# Classification of Elements and Periodicity in Properties – 5

**1)** A negative value for the packing fraction shows that the isotope is

**a)** Unstable

**b)** Very stable

**c)** Artificial

**d)** Stable

**2)** The packing fraction may be

**a)** Positive

**b)** Negative

**c)** Zero

**d)** All the above

**3)** Which does not take place by disintegration?

**a)**

**b)**

**c)**

**d)**

**4)** By removing one -particle from a radioactive isotope, the atomic number

**a)** Increases by one

**b)** Decreases by one

**c)** No change

**d)** Decreases by two

**5)** An isotone of _{32}Ge^{76} is

**a)**

**b)**

**c)**

**d)**

**6)** Bismuth is the end product of the radioactive disintegration series

**a)** 4n

**b)** 4n+l

**c)** 4n+2

**d)** 4n+3

**7)** An isobar of ^{40}_{20}Ca is

**a)**

**b)**

**c)**

**d)**

**8)** A radioisotope has a half-life of 10 years. What percentage of the original amount of it would you expect to remain after 20 years?

**a)** 0

**b)** 12.5

**c)** 25

**d)** 8

**9)** The thorium series starts from Th-232 and ends at

**a)** Pb-208

**b)** Bi-209

**c)** Pb-206

**d)** Pb-207

**10)** With the passage of time, radioactive disintegration

**a)** Increases

**b)** Decreases

**c)** Remains same

**d)** May increase or decrease

**11)** The most radioactive element is

**a)** Uranium

**b)** Thorium

**c)** Radium

**d)** Polonium

**12)** The loss of an -particle is equivalent to the

**a)** Loss of two neutrons

**b)** Loss of two protons

**c)** Loss of two neutrons and two protons

**d)** None of these

**13)** The loss of a -particle is equivalent to the

**a)** Increase of one proton

**b)** Decrease in one neutron

**c)** Combination of both (a) and (b)

**d)** None of these

**14)** How many a-particles are emitted in the nuclear transformation

^{215}_{84}Po^{211}_{82}Pb?

**a)** One

**b)** Two

**c)** Three

**d)** Four

**15)** Of the following radiations, the one most easily stopped by air is

**a)** X-rays

**b)**

**c)**

**d)** Ultra violet rays

**16)** @ The packing fraction is related to the

**a)** Mass defect

**b)** Atomic number

**c)** Nuclear spin

**d)** None

**17)** The total energy given out during the binding up of nucleons in the nucleus is called the

**a)** Nuclear energy

**b)** Binding energy

**c)** Packing fraction

**d)** Fusion energy

**18)** An atom of radium combines with two atoms of chlorine to form RaCl_{2} molecule. The radioactivity of RaCl_{2} will be

**a)** Zero

**b)** One half of the same quantity of Ra

**c)** One third of the same quantity of Ra

**d)** As much as that of the same quantity of Ra

**19)** Which of the following nuclear reactions is correct?

**a)**

**b)**

**c)**

**d)**

**20)** The disintegration constant of a radioactive element is 1.7 x 10^{-6} per sec. Its half-life will be

**a)** 5 days

**b)** 5 hours

**c)** 5 months

**d)** 5 years

**21)** The half-life period of a radioactive nuclide is 3 hours. In 9 hours its activity will be reduced to

**a)** 1/9

**b)** 7/8

**c)** 1/27

**d)** 1/6

**22)** If 12 g of a sample is taken, then 6 g of a sample decays in one hour. The amount of sample showing decay in the next hour is

**a)** 6 g.

**b)** 12 g.

**c)** 3 g.

**d)** 1 g.

**23)** Which of the following are isotonic?

**a)**

**b)**

**c)**

**d)**

**24)** A sample of a radioactive element with t_{1/2} of 11.2 years weighs 2.0 g. After 33.6 years how much is left?

**a)** 0.25 g

**b)** 0.5 g.

**c)** 0.75 g.

**d)** 1.0 g.

**25)** The emission of a beta particle by an atom of an element results in the formation of an

**a)** Isotope

**b)** Isomer

**c)** Isomorph

**d)** Isobar

**26)** The age of rocks on earth or the samples of rocks and dust brought back from the moon can be found by determining the proportion of radioactive … in the rock or dust

