Chapter 13: Nuclei Short Questions | physics Class 12 CBSE

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Chapter 13: Nuclei Short Questions | physics Class 12 CBSE | ByteNotes.in

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Question 1

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Define atomic mass unit (u). What is its value in kilograms?

The atomic mass unit (u) is defined as 1/12th of the mass of one carbon-12 (¹²C) atom.
Its value in SI units is 1 u = 1.660539 × 10⁻²⁷ kg.
This unit is used because atomic masses are extremely small and kilograms are inconvenient for such measurements.

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Question 2

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Distinguish between isotopes, isobars, and isotones with one example each.

Isotopes are nuclides of the same element with same Z but different A (different neutrons), e.g., ¹H and ²H.
Isobars are nuclides of different elements with same A but different Z, e.g., ³₁H and ³₂He.
Isotones are nuclides with same neutron number N but different Z, e.g., ¹⁹⁸₈₀Hg and ¹⁹⁷₇₉Au.

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Question 3

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State the relation between nuclear radius and mass number. What does this imply about nuclear density?

The nuclear radius is given by R = R₀A^(1/3), where R₀ = 1.2 × 10⁻¹⁵ m and A is the mass number.
Since R³ ∝ A, the volume of the nucleus is proportional to A.
Because mass is also proportional to A (each nucleon has nearly equal mass), the nuclear density is constant and independent of A, approximately 2.3 × 10¹⁷ kg/m³.

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Question 4

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What is mass defect? Write the expression for mass defect of a nucleus with atomic number Z and mass number A.

Mass defect is the difference between the sum of masses of individual nucleons and the actual measured mass of the nucleus.
It is expressed as ΔM = [Zmp + (A–Z)mn] – M, where mp is proton mass, mn is neutron mass, and M is the nuclear mass.
The mass defect arises because some mass is converted into binding energy that holds the nucleus together, consistent with E = mc².

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Question 5

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What is nuclear binding energy? How is it related to mass defect?

Nuclear binding energy Eb is the energy required to completely separate a nucleus into its constituent protons and neutrons.
It is related to the mass defect ΔM by the Einstein relation: Eb = ΔMc².
Equivalently, it is the energy released when free nucleons combine to form the nucleus; it represents the stability of the nucleus.

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Question 6

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Define binding energy per nucleon. Why is it considered a better measure of nuclear stability than total binding energy?

Binding energy per nucleon, Ebn = Eb/A, is the average energy per nucleon required to completely disintegrate a nucleus into free nucleons.
It is a better measure because it allows direct comparison of stability across nuclei of different sizes.
A higher Ebn indicates a more tightly bound and more stable nucleus; it accounts for the varying number of nucleons in different nuclei.

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Question 7

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State any two main features of the binding energy per nucleon versus mass number curve.

Ebn is nearly constant at about 8 MeV/nucleon for nuclei with mass number between 30 and 170, indicating comparable stability in this range.
Ebn peaks at about 8.75 MeV near A = 56 (iron), making iron-group nuclei the most stable.
Ebn is lower for both very light nuclei (A < 30) and very heavy nuclei (A > 170), which explains why both fission and fusion can release energy.

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Question 8

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Write the energy equivalent of 1 atomic mass unit in MeV. How is this useful in nuclear physics?

1 u = 931.5 MeV/c², meaning 1 atomic mass unit is equivalent to an energy of 931.5 MeV.
This conversion is derived using E = mc² with m = 1.6605 × 10⁻²⁷ kg, giving energy ≈ 1.4924 × 10⁻¹⁰ J = 931.5 MeV.
This is extremely useful for calculating binding energies and Q-values of nuclear reactions directly from mass data given in atomic mass units.

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Question 9

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List any four important properties of nuclear force.

The nuclear force is much stronger than both the Coulomb force and gravitational force, enabling it to overcome electrostatic repulsion between protons.
It is short-ranged, becoming negligible for nucleon separations greater than a few femtometres (about 2–3 fm).
It is charge-independent: the force between neutron-neutron, proton-neutron, and proton-proton pairs is approximately equal.
The nuclear force is attractive for separations greater than 0.8 fm and repulsive for separations less than 0.8 fm, preventing nucleons from merging.

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Question 10

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How was the neutron discovered? Who discovered it and in which year?

James Chadwick discovered the neutron in 1932 by bombarding beryllium nuclei with alpha particles, which produced neutral radiation.
This neutral radiation could knock out protons from light nuclei (He, C, N). Chadwick showed it could not be photons, as energy-momentum conservation would require impossibly high photon energies.
He concluded the radiation consisted of neutral particles with mass nearly equal to that of a proton — these were named neutrons. He was awarded the Nobel Prize in 1935 for this discovery.

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Question 11

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What is radioactivity? Who discovered it and under what circumstances?

Radioactivity is the spontaneous emission of alpha, beta, or gamma radiation by the nucleus of an unstable atom.
It was discovered by A.H. Becquerel in 1896 accidentally while studying fluorescence of uranium-potassium sulphate compounds.
Becquerel noticed that uranium compounds could blacken a photographic plate even through black paper and silver, indicating penetrating spontaneous radiation not dependent on external stimulation.

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Question 12

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Name the three types of radioactive decay. State the particle emitted in each type.

Alpha (α) decay: the nucleus emits a helium-4 nucleus (⁴₂He), which consists of 2 protons and 2 neutrons.
Beta (β) decay: the nucleus emits an electron (β⁻) or a positron (β⁺), along with an antineutrino or neutrino respectively.
Gamma (γ) decay: the nucleus in an excited state emits high-energy electromagnetic photons (gamma rays) with wavelengths shorter than X-rays.

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Question 13

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What is the Q-value of a nuclear reaction? How is it expressed in terms of masses?

The Q-value of a nuclear reaction is defined as: Q = (final kinetic energy) – (initial kinetic energy) of the particles involved.
By conservation of mass-energy, it can also be written as: Q = (sum of initial masses – sum of final masses) × c².
If Q > 0, the reaction is exothermic (energy is released); if Q < 0, the reaction is endothermic (energy must be supplied).

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Chapter 13: Nuclei | Short Questions

Define atomic mass unit (u). What is its value in kilograms?

Distinguish between isotopes, isobars, and isotones with one example each.

State the relation between nuclear radius and mass number. What does this imply about nuclear density?

What is mass defect? Write the expression for mass defect of a nucleus with atomic number Z and mass number A.

What is nuclear binding energy? How is it related to mass defect?

Define binding energy per nucleon. Why is it considered a better measure of nuclear stability than total binding energy?

State any two main features of the binding energy per nucleon versus mass number curve.

Write the energy equivalent of 1 atomic mass unit in MeV. How is this useful in nuclear physics?

List any four important properties of nuclear force.

How was the neutron discovered? Who discovered it and in which year?

What is radioactivity? Who discovered it and under what circumstances?

Name the three types of radioactive decay. State the particle emitted in each type.

What is the Q-value of a nuclear reaction? How is it expressed in terms of masses?

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