Nuclear Chemistry Questions and Answers

Q: Which among the following nuclei are fissile ?(I) 233U(II) 238U(III) 239Pu(IV) 235U

The fissile nuclei are (I), (III) and (IV): 233U, 239Pu and 235U.A fissile nucleus can be split by slow (thermal) neutrons and sustain a chain reaction. Nuclei with an odd number of neutrons, such as 233U (141 neutrons), 235U (143) and 239Pu (145), gain enough pairing energy on capturing a neutron to exceed the fission barrier, so even a slow neutron triggers fission.238U, with 146 neutrons, is not fissile. It fissions only with fast neutrons and is called fertile, because neutron capture turns it into 239Pu; likewise 232Th breeds 233U. Every option that includes (II) therefore lists a non-fissile nucleus.

Q: Which detector works on the principle of ionization of gas molecules ?

The GM (Geiger-Müller) counter works by ionization of gas molecules.A GM tube is filled with an inert gas such as argon with a quenching additive, at low pressure, with a high voltage between a central wire and the wall. Radiation entering the tube ionizes gas molecules; the electrons are accelerated towards the wire and cause an avalanche of further ionization, producing a large pulse that is counted.A scintillation counter detects flashes of light emitted by a phosphor (such as NaI(Tl)) when radiation strikes it, converted to pulses by a photomultiplier. Semiconductor and surface barrier detectors are solid-state devices in which radiation creates electron-hole pairs in silicon or germanium, not ions in a gas.

Q: As radioactivity is a random phenomenon, the error in measuring activity in CPM can be reduced by :

The error is reduced by measuring the activity for a longer period of time.Radioactive decay is random and follows Poisson statistics: if N counts are recorded, their standard deviation is √N. The relative error therefore falls as more counts are collected:\[ \frac{\sigma}{N} = \frac{\sqrt{N}}{N} = \frac{1}{\sqrt{N}} \]For example, 100 counts carry a 10% error, but 10,000 counts only 1%. Counting for longer collects more counts and so gives a more precise count rate in CPM.A short counting time collects fewer counts and increases the relative error. Counting at intermittent intervals does not by itself increase the total counts, and a detector's 'current' does not change the randomness of decay.

Q: Thermonuclear reactions are responsible for energy production in :

Thermonuclear reactions produce the energy of stars.Thermonuclear reactions are fusion reactions, in which light nuclei such as hydrogen isotopes join to form heavier ones. They need temperatures of millions of kelvin so that the nuclei move fast enough to overcome their electrical repulsion. Such conditions exist in the cores of stars, including the Sun, where hydrogen is fused into helium.Nuclear reactors and atom bombs release energy by fission, the splitting of heavy nuclei such as 235U and 239Pu. Volcanoes are driven by the Earth's internal heat, much of which comes from radioactive decay, not fusion.

Q: The radionuclide 135Te undergoes four successive β− emission to give the stable isotope :

The stable product is 135Ba.Each β− emission turns a neutron into a proton: the mass number stays at 135 and the atomic number rises by one. Tellurium is element 52, so four successive emissions give:\[ \begin{aligned} & {}^{135}_{52}\mathrm{Te} \rightarrow {}^{135}_{53}\mathrm{I} \rightarrow {}^{135}_{54}\mathrm{Xe} \\ & \rightarrow {}^{135}_{55}\mathrm{Cs} \rightarrow {}^{135}_{56}\mathrm{Ba} \end{aligned} \]135I, 135Xe and 135Cs are the intermediates after one, two and three emissions; each is radioactive. 135Ba, element 56, is the stable end of this fission-product chain.

Q: The radionuclide \({}^{22}_{11}\mathrm{Na}\) decays by a positron emission. The ratio of the atomic mass and atomic number of the resulting nuclide is given by :

The ratio is 22/10.In positron emission a proton becomes a neutron, emitting a positron and a neutrino. The mass number is unchanged and the atomic number falls by one:\[ \begin{aligned} & {}^{22}_{11}\mathrm{Na} \rightarrow \\ & {}^{22}_{10}\mathrm{Ne} + e^{+} + \nu \end{aligned} \]The product is neon-22, with mass number 22 and atomic number 10. 22/11 is the parent itself, and 21/10 or 21/11 would require the loss of a nucleon, which positron emission does not involve.

Q: The correct order of increasing half-life of radioactive isotopes of 93Np239, 90Th232, 88Ra226 and 6C14 is :

The increasing order of half-life is 93Np239 \(\lt\) 88Ra226 \(\lt\) 6C14 \(\lt\) 90Th232.The half-lives are: neptunium-239, about 2.4 days; radium-226, about 1,600 years; carbon-14, about 5,730 years; and thorium-232, about 14 billion years, longer than the age of the universe.The options putting carbon-14 first treat it as the shortest-lived, but it outlasts radium-226 by more than three times and Np-239 by about a million times. The option ending with Np-239 reverses the scale, since Np-239 is by far the shortest-lived isotope in the list.