**a)** Potassium and stable calcium

**b)** Uranium and stable lead

**c)** Carbon and stable carbon

**d)** Radium and stable lead

**27)** Which is different in isotopes of an element?

**a)** Atomic number

**b)** Mass number

**c)** Number of protons

**d)** Number of electrons

**28)** The reaction ^{27}_{13}Al+ ^{4}_{2}He ^{30}_{14}Si+ ^{1}_{1}H is of the type called

**a)** Nuclear fusion

**b)** Nuclear fission

**c)** Chemical

**d)** Transmutation

**29)** When ^{7}_{3}Li is bombarded with protons, – rays are produced. The nuclide formed is

**a)**

**b)**

**c)**

**d)**

**30)** A certain radioactive isotope has a half-life of 50 days. The fraction of the material left behind after 100 days will be

**a)** 50%

**b)** 75%

**c)** 125%

**d)** 25%

**31)** ^{14}_{6}C in the upper atmosphere is formed by the action of neutrons on

**a)**

**b)**

**c)**

**d)**

**32)** Atoms with the same mass number but having different nuclear charges are called

**a)** Isotopes

**b)** Isobars

**c)** Isochores

**d)** Isotones

**33)** One Curie of activity is equivalent to

**a)**

**b)**

**c)**

**d)** None

**34)** When a radioactive substance is subjected to a vacuum, the rate of disintegration per second

**a)** Increases considerably

**b)** Increases only if the products are gaseous

**c)** Is not affected

**d)** Suffers a slight decrease

**35)** In – decay, the

**a)** N/P decreases

**b)** N/P increases

**c)** N/P remains constant

**d)** None

**36)** The half-life of a radioactive isotope is 1.5 hours. The mass of it that remains undecayed after 6 hours is (the initial mass of the isotope is 32 g)

**a)** 32g

**b)** 16g

**c)** 8g

**d)** 2g

**37)** -rays given off by a radioactive element are from the

**a)** Nucleus

**b)** Valency electrons

**c)** Outer orbit

**d)** All the orbits

**38)** The half-life period of a substance is 1600 minutes. What fraction of the substance will remain after 6400 minutes?

**a)** 1/16

**b)** 1/4

**c)** 1/8

**d)** 1/2

**39)** ^{238}_{92}U is an isotope of uranium. The number of neutrons it can contain is

**a)** 156

**b)** 126

**c)** 146

**d)** 136

**40)** A radioactive element has a half-life of 4.5 x 10^{9} years. If 80 g of this was taken, the time taken for it to decay to 40 g will be

**a)**

**b)**

**c)**

**d)**

**41)** If 8.0 g. of radioactive isotope has a half-life of 10 hours, the half-life of 2.0 g. of the same substance is

**a)** 2.5 hours

**b)** 5.0 hours

**c)** 10 hours

**d)** 40 hours

**42)** A substance is kept for 2 hours, and three fourth of it disintegrates during this period. The half-life of the substance is

**a)** 2 hours

**b)** 1 hour

**c)** 30 minutes

**d)** 2 hours

**43)** 16 g of a radioactive isotope is reduced to 1.0g after one hour. The half-life period of the substance is

**a)** 15 minutes

**b)** 30 minutes

**c)** 45 minutes

**d)** None

**44)** Starting from radium, the radioactive disintegration process terminates when the following is obtained:

**a)** Lead

**b)** Radon

**c)** Radium A

**d)** Radium B

**45)** During – decay,

**a)** An atomic electron is ejected

**b)** An electron which is already present within the nucleus is ejected

**c)** A neutron in the nucleus decays emitting an electron

**d)** A part of the bending of the nucleus is converted into an electron

**46)** The order of penetration of three radioactive rays through a metal sheet is

**a)**

**b)**

**c)**

**d)** None

**47)** Two nuclei are not identical but have the same number of nucleons. These are called

**a)** Isotopes

**b)** Isobars

**c)** Isotones

**d)** None

**48)** The half-life of a radioactive nuclide is 10 months. The fraction of the substance left behind after 40 months is

