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Nuclei is a short chapter with a very steady return. Binding energy and mass defect appear almost every year, and both are straightforward once the formulas are in place. The single idea that ties the chapter together is the binding energy per nucleon curve: it rises to iron and falls after it, and that one shape explains both fission and fusion. Ten questions below. Give them ten honest minutes.
What this chapter carries in NEET(UG)
- Usually 2 questions every year, which is 8 marks.
- Binding energy and mass defect are asked almost every year.
- Nuclear size, nuclear density, fission and fusion fill the rest.
What the syllabus says
In the syllabus (Unit 18, Atoms and Nuclei): Composition and size of the nucleus, atomic masses, the mass–energy relation, mass defect, binding energy per nucleon and how it varies with mass number, nuclear fission and fusion.
Taken out in 2024: isotopes, isobars and isotones; radioactivity — alpha, beta and gamma rays and their properties; the radioactive decay law, and with it half-life, mean life and activity.
Taken from the NMC syllabus for NEET (UG) 2026, the latest one published. See the full syllabus, unit by unit.
Formulas you must know by heart
| Nuclear radius | R = R0A1/3, R0 = 1.2 fm |
| Nuclear density | about 2.3 × 1017 kg/m3, the same for every nucleus |
| Mass defect | Δm = [Zmp + (A − Z)mn] − M |
| Binding energy | BE = Δm × 931.5 MeV |
| Mass–energy equivalence | 1 u = 931.5 MeV |
| Q value of a reaction | Q = (mass of reactants − mass of products) × 931.5 MeV |
| Energy released per fission of uranium-235 | about 200 MeV |
| Fusion of four hydrogen nuclei into helium | about 26 MeV |
The binding energy per nucleon curve peaks near iron at about 8.8 MeV. Everything lighter can release energy by joining together, and everything heavier by splitting apart. That is fusion and fission in one sentence.
Ten questions. Give yourself ten minutes.
Write your answers down before you scroll. A question you looked up is a question you have not answered.
Q1. The radius of a nucleus of mass number A is proportional to
- (a) A
- (b) A1/3
- (c) A2
- (d) Z
Q2. The density of nuclear matter
- (a) increases with mass number
- (b) decreases with mass number
- (c) is the same for all nuclei
- (d) is proportional to the atomic number
Q3. One atomic mass unit is equivalent to an energy of about
- (a) 931.5 MeV
- (b) 1.6 MeV
- (c) 13.6 eV
- (d) 8 MeV
Q4. The binding energy per nucleon is largest for a nucleus of
- (a) hydrogen
- (b) iron
- (c) uranium
- (d) helium
Q5. The mass defect of a nucleus is 0.05 u. Its binding energy is about
- (a) 46.6 MeV
- (b) 9.3 MeV
- (c) 93.1 MeV
- (d) 4.7 MeV
Q6. Energy is released in nuclear fission because
- (a) the mass of the products is more than the mass of the parent nucleus
- (b) the binding energy per nucleon of the products is higher than that of the parent
- (c) the number of nucleons increases
- (d) the nuclear force becomes weaker
Q7. Two nuclei have mass numbers 27 and 125. The ratio of their radii is
- (a) 27 : 125
- (b) 3 : 5
- (c) 5 : 3
- (d) 9 : 25
Q8. In a nuclear reaction the total mass of the products is less than the total mass of the reactants by Δm. The energy released is
- (a) Δm c
- (b) Δm c²
- (c) Δm / c²
- (d) 2Δm c²
Q9. The energy of the sun is produced mainly by
- (a) nuclear fission
- (b) nuclear fusion
- (c) chemical burning
- (d) gravitational contraction
Q10. The force that binds the protons and neutrons inside a nucleus is
- (a) gravitational
- (b) electrostatic
- (c) the strong nuclear force
- (d) magnetic
Answer key, with a hint for each
| Q1 | (b) | R = R0A1/3. Since volume goes as R3, the volume is proportional to A, which is why the density comes out the same for all nuclei. |
| Q2 | (c) | Follows straight from the line above. Mass goes as A and so does volume, so the ratio is constant. |
| Q3 | (a) | 1 u = 931.5 MeV. This is the number that turns a mass defect into a binding energy. |
| Q4 | (b) | About 8.8 MeV per nucleon near A = 56. That is why iron is the most stable nucleus. |
| Q5 | (a) | Binding energy = mass defect in u × 931.5 MeV. Here 0.05 × 931.5 ≈ 46.6 MeV. |
| Q6 | (b) | Moving towards the peak of the binding energy per nucleon curve always releases energy. |
| Q7 | (b) | R ∝ A^(1/3). Take the cube root of each mass number: 3 and 5. |
| Q8 | (b) | Einstein’s mass–energy relation. Keep Δm in kg with c in m/s, or use 931.5 MeV for each atomic mass unit. |
| Q9 | (b) | Light nuclei joining together. Four hydrogen nuclei build one helium nucleus and the lost mass appears as energy. |
| Q10 | (c) | It is far stronger than the electrostatic repulsion between protons, but it acts only over a very short range. |
Now mark yourself honestly
NEET(UG) marks the way it marks. Use the same rule here:
- +4 for a right answer
- −1 for a wrong one
- 0 for a blank
So ten questions here are out of 40. In the real paper, 45 Physics questions are out of 180.
If you scored 32 or more, the basics of this chapter are secure. Move to harder questions, not more of these.
If you scored below 24, do not feel bad about it. You have just found out something useful before the exam did. Go back to the formula table above, learn it properly, and sit these ten again tomorrow.
And if you guessed a few and they landed, count those as wrong. A guess that works once will not work in May.
The full chapter, on a clock
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