Q: The atoms having same atomic mass but different atomic number are called :

Such atoms are isobars.Isobars have the same mass number but different atomic numbers, so they are different elements, for example \(^{40}_{18}\text{Ar}\) and \(^{40}_{20}\text{Ca}\).Isotopes have the same atomic number but different mass numbers (\(^{12}\text{C}\) and \(^{14}\text{C}\)). Isotones have the same number of neutrons. Isomers are compounds with the same molecular formula but different structures.

Q: The radionuclide 229Th belongs to the :

229Th belongs to the neptunium decay series.Alpha decay lowers the mass number by 4 and beta decay leaves it unchanged, so every member of a natural decay series has the same remainder when its mass number is divided by 4. The four series are the thorium (4n), neptunium (4n + 1), uranium (4n + 2) and actinium (4n + 3) series.\[ 229 = 4 \times 57 + 1 \]229 is of the 4n + 1 type, the neptunium series, which starts from 237Np. The name of the element is the trap: 229Th is not in the thorium series, which contains 232Th and other 4n nuclides.

11.

Which among the following nuclei are fissile ?

(I) ²³³U
(II) ²³⁸U
(III) ²³⁹Pu
(IV) ²³⁵U

A.(I) and (II)
B.(II) and (III)
C.(I), (III) and (IV)
D.(II), (III) and (IV)

Exam Year(s)

MH SET - 2021, SET Exam - 2021,

Answer: C

The fissile nuclei are (I), (III) and (IV): ²³³U, ²³⁹Pu and ²³⁵U.

A fissile nucleus can be split by slow (thermal) neutrons and sustain a chain reaction. Nuclei with an odd number of neutrons, such as ²³³U (141 neutrons), ²³⁵U (143) and ²³⁹Pu (145), gain enough pairing energy on capturing a neutron to exceed the fission barrier, so even a slow neutron triggers fission.

²³⁸U, with 146 neutrons, is not fissile. It fissions only with fast neutrons and is called fertile, because neutron capture turns it into ²³⁹Pu; likewise ²³²Th breeds ²³³U. Every option that includes (II) therefore lists a non-fissile nucleus.

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12.

Which detector works on the principle of ionization of gas molecules ?

A.Semiconductor detector
B.Scintillation counter
C.GM counter
D.Surface barrier detector

Exam Year(s)

MH SET - 2021, SET Exam - 2021,

Answer: C

The GM (Geiger-Müller) counter works by ionization of gas molecules.

A GM tube is filled with an inert gas such as argon with a quenching additive, at low pressure, with a high voltage between a central wire and the wall. Radiation entering the tube ionizes gas molecules; the electrons are accelerated towards the wire and cause an avalanche of further ionization, producing a large pulse that is counted.

A scintillation counter detects flashes of light emitted by a phosphor (such as NaI(Tl)) when radiation strikes it, converted to pulses by a photomultiplier. Semiconductor and surface barrier detectors are solid-state devices in which radiation creates electron-hole pairs in silicon or germanium, not ions in a gas.

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13.

As radioactivity is a random phenomenon, the error in measuring activity in CPM can be reduced by :

A.Measuring the activity for a longer period of time
B.Measuring the activity for a short period of time
C.Measuring the activity by increasing the current
D.Measuring the activity at intermittent time intervals

Exam Year(s)

MH SET - 2021, SET Exam - 2021,

Answer: A

The error is reduced by measuring the activity for a longer period of time.

Radioactive decay is random and follows Poisson statistics: if N counts are recorded, their standard deviation is √N. The relative error therefore falls as more counts are collected:

\[ \frac{\sigma}{N} = \frac{\sqrt{N}}{N} = \frac{1}{\sqrt{N}} \]

For example, 100 counts carry a 10% error, but 10,000 counts only 1%. Counting for longer collects more counts and so gives a more precise count rate in CPM.

A short counting time collects fewer counts and increases the relative error. Counting at intermittent intervals does not by itself increase the total counts, and a detector's 'current' does not change the randomness of decay.

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14.

Thermonuclear reactions are responsible for energy production in :

A.Nuclear reactors
B.Atom bombs
C.Volcanoes
D.Stars

Exam Year(s)

MH SET - 2021, SET Exam - 2021,

Answer: D

Thermonuclear reactions produce the energy of stars.

Thermonuclear reactions are fusion reactions, in which light nuclei such as hydrogen isotopes join to form heavier ones. They need temperatures of millions of kelvin so that the nuclei move fast enough to overcome their electrical repulsion. Such conditions exist in the cores of stars, including the Sun, where hydrogen is fused into helium.

Nuclear reactors and atom bombs release energy by fission, the splitting of heavy nuclei such as ²³⁵U and ²³⁹Pu. Volcanoes are driven by the Earth's internal heat, much of which comes from radioactive decay, not fusion.

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15.

The radionuclide ¹³⁵Te undergoes four successive β⁻ emission to give the stable isotope :

A.\({}^{135}\mathrm{I}\)
B.\({}^{135}\mathrm{Xe}\)
C.\({}^{135}\mathrm{Cs}\)
D.\({}^{135}\mathrm{Ba}\)

Exam Year(s)

MH SET - 2023, SET Exam - 2023,

Answer: D

The stable product is ¹³⁵Ba.