**a)** 1/2

**b)** 1/4

**c)** 1/8

**d)** 1/16

**49)** The reciprocal of the radioactive decay constant is called the

**a)** Half life period

**b)** Whole life period

**c)** Average life period

**d)** None.

**50)** Which of the following statements is false?

**a)** The atom has a tiny, dense centre or core called nucleus.

**b)** Most of the volume of the atom is empty space

**c)** The nucleus has a small negative electrical charge.

**d)** The electrons revolve around the nucleus.

**51)** Radium has an atomic weight 226 and a half-life of 1600 years. The number of disintegration produced per sec from one g are

**a)** ^{}

**b)**

**c)**

**d)**

**52)** In radioactive decay an emitted electron comes from

**a)** The nucleus of the atom

**b)** The orbit with principal quantum number

**c)** The inner orbital of the atom

**d)** The outermost orbit of the atom

**53)** Which of the following has a magic number of neutrons?

**a)**

**b)**

**c)**

**d)**

**54)** The atomic number of bromine is 35 and its mass number is 80. Two isotopes of bromine are present in equal amounts. Which of the following statement represents the correct number of neutrons in the first and second isotope respectively?

**a)** 34, 36

**b)** 44, 46

**c)** 45, 47

**d)** 79, 81

**55)** In nuclear reactors control rods are used to

**a)** Increase the rate of reaction.

**b)** Decrease the rate of reaction.

**c)** Increase or decrease the rate of reaction .

**d)** Control the temperature

**56)** What is enriched uranium?

**a)** U-238

**b)** U-235

**c)** U-235 + Radium

**d)** U-235 + U-238

**57)** For a nuclear fusion reaction to occur, the temperature required is of the order of

**a)**

**b)**

**c)**

**d)**

**58)** Magic number elements are those isotopes of elements which

**a)** have the number of protons or neutrons as 2, 8, 20, 28, 50, 82, or 126.

**b)** Are relatively more abundant.

**c)** Are unusually stable.

**d)** All of these

**59)** has n/p ratio too low for stability. Its stability can be increased by

**a)** Positron emission

**b)** Beta – decay

**c)** Alpha – decay

**d)** Electron capture

**60)** The projectile experiencing the minimum repulsion on approaching a nucleus is a

**a)**

**b)** Proton

**c)** Neutron

**d)** Positron

**61)** Lead containers are generally used to store radioactive materials, because

**a)** Pb is stable

**b)** Pb has high atomic mass

**c)** Pb is poor conductor

**d)** Pb is a good absorber

**62)** The half-life is 20 days. After 40 days, the amount left is

**a)** 1/2

**b)** 1/4

**c)** 1/8

**d)** 1/16

**63)** A radioactive nuclide dissociates into 1/16th of the initial amount in 2 hours. Its half-life is

**a)** 1 hour

**b)** 30 minutes

**c)** 15 minutes

**d)** 45 minutes

**64)** A radioactive element has a half-life of one day. After three days the amount of the element left will be

**a)** 1/2 of the original

**b)** 1/4 of the original

**c)** 1/8 of the original

**d)** 1/16 of the original

**65)** The missing particle in the reaction + — is a

**a)** Deutron

**b)** Proton

**c)** Neutron

**d)**

**66)** If 75% of a first order reaction is completed in 32 minutes, then 50% of the reaction would be completed in?

**a)** 24 Minutes

**b)** 16 Minutes

**c)** 8 Minutes

**d)** 4 Minutes

**67)** The half life of ^{226}Ra is 1620 years. The time taken to convert 10 g of radium to1.25 g is

**a)** 810 years

**b)** 1620 years

**c)** 3240 years

**d)** 4860 years

**68)** A radioactive substance has a half-life of 5 days. After 20 days it was found that 3 g of the isotope was left in the container. The initial weight of the isotope was

**a)** 48g

**b)** 36g

**c)** 18 g

**d)** 24 g

**69)** 3/4 of a radioactive material decays in 2.5 days. How long will it take for 15/16th of the material to decay?

**a)** 2 days

**b)** 5 days

**c)** 7.5 days

**d)** 10 days

**70)** If the amount of radioactive substance is increased three times, the number of disintegrating atoms per unit time will

