Nuclei — Class 12 MCQs with Answers
Class 12 CBSE Physics · Chapter 13
90 practice questions · 30 Easy · 30 Medium · 30 Hard · Updated
Practise the most important Class 12 CBSE Physics questions from Chapter 13, "Nuclei". You get 9 timed quizzes made from 90 NCERT-based MCQs, with answers and explanations. The questions are split into 30 Easy, 30 Medium and 30 Hard. Warm up on the basics, then move on to the exam-level questions that set top scorers in CBSE & Maharashtra HSC Board exams, JEE Main, MHT-CET, JEE Advanced and NEET UG apart.
To score well in "Nuclei", focus on numericals, derivations and applying concepts. Each MCQ here is timed and uses exam-style marking (+4 correct, −1 wrong, 0 skipped). This trains you to stay accurate under time pressure, as real papers need. Every question has a short explanation, so a wrong answer becomes a quick lesson. It is the fastest way to fix gaps before a test.
Use this chapter for focused revision. Start with the Easy set to check your basics on Nuclei, then move to Medium and Hard to practise applying them. Your accuracy, streaks and XP save automatically. This chapter also adds to your overall Class 12 Physics mastery score. 14 sample questions are solved in full below, with the answer and a worked explanation. Sign in free to start practising.
Key concepts: Nuclei (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.
Nuclei — important questions & MCQs with answers (Class 12 Physics)
14 solved questions from this chapter's difficulty levels, each with its answer and explanation. The other 76 are timed and scored when you sign in.
- Q1Easy
Nucleons are:
A.Protons onlyB.ElectronsC.Protons and neutrons✓ CorrectD.Neutrons onlyAnswer: C. 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.
- Q2Easy
In α-decay, mass number A and atomic number Z change by:
A.A unchanged, Z by +1B.A by −4, Z by −2✓ CorrectC.A unchanged, Z by −1D.A by −2, Z by −4Answer: B. 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.
- Q3Easy
Isotopes have same:
A.Mass number, different ZB.Atomic number, different mass number✓ CorrectC.Atomic and mass numbersD.Charge onlyAnswer: B. Atomic number, different mass number
Explanation: Isotopes share the same atomic number Z (proton count, same element) but differ in mass number A due to different neutron counts.
- Q4Easy
In β⁻ decay, atomic number changes by:
A.−2B.−1C.+1✓ CorrectD.0Answer: C. +1
Explanation: In β⁻ decay a neutron converts to a proton plus an electron and antineutrino, so Z increases by 1 while A stays the same — don't decrease Z here.
- Q5Easy
Isobars have same:
A.Neutron numberB.Atomic number, different mass numberC.Mass number, different atomic number✓ CorrectD.All propertiesAnswer: C. Mass number, different atomic number
Explanation: Isobars share the same mass number A but different atomic number Z — same total nucleon count, different element identity.
- Q6Easy
In β⁺ decay, atomic number changes by:
A.−1✓ CorrectB.+1C.−2D.0Answer: A. −1
Explanation: In β⁺ decay a proton converts to a neutron plus a positron and neutrino, so Z decreases by 1 while A is unchanged.
- Q7Medium
Radius of ²⁷Al nucleus is approximately (R₀ = 1.2 fm):
A.3.6 fm✓ CorrectB.1.2 fmC.5.4 fmD.32.4 fmAnswer: A. 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.
- Q8Medium
α-particle is:
A.PhotonB.ElectronC.Helium nucleus (2p, 2n)✓ CorrectD.NeutronAnswer: C. Helium nucleus (2p, 2n)
Explanation: An α-particle is a bare helium-4 nucleus: 2 protons + 2 neutrons with no electrons, giving it charge +2e and mass number 4.
- Q9Medium
Ratio of nuclear volumes of nuclei with A=216 and A=27:
A.27 : 1B.2 : 1C.3 : 1D.8 : 1✓ CorrectAnswer: D. 8 : 1
Explanation: Volume scales as A since R∝A^(1/3) makes R³∝A: V₂/V₁ = A₂/A₁ = 216/27 = 8, a direct ratio of mass numbers, not of radii.
