Nuclei — Class 12 MCQs with Answers

Nuclei — important questions & MCQs with answers (Class 12 Physics)

6 solved questions from this chapter's difficulty levels, each with its answer and explanation. Tap an option to check yourself.

0 of 6 answered

1. Q1 of 6Easy

   Nucleons are:

   AProtons onlyBElectronsCProtons and neutronsDNeutrons only
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   Answer: Protons and neutrons

   Explanation: Nucleons are the particles that make up the nucleus — protons and neutrons together; electrons orbit outside and aren't nucleons at all.

2. Q2 of 6Easy

   In α-decay, mass number A and atomic number Z change by:

   AA unchanged, Z by +1BA by −4, Z by −2CA unchanged, Z by −1DA by −2, Z by −4
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   Answer: A by −4, Z by −2

   Explanation: α-decay ejects a helium nucleus (2 protons, 2 neutrons), so mass number drops by 4 and atomic number drops by 2: A→A−4, Z→Z−2.

3. Q3 of 6Medium

   Radius of ²⁷Al nucleus is approximately (R₀ = 1.2 fm):

   A3.6 fmB1.2 fmC5.4 fmD32.4 fm
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   Answer: 3.6 fm

   Explanation: R = R₀A^(1/3) = 1.2 × 27^(1/3) = 1.2 × 3 = 3.6 fm, since 27^(1/3)=3 exactly — a handy cube-root shortcut for aluminium-27.

4. Q4 of 6Medium

   Assertion (A): Energy is released when a nucleus of mass number about 240 splits into two nuclei of mass number about 120. Reason (R): The binding energy per nucleon of a nucleus with A ≈ 120 is greater than that of a nucleus with A ≈ 240.

   ABoth A and R are true and R is the correct explanation of ABBoth A and R are true but R is not the correct explanation of ACA is true but R is falseDA is false but R is true
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   Answer: Both A and R are true and R is the correct explanation of A

   Explanation: A is true: this is nuclear fission, which releases about 200 MeV per nucleus. R is true (about 8.5 MeV against 7.6 MeV per nucleon) and explains A: the nucleons end up more tightly bound in the fragments, and the gain in binding energy is released.

5. Q5 of 6Hard

   Two nuclei have radius ratio 1:2. Ratio of their mass numbers:

   A1 : 2B1 : 8C1 : 4D2 : 1
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   Answer: 1 : 8

   Explanation: R ∝ A^(1/3), so A ∝ R³: a radius ratio of 1:2 cubes to a mass-number ratio of 1³:2³ = 1:8 — don't just double the ratio linearly.

6. Q6 of 6Hard

   Assertion (A): The reaction ¹²₆C + ¹²₆C → ²⁰₁₀Ne + ⁴₂He releases energy. Reason (R): The total mass of a ²⁰Ne atom and a ⁴He atom (19.992439 u + 4.002603 u) is less than the total mass of two ¹²C atoms (24.000000 u).

   ABoth A and R are true and R is the correct explanation of ABBoth A and R are true but R is not the correct explanation of ACA is true but R is falseDA is false but R is true
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   Answer: Both A and R are true and R is the correct explanation of A

   Explanation: A is true: Q = (24.000000 − 19.992439 − 4.002603) × 931.5 MeV = 0.004958 × 931.5 ≈ 4.62 MeV, which is positive. R is true (19.992439 + 4.002603 = 23.995042 u) and explains A: the products have less rest mass than the reactants, so Q = (initial mass − final mass)c² is positive and that much energy is released.

Nuclei notes: key concepts (Class 12 Physics)

The structure of the nucleus: its composition and size, the atomic mass unit and mass-energy equivalence, mass defect and binding energy with the curve that explains both fission and fusion, the three modes of radioactive decay and how each changes the nucleus, and the exponential decay law with half-life and mean life.

Composition of the nucleus

Protons and neutrons, collectively called nucleons. The atomic number counts protons and the mass number counts total nucleons.

Isotopes, isobars and isotones

Isotopes share the atomic number, isobars share the mass number, and isotones share the neutron number. Only isotopes have identical chemistry.

Size of the nucleus

The radius grows as the cube root of mass number, so nuclear VOLUME is directly proportional to the number of nucleons.

Nuclear density

Because volume scales with A, density is the same for every nucleus — about 2.3 × 10¹⁷ kg m⁻³, some fourteen orders of magnitude above ordinary matter.

Atomic mass unit

One twelfth of the mass of a carbon-12 atom, equal to 1.66 × 10⁻²⁷ kg and equivalent to 931.5 MeV of energy.

Mass defect

A nucleus weighs LESS than the sum of its free nucleons. The missing mass was converted into the energy that binds them together.

Binding energy

The energy equivalent of the mass defect — the work needed to pull a nucleus apart into free nucleons, or released when it is assembled.

Binding energy per nucleon

The proper measure of nuclear stability, peaking near 8.8 MeV around iron (A ≈ 56) and falling off for both very light and very heavy nuclei.

Nuclear fission

A heavy nucleus splits into two lighter ones, releasing energy because the fragments sit higher on the binding-energy curve. It powers nuclear reactors.

Nuclear fusion

Light nuclei combine into a heavier one, again moving up the binding-energy curve. It powers the Sun and needs enormous temperatures to overcome repulsion.

Radioactivity

The spontaneous disintegration of unstable nuclei, entirely unaffected by temperature, pressure or chemical combination.

Alpha decay

Emission of a helium nucleus, reducing mass number by four and atomic number by two. Alpha particles are the least penetrating radiation.

Beta decay

In beta-minus a neutron becomes a proton and the atomic number RISES by one; in beta-plus a proton becomes a neutron and it falls by one. Mass number is unchanged in both.

Gamma decay

Emission of a high-energy photon from an excited nucleus. Neither mass number nor atomic number changes — only the energy state does.

Law of radioactive decay

The number of undecayed nuclei falls exponentially, since the rate of decay at any moment is proportional to how many remain.

Half-life and mean life

Half-life is the time for half the nuclei to decay; mean life is the average lifetime and is always the longer of the two.