**a)** Be doubled

**b)** Triple

**c)** Be one third

**d)** Be unchanged

**71)** The end product of the (4n+2) radioactive disintegration series is

**a)**

**b)**

**c)**

**d)**

**72)** The radioactive isotope which is used to estimate the age of ancient geological formations is

**a)**

**b)**

**c)**

**d)**

**73)** In the following radioactive decay, , how many are ejected from X to form Y?

**a)**

**b)**

**c)**

**d)**

**74)** The symbol X in the following equation is:

**a)** A neutron

**b)** A deutron

**c)** A position

**d)**

**75)** Which element is the end product of every natural radioactive series?

**a)** Pb

**b)** Sn

**c)** C

**d)** Bi

**76)** In radioactive decay, which one of the following moves the fastest?

**a)**

**b)**

**c)**

**d)** Positron

**77)** Which one of the following notations shows the product incorrectly ?

**a)**

**b)** ^{}

**c)**

**d)**

**78)** Which is not emitted by a radioactive substance?

**a)**

**b)**

**c)** Positron

**d)** Proton

**79)** A radioactive element X emits and forms. The element X is

**a)**

**b)**

**c)**

**d)**

**80)** Which of the following is used as a control rod?

**a)** Cadmium rod

**b)** Graphite rod

**c)** Boron

**d)** Both (a) and (c)

**81)** After emission of one alpha and one particle from the number of neutrons in the atom will be

**a)** 142

**b)** 140

**c)** 144

**d)** 143

**82)** @ 1 g of a radioactive element reduces to 125 mg. after 24 hours. The half-life of the isotope is —-?

**a)** 8 hours

**b)** 24 hours

**c)** 6 hours

**d)** 4 hours

**83)** In the reaction , the x is a

**a)** Proton

**b)** Positron

**c)** Electron

**d)** Neutron

**84)** In nuclear reactors, the speed of the neutron is slowed down by

**a)** Heavy water

**b)** Ordinary water

**c)** Zinc rod

**d)** Molten caustic potash.

**85)** In the reaction represented by the sequence, the decays in the sequence are:

**a)**

**b)**

**c)**

**d)** ^{+}

**86)** Of the three isotopes of hydrogen and .,

**a)** All are stable

**b)** Two are unstable

**c)**

**d)**

**87)** The radiation which can penetrate through a 20-cm thick steel plate is

**a)**

**b)**

**c)**

**d)** Positive rays

**88)** In the sun the source of energy is

**a)** Fusion of heavy nuclei

**b)** Fission of heavy nuclei

**c)** Fusion of hydrogen atom

**d)** Fission of uranium atom

**89)** Thermal neutrons have energy of the order of

**a)** 1 Mev

**b)** 1 eV

**c)** 2 eV

**d)** 100 eV

**90)** The huge amount of energy, which is released during atomic fission, is due to the

**a)** Loss of mass

**b)** Loss of proton

**c)**

**d)** Loss of electrons

**91)** In nuclear reactors heavy water is used as a

**a)** Fuel

**b)** Projectile

**c)** Moderator

**d)** Arestor

**92)** The half-life of radioactive radon is 3.8 days. The time at the end of which 1/20th of the radon sample will remain undecayed is about

**a)** 1.6 days

**b)** 16.4 days

**c)** 20 days

**d)** 3.8 x 20 years

**93)** A radioactive element has a t_{1/2} of 60 minutes. The amount left after 3 hours is :

**a)** 17.5%

**b)** 12.5%

**c)** 25%

**d)** 50%

**94)** @ The ratio of the radius of the atom to the nucleus is

**a)**

**b)**

**c)**

**d)**

**95)** A certain nuclide has a half-life of 30 minutes. If one starts with 48 g of it, the amount left at the end of 30 minutes will be

**a)** 24 g

**b)** 16 g

**c)** 8 g

**d)** 6 g

**96)** The binding energy of an element is 64 MeV. If the binding energy per nucleon is 6.4, the number of nucleons is

**a)** 64

**b)** 6

**c)** 10

**d)** 16

**97)** A radioactive isotope having a half-life of three days was received after 12 days. It was found that there was 3 g left in the container. The initial weight of the isotope when packed was

