiations.
Ans. (i) Properties of α-particles
(a) They are doubly ionised helium nuclei having a mass equal to
6.64 × 10–27 kg and charge equal to 3.2 × l0–19 C.
(b) They have the highest ionising power.
(c) They have the least penetrating power.
(d) They produce fluorescence in the zinc sulphide screen.
(ii) Properties of β-particles.
(a) They are negatively charged particles, having a charge equal to
–1.6 × 10– 19 C and mass 9.1 × 10–31 kg.
(b) They have more penetration power than α-particles.
(c) They have less ionising power than α-particles.
(d) They affect photographic plate.
(iii) Properties of γ-radiations.
(a) They are electromagnetic waves, having wavelength less than 0.01 Å.
(b) They have a very high penetration power.
(c) They have a very low ionising power.
(d) They are not affected by electric or magnetic fields.
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19. Compare the : (a) ionising power, (b) penetration power of α, β and γ-particles.
Ans. (a) Ionising power : If one unit is ionising power of γ-radiations, then 100 units
is ionising power of β-particles and 10000 units is the ionising power of
α-particles.
(b)Penetration power : γ-radiations can penetrate even 30 cm of iron. β-particles
can penetrate 1mm of lead or 5 mm of aluminium. α-particles can penetrate
2.7 to 8.6 cm of air.
20. When does the nucleus of an atom tends to be radioactive ?
Ans. (i) When the atomic number of an element is more than 82, it tends to be
radioactive.
(ii) When the nucleus of any element has imbalance of neutrons as compared to
normal atoms, it tends to be radioactive.
21. (i) Name three types of radiations emitted by a radioactive substance.
(ii) Name the radiations which have charge.
(iii) Name the type of radiations which are most penetrating.
(iv) Name the type of radiations which travel with the speed of light.
Ans. (i) α-radiations; β-radiations and γ- radiations
(ii) α and β-radiations
(iii) γ- radiations
(iv) γ-radiations
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22. (i) What is the nature of gamma radiations ?
(ii) How is it possible for an element to decay into another element of higher
atomic number ?
(iii) Is it possible for hydrogen atom isotope to emit alpha particle ? Explain.
Ans. (i) γ-radiations are electromagnetic in nature and as such obey the laws of
reflection, refraction and are not affected by electric or magnetic fields.
(ii) By ejecting a β-particle, the nucleus of an element can decay into another
element higher in atomic number.
(iii) No, it is not possible. It is because alpha particle consists of two protons and
two neutrons. However, none of the isotopes of hydrogen has the same number
or more of protons and neutrons in its nuclei. Thus, they cannot emit an alpha
particle.
23. What are α, β and γ radiations composed of ? Which of these rays have the least
penetration power?
Ans. Refer to answer no. 18.
α-radiations have the least penetrating power.
24. Radioactive isotope of sodium
24
11 Na ,
changes to stable
24
12 Mg .
Which particle
does it eject ? Represent nuclear reaction in the form of equation.
Ans. Radioactive sodium ejects β -particle
24
11 Na
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10
24
12 Mg
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25. How do infra-red and γ -rays differ in their properties?
Ans. (i) The wavelength of infra-red rays is of the order of 10–6 m or more whereas
wavelength of γ-rays is of the order of 10–13 m or less.
(ii) γ-rays are highly penetrating as compared to infra-red radiation.
26. State two differences and two similarities between X-rays and γ-rays.
Ans. Differences :
(i) The wavelength of X-rays is of the order of 1Å, whereas wavelength of γ-rays
is of the order of 10–3Å.
(ii) X-rays are less penetrating than γ-rays.
Similarities :
(i) Both are electromagnetic waves.
(ii) Both travel with the speed of 3 × 108 ms–1.
27. A radioactive sample is kept in the centre of a large evacuated sphere. How safe
will it be? What changes do you suggest for more safety?
Ans. The radioactive source is likely to emit α , β and γ radiations.
(i) The alpha radiations have the least penetrating power and hence are likely to
be absorbed by the walls of the sphere.
(ii) The beta radiations have a higher penetrating power and hence will pass out
of the walls of sphere. However, if the sphere is bigger and not evacuated, the
beta radiations will not pass out of it.
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(iii) γ -radiations have highest penetrating power, hence will easily pass out of the
wall of sphere. The radioactive-radiation can be stopped if
(i) sphere is made of thick lead walls, (ii) is filled with air, and (iii) has large
diameter.
28. A radioactive element AZ X loses two successive β-particles, followed by an
α-particle, such that the resulting nuclides is PQY. Find the values of P and Q.