Each β⁻ emission turns a neutron into a proton: the mass number stays at 135 and the atomic number rises by one. Tellurium is element 52, so four successive emissions give:

\[ \begin{aligned} & {}^{135}_{52}\mathrm{Te} \rightarrow {}^{135}_{53}\mathrm{I} \rightarrow {}^{135}_{54}\mathrm{Xe} \\ & \rightarrow {}^{135}_{55}\mathrm{Cs} \rightarrow {}^{135}_{56}\mathrm{Ba} \end{aligned} \]

¹³⁵I, ¹³⁵Xe and ¹³⁵Cs are the intermediates after one, two and three emissions; each is radioactive. ¹³⁵Ba, element 56, is the stable end of this fission-product chain.

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16.

The radionuclide \({}^{22}_{11}\mathrm{Na}\) decays by a positron emission. The ratio of the atomic mass and atomic number of the resulting nuclide is given by :

A.\(\frac{22}{10}\)
B.\(\frac{22}{11}\)
C.\(\frac{21}{10}\)
D.\(\frac{21}{11}\)

Exam Year(s)

MH SET - 2023, SET Exam - 2023,

Answer: A

The ratio is 22/10.

In positron emission a proton becomes a neutron, emitting a positron and a neutrino. The mass number is unchanged and the atomic number falls by one:

\[ \begin{aligned} & {}^{22}_{11}\mathrm{Na} \rightarrow \\ & {}^{22}_{10}\mathrm{Ne} + e^{+} + \nu \end{aligned} \]

The product is neon-22, with mass number 22 and atomic number 10. 22/11 is the parent itself, and 21/10 or 21/11 would require the loss of a nucleon, which positron emission does not involve.

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17.

The correct order of increasing half-life of radioactive isotopes of ₉₃Np²³⁹, ₉₀Th²³², ₈₈Ra²²⁶ and ₆C¹⁴ is :

A.\({}_{6}\mathrm{C}^{14}\) \(\lt\) \({}_{93}\mathrm{Np}^{239}\) \(\lt\) \({}_{88}\mathrm{Ra}^{226}\) \(\lt\) \({}_{90}\mathrm{Th}^{232}\)
B.\({}_{90}\mathrm{Th}^{232}\) \(\lt\) \({}_{88}\mathrm{Ra}^{226}\) \(\lt\) \({}_{93}\mathrm{Np}^{239}\) \(\lt\) \({}_{6}\mathrm{C}^{14}\)
C.\({}_{93}\mathrm{Np}^{239}\) \(\lt\) \({}_{88}\mathrm{Ra}^{226}\) \(\lt\) \({}_{6}\mathrm{C}^{14}\) \(\lt\) \({}_{90}\mathrm{Th}^{232}\)
D.\({}_{88}\mathrm{Ra}^{226}\) \(\lt\) \({}_{90}\mathrm{Th}^{232}\) \(\lt\) \({}_{6}\mathrm{C}^{14}\) \(\lt\) \({}_{93}\mathrm{Np}^{239}\)

Exam Year(s)

MH SET - 2023, SET Exam - 2023,

Answer: C

The increasing order of half-life is ₉₃Np²³⁹ \(\lt\) ₈₈Ra²²⁶ \(\lt\) ₆C¹⁴ \(\lt\) ₉₀Th²³².

The half-lives are: neptunium-239, about 2.4 days; radium-226, about 1,600 years; carbon-14, about 5,730 years; and thorium-232, about 14 billion years, longer than the age of the universe.

The options putting carbon-14 first treat it as the shortest-lived, but it outlasts radium-226 by more than three times and Np-239 by about a million times. The option ending with Np-239 reverses the scale, since Np-239 is by far the shortest-lived isotope in the list.

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18.

The atoms having same atomic mass but different atomic number are called :

A.Isotopes
B.Isobars
C.Isomers
D.Isotones

Exam Year(s)

MH SET - 2024, SET Exam - 2024,

Answer: B

Such atoms are isobars.

Isobars have the same mass number but different atomic numbers, so they are different elements, for example \(^{40}_{18}\text{Ar}\) and \(^{40}_{20}\text{Ca}\).

Isotopes have the same atomic number but different mass numbers (\(^{12}\text{C}\) and \(^{14}\text{C}\)). Isotones have the same number of neutrons. Isomers are compounds with the same molecular formula but different structures.

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19.

The radionuclide ²²⁹Th belongs to the :

A.Thorium decay series
B.Uranium decay series
C.Actinium decay series
D.Neptunium decay series

Exam Year(s)

MH SET - 2025, SET Exam - 2025,

Answer: D

²²⁹Th belongs to the neptunium decay series.

Alpha decay lowers the mass number by 4 and beta decay leaves it unchanged, so every member of a natural decay series has the same remainder when its mass number is divided by 4. The four series are the thorium (4n), neptunium (4n + 1), uranium (4n + 2) and actinium (4n + 3) series.

\[ 229 = 4 \times 57 + 1 \]

229 is of the 4n + 1 type, the neptunium series, which starts from ²³⁷Np. The name of the element is the trap: ²²⁹Th is not in the thorium series, which contains ²³²Th and other 4n nuclides.

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