**a)** 12 g

**b)** 24 g

**c)** 36 g

**d)** 48 g

**98)** A sample of radioactive isotope with a half-life of 20 days weighs 1.0 g. After 40 days, the weight of the remaining element is

**a)** 0.5 g

**b)** Zero

**c)** 0.25 g

**d)** 1/6 g

**99)** If 2.0 g of an isotope has a half-life of 20 days, the half-life of 2.3 g of the same sample is

**a)** 20 days

**b)** 10 days

**c)** 40 days

**d)** 5 days

**100)** If 5.0 g. of a radioactive substance has t_{1/2} =14 hours, 20 g. of the same sample will have a t_{1/2 }of

**a)** 56 hr

**b)** 3.5 hr

**c)** 14 hr

**d)** 28 hr

Answer

**Ans 1)** d

**Ans Desc 1)** For stable elements the packing fraction is either zero or negative.

**Ans 2)** d

**Ans Desc 2)** The packing fraction may be zero, positive or negative.

**Ans 3)** d

**Ans Desc 3)** ^{A}_{Z}X – _{2}He^{4}_{ Z-2}Y^{ A-4}.

**Ans 4)** a

**Ans Desc 4)** The emission of a -particle increases the atomic number by one unit, but the mass number remains the same.

**Ans 5)** b

**Ans Desc 5)** Isotones have same number of neutrons. Both ^{76}_{32}Ge and ^{77}_{33}As have the same number (44) of neutrons.

**Ans 6)** b

**Ans Desc 6)** Bismuth is the end product of the 4n+l series, also known as the neptunium series.

**Ans 7)** a

**Ans Desc 7)** Isobars have same mass number but different atomic number.

**Ans 8)** c

**Ans Desc 8)** Number of half lives n = = 2. Amount left = = = = 0.25 or 25%.

**Ans 9)** a

**Ans Desc 9)** Thorium or 2n series ends at Pb-208.

**Ans 10)** b

**Ans Desc 10)** Because the rate of disintegration follows first order reaction kinetics.

**Ans 11)** c

**Ans Desc 11)**

**Ans 12)** c

**Ans Desc 12)** The loss of an -particle, ^{4}_{2}He, decreases the mass number by 4 units and the atomic number by 2 units. There is thus the loss of 2 protons and 2 neutrons.

**Ans 13)** a

**Ans Desc 13)** The loss of a beta particle increases the atomic number by one unit.

**Ans 14)** a

**Ans Desc 14)** ^{215}_{84}Po – ^{4}_{2}He^{211}_{82}Pb.

**Ans 15)** b

**Ans Desc 15)**

**Ans 16)** a

**Ans Desc 16)** Packing fraction = x 10^{4}.

**Ans 17)** b

**Ans Desc 17)**

**Ans 18)** d

**Ans Desc 18)** Radioactivity is a characteristic property of the unstable nuclei of Ra.

**Ans 19)** b

**Ans Desc 19)** The mass number and atomic number are the same on both the sides

**Ans 20)** a

**Ans Desc 20)** Half life (t_{1/2}) = = 0.408 x10^{6} sec

^{=} 4.72 days.

**Ans 21)** b

**Ans Desc 21)** No. of half lives, n = 9/3 = 3. Amount left = 1/2^{3} = 1/8.

Thus the activity is reduced by 7/8

**Ans 22)** c

**Ans Desc 22)** The half life is 1 hour. In each half-life, half of the sample (3.0 g.) decays.

**Ans 23)** a

**Ans Desc 23)** All contain 8 neutrons.

**Ans 24)** a

**Ans Desc 24)** The number of half lives n = = 3

Amount left = 2/2^{3} = 2/8 = 1/4 = 0.25 g.