–β
Ans. AZ X ⎯⎯→
∴ PQY =
A
z +1 X1
–β
⎯⎯→ z +
–α
A
z +2 X 2
⎯⎯→
A–4
z X3
A–4
z X3
∴ P = A – 4; Q = Z.
29. An imaginary radioactive element
235
92
X decays to form X1, X2, X3, X4, X5, and
X6 nuclei, by ejecting two β-particles, followed by α-particle and again two
β- particles, followed by an α-particle. Represent the various nuclear changes
in the form of an equation. State the mass number and atomic number of X6.
List the isotopes and isobars in the above nuclear equation.
Ans.
235
92 X
–β
–β
235
93 X1
231
93 X 4
–β
–β
–
235
94 X 2
231
94 X 5
–
231
92 X 3
227
92 X 6
Mass number of X6 is 227 and atomic number of X6 is 92.
Isotopes {(a)
Isobars {(a)
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235
231
227
92 X; 92 X 3; 92 X 6
235
235
235
92 X; 93 X1; 94 X 2
(b)
(b)
235
231
94 X 2; 94 X 5
231
231
231
92 X 3; 93 X 4; 94 X 5 }
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30. Briefly describe Geiger counter.
Ans. Geiger counter consists of a metal cylinder in which is fixed to a thin metallic
wire along its axis. The mouth of cylinder is closed by a thin glass or mica
window. In the cylinder argon gas is filled with traces of chlorine at a pressure
of 100 mm.
The electrical contacts are made as shown in the diagram and potential difference
between cylinder and wire is kept slightly below than necessary to produce
discharge in gas.
When any charged particle enters the cylinder through glass window, it ionises
the gas, thereby producing ionisation current which is momentary in nature.
This momentary current is amplified which in turn advances electronic counter.
31. What is background radiation? How can you detect background radiation?
Ans. The radiation which is omnipresent everywhere on the Earth because of
radioactive substances present in the Earth and the cosmic particles in the
atmosphere is called background radiation. These radiations can be detected
by Geiger counter and their rate is constant for a given place.
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32. How will you count the true rate of emission from a radioactive source by
Geiger counter?
Ans. First, record the background radiation by Geiger counter. Let the count rate
be ‘n’. Now count the rate of emission from a given radioactive source.
Let the count rate be N.
True rate of emission = N – n.
33. (i) What is the radioactivity?
(ii) A radioactive substance is oxidised. What change would you expect to take
place in the nature of radioactivity? Give one reason for your answer.
Ans. (i) The property due to which the nucleus of an element decays into the nucleus
of some other element, with the ejection of alpha particles or beta particles or
gamma radiations is called radioactivity.
(ii) No change will take place in the nature of radioactivity. It is because
oxidation is the property of electrons in the outermost shell of atoms
where the nucleus is not affected by any means.
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34. (i) What are beta rays and gamma rays?
(ii) Explain briefly what changes take place within the nuclei, when the beta
particle is emitted by radioactive substance.
Ans. (i) β -particles are negatively charged particles having a charge of –1.6 × 10–19C
and mass 9.106 × 10–31 kg. They are emitted by the nucleus of the atom
when a neutron decays into a proton and electron.
γ -rays are emitted by the nucleus of element when radioactive decay takes
place. They are formed when a part of nuclear mass changes into energy.
(ii) The electron is ejected as β -particle, but proton is retained by nucleus. Thus,
element decays into another element whose position is one place ahead in the
periodic table.
35. Why are the alpha particles not used in radiotherapy?
Ans. The alpha particles have very large atomic volume and hence do notpenetrate
human skin.
36. Why do we usually use isotopes emitting gamma radiation as radioactive
tracers in medical science?
Ans.Gamma radiations are highly penetrating. Thus, when isotopes emitting gamma
radiation are injected in the body, the γ -radiation emitted by them can be easily
detected in different parts of the body.
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37. What are radio-isotopes? State one use of radio-isotopes.
Ans. The isotopes of some elements having atomic number less than 82, such that
they emit radioactive radiations are called radio-isotopes.
Radio-sodium chloride is used as traces in the detection of cancer and brain
tumour.
38. Which radiation produces maximum biological damage. What precautions are
necessary while handling the source of their radiation? State safety precautions
in handling radioactive material.
Ans.Gamma radiation produces the maximum biological damage. Following
precautions are necessary in handling the source of gamma radiation.
(1) The radioactive substance should be kept in thick walled lead containers with
a very narrow opening, which could be plugged by a lead stopper.
(2) The radioactive material should be handled with lead tongs and the handler
must wear lead lined clothes.
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