- Q10Medium
Half-life of a radioactive sample:
A.Half its volumeB.Total lifeC.Half its massD.Time for half the nuclei to decay✓ CorrectAnswer: D. Time for half the nuclei to decay
Explanation: Half-life is the time for half of the radioactive nuclei present to decay — not half the mass or volume — related to λ by T₁/₂ = 0.693/λ.
- Q11Medium
Ratio of radii of nuclei with mass numbers 27 and 125:
A.5 : 3B.3 : 5✓ CorrectC.27 : 125D.1 : 5Answer: B. 3 : 5
Explanation: R∝A^(1/3), so R₁:R₂ = 27^(1/3):125^(1/3) = 3:5 — take the cube root of each mass number, don't just ratio A directly.
- Q12Hard
Two nuclei have radius ratio 1:2. Ratio of their mass numbers:
A.1 : 2B.1 : 8✓ CorrectC.1 : 4D.2 : 1Answer: B. 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.
- Q13Hard
Mass defect Δm relates to binding energy as:
A.E = Δm·c²✓ CorrectB.E = Δm/cC.E = Δm + cD.E = Δm·cAnswer: A. E = Δm·c²
Explanation: Mass-energy equivalence gives binding energy directly from mass defect: E = Δm·c², with c² supplying the huge 931.5 MeV/u conversion factor.
- Q14Hard
Compared to nuclear density, density of water is roughly smaller by factor:
A.10¹⁴✓ CorrectB.10²C.10⁶D.10²³Answer: A. 10¹⁴
Explanation: Nuclear density (~10¹⁷ kg/m³) versus water's ~10³ kg/m³ gives a ratio of about 10¹⁴ — nuclear matter is fourteen orders of magnitude denser than everyday matter.
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Start this chapter free →Nuclei — FAQs
What are the key concepts in Class 12 Physics Nuclei?+
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. Key ideas include Composition of the nucleus, Isotopes, isobars and isotones, Size of the nucleus, Nuclear density.
What does Class 12 Physics Chapter 13 (Nuclei) cover on XamBaaz?+
It has 90 NCERT-based MCQs on "Nuclei": 30 Easy, 30 Medium and 30 Hard. Together they make 9 timed quizzes, and you never get the same set twice. Every question has an instant explanation. They help you prepare for CBSE & Maharashtra HSC Board exams, JEE Main, MHT-CET, JEE Advanced and NEET UG.
Are these "Nuclei" questions free to practise?+
Yes. Sign in with Google to practise "Nuclei" free. Full unlimited access is ₹999/year on a launch offer until 1 December 2026. No chapter is charged separately.
How should I revise "Nuclei" for the exam?+
Start with the Easy quiz to check your basics, then try Medium and Hard to practise applying them. There are 9 timed quizzes on this chapter, so you can come back for a fresh set instead of one you have seen. Read each explanation, retry the questions you miss, and track your accuracy until it stays high.
Are these "Nuclei" MCQs available with answers?+
Yes. 14 sample questions are shown here in full, each with the correct option and a step-by-step "Why" explanation. Sign in free with Google to start practising, with instant scoring.
Is there negative marking in the "Nuclei" quizzes?+
Yes. The timed quizzes use exam-style marking: +4 for a right answer, −1 for a wrong one and 0 for a skip, the same negative marking as JEE Main, JEE Advanced and NEET UG. MHT-CET and CBSE board papers have no negative marking. Our mocks for those are scored their way.
What are the important questions from Nuclei (Class 12 Physics)?+
The questions that matter most test Composition of the nucleus, Isotopes, isobars and isotones, Size of the nucleus, Nuclear density. This page shows 14 solved important MCQs with answers and explanations; all 90 questions on the chapter are available as timed quizzes once you sign in.
Is there an online quiz for Nuclei?+
Yes — Class 12 Physics Nuclei has timed online quizzes at Easy, Medium and Hard levels, with instant scoring and a worked explanation on every question. The first quiz on the chapter is free.