**Ans 25)** d

**Ans Desc 25)** Self explanatory

**Ans 26)** b

**Ans Desc 26)** Uranium–lead dating technique.

**Ans 27)** b

**Ans Desc 27)** Isotopes have the same atomic number but different mass numbers.

**Ans 28)** d

**Ans Desc 28)** The reaction is artificial transmutation.

**Ans 29)** b

**Ans Desc 29)** ^{7}_{3}Li + ^{1}_{1}H ^{8}_{4}Be + -rays.

**Ans 30)** d

**Ans Desc 30)** No. of half lives n = = 2

Amount left = = = = 0.25 or 25%

**Ans 31)** a

**Ans Desc 31)** ^{14}_{7}N + ^{1}_{0}n^{14}_{6}C +^{1}_{1}H.

**Ans 32)** b

**Ans Desc 32)** Isobars have same mass number but different atomic number (nuclear charge)

**Ans 33)** b

**Ans Desc 33)**

**Ans 34)** c

**Ans Desc 34)** The rate of decay is independent of all external factors.

**Ans 35)** b

**Ans Desc 35)** In-decay N/P value increases. ^{A}_{Z}A – ^{4}_{2}He ^{A-4}_{Z-2}B

**Ans 36)** d

**Ans Desc 36)** No. of half lives = 6/1.5 = 4

Amount left = = = 2.0 g.

**Ans 37)** a

**Ans Desc 37)** The emission of -particles from a radioactive nucleus is due to the conversion of a neutron into a proton. ^{1}_{0}n^{1}_{1}H + ^{0}_{-1}e.

**Ans 38)** a

**Ans Desc 38)** No. of half lives n = 6400/1600 = 4. Amount left = 1/2^{4} = 1/16.

**Ans 39)** c

**Ans Desc 39)** No. of neutrons = 238 – 82 = 146

**Ans 40)** b

**Ans Desc 40)**

**Ans 41)** c

**Ans Desc 41)** Half-life is independent of the mass of the substance.

**Ans 42)** b

**Ans Desc 42)**

**Ans 43)** a

**Ans Desc 43)** 0.1 g. 16/2^{n }or 2^{n} = 16 or n=4. Half-life period = = 15 min.

**Ans 44)** a

**Ans Desc 44)**

**Ans 45)** c

**Ans Desc 45)** During decay a neutron in the nucleus is converted into electron,

**Ans 46)** a

**Ans Desc 46)**

**Ans 47)** b

**Ans Desc 47)** Isobars have the same number of nucleons (neutrons + protons). For example, and are isobars. The number of protons + neutrons in Ca isotope = 20 + 20 = 40. Similarly the number of protons + neutrons in an argon isotope = 18 + 22 =40.

**Ans 48)** d

**Ans Desc 48)** No. of half lives = 40/10 = 4.Amount left = 1/2^{4} = 1/16.

**Ans 49)** c

**Ans Desc 49)**

**Ans 50)** c

**Ans Desc 50)** The nucleus has no negative charge

**Ans 51)** b

**Ans Desc 51)**

**Ans 52)** a

**Ans Desc 52)** In radioactive decay a neutron in the nucleus is converted into a proton and an electron nH + e.

**Ans 53)** b

**Ans Desc 53)** The magic number for neutrons is 126.Bi has 209 – 83 = 126 neutrons.

**Ans 54)** b

**Ans Desc 54)** Two isotopes of Br and Br will result in an average mass of 80 if present in equal ratio.

**Ans 55)** c

**Ans Desc 55)** To control the rate of reaction.

**Ans 56)** b

**Ans Desc 56)** Enriched uranium is ^{235}U. It is refined uranium obtained from a mixture of ^{235}U and^{238}U.

**Ans 57)** d

**Ans Desc 57)** Nuclear fusion occurs at a very high temperature.

**Ans 58)** d

**Ans Desc 58)**

**Ans 59)** a

**Ans Desc 59)** Positron emission increases N/P ratio.

**Ans 60)** c

**Ans Desc 60)** Because it has no charge.

**Ans 61)** d

**Ans Desc 61)** Lead is a good absorber of radiations.

**Ans 62)** b

**Ans Desc 62)** 40/20 = 2, Amount left = = = 25%

**Ans 63)** b

**Ans Desc 63)**

**Ans 64)** c

**Ans Desc 64)** No. of half lives n = 3/1 = 3. Amount left = 1/2^{3} = 1/8.

**Ans 65)** c

**Ans Desc 65)**

**Ans 66)** b

**Ans Desc 66)**

**Ans 67)** d

**Ans Desc 67)** 10 g. 5g 2.5 g. 1.25 g. Total time1620 + 1620 + 1620 = 4860 years.

**Ans 68)** a

**Ans Desc 68)** No. of half lives = 20/5 = 4

Amount left = or 3.0 = or Initial concentration = 48 g.

**Ans 69)** b

**Ans Desc 69)** Or

**Ans 70)** b

**Ans Desc 70)** The number of disintegrating atoms per unit time is directly proportional to the amount of radioactive substance present.

**Ans 71)** b

**Ans Desc 71)** The end product of (4n+2) radioactive series is _{82}Pb^{206}

**Ans 72)** d

**Ans Desc 72)** C^{14} is used for the purpose.

**Ans 73)** a

**Ans Desc 73)** Decrease in atomic number = 92-89 = 3.

Decrease in mass number = 232-220 = 12.

Hence number of -particles () = 12/4 =3.

Number of -particles = 3×2-3 = 3.

**Ans 74)** a

**Ans Desc 74)** . The X is a neutron.

**Ans 75)** a

**Ans Desc 75)**

**Ans 76)** c

**Ans Desc 76)** The one having the least mass would move the fastest.

**Ans 77)** a

**Ans Desc 77)**

**Ans 78)** d

**Ans Desc 78)** A proton is not emitted by a radioactive substance.

**Ans 79)** a

**Ans Desc 79)** Y + 3(He) + 1(e) +. Taking atomic masses and atomic numbers into consideration we see that atomic number of X =81 and mass number of X is 237. So X is .

**Ans 80)** d

**Ans Desc 80)** Both cadmium and boron rods are used in nuclear reactors to prevent the fission chain reaction from becoming too violent and hence to prevent damage to the reactor by an explosion. The control rods also serve many other purposes.

**Ans 81)** d

**Ans Desc 81)** X – He XX. The number of neutrons = 234-91 = 143.

**Ans 82)** a

**Ans Desc 82)** N = [ ]^{n }N_{o }or 125 = [ ]^{n }1000 mg

[]^{n }= = or [ ]^{3
}Thus n = 3 (Half lives). The total time is 24hours. Â Half-life time = 24/3 = 8 hours.

**Ans 83)** c

**Ans Desc 83)** Np Pu + e. The X is an electron.

**Ans 84)** a

**Ans Desc 84)** Heavy water (D_{2}O) acts as a moderator.

**Ans 85)** d

**Ans Desc 85)** The sequence is .

**Ans 86)** d

**Ans Desc 86)**

**Ans 87)** a

**Ans Desc 87)** Gamma rays have the maximum power of penetration.

**Ans 88)** c

**Ans Desc 88)** The source of energy in sun is nuclear fusion, which is continuously going on.

**Ans 89)** b

**Ans Desc 89)** Self explanatory

**Ans 90)** a

**Ans Desc 90)** During atomic fission mass is converted into energy.

**Ans 91)** c

**Ans Desc 91)** D_{2}O is used as a moderator, the function of which is to slow down the speed of neutrons.

**Ans 92)** b

**Ans Desc 92)** T = log ; N = .

**Ans 93)** b

**Ans Desc 93)** Amount left = = = = 12.5%.

**Ans 94)** a

**Ans Desc 94)** Radius of atom – 10^{-10 }m; Radius of nucleus = 10^{-14 }m.

= = 10^{4 }: 1

**Ans 95)** a

**Ans Desc 95)** The half-life is the time in which half of the substance is decayed. So, amount left = 48/2=24g.

**Ans 96)** c

**Ans Desc 96)** BE per nucleon =

Thus, no. of nucleons = = 10

**Ans 97)** d

**Ans Desc 97)** = No. of half lives = = 4.

Now amount left = =

Thus initial conc = Amount left 16 = 316 = 48 g.

**Ans 98)** c

**Ans Desc 98)** No. of half lives n = 40/20 = 2. Amount left = 1/2^{2} = 1/4 = 0.25 g.

**Ans 99)** a

**Ans Desc 99)** Half-life does not depend on mass number.

**Ans 100)** c

**Ans Desc 100)** Half-life = 0.693/K. It is independent of the initial amount.