Nuclei: Class 12 Important Questions with Answers

Practice set: test yourself

Try every question before looking at the answers below. 64 or more out of 80 means you are ready for the board.

C. Very short answer (1 mark each)

1. Define atomic mass unit.
2. Define mass defect.
3. Give one example of isobars.
4. What is R₀?
5. Why is Fe-56 very stable?
6. What is a chain reaction?
7. Write the energy equivalent of 1 g.
8. What is thermonuclear fusion?
9. Name the three kinds of radioactive decay.
10. Why is the BE/A curve flat in the middle?

D. Short answer I (2 marks each)

1. Show that nuclear density is independent of A.
2. Find the binding energy of ²H (2.014102 u).
3. Draw the PE of two nucleons against distance; state one conclusion.
4. Find the radius of ²⁷Al and ⁶⁴Cu.
5. Explain why energy is released in fission using the BE/A curve.
6. Write four features of nuclear force.
7. Find Q for ²H + ²H → ³H + ¹H (³H 3.016049, ¹H 1.007825 u).
8. Differentiate isotopes and isotones with examples.

Rough work

E. Short answer II (3 marks each)

1. Define mass defect and binding energy. Calculate the BE per nucleon of ¹⁶O (15.994915 u).
2. Draw the BE/A curve. Give two features and explain the energy release in fusion.
3. Find the energy released by fission of 1 g of ²³⁵U (200 MeV per fission), in kWh.
4. Explain the proton–proton cycle and find the total energy.
5. Differentiate fission and fusion (three points).
6. State three features of nuclear force and draw the PE graph.

F. Long answer (5 marks each)

1. (a) Define binding energy and mass defect. (b) Draw the BE/A curve and explain fission and fusion with it. (c) Find the BE of ⁵⁶Fe (55.934939 u).
2. (a) Show that R = R₀A^⅓ implies constant density. (b) Write the features of nuclear forces. (c) Find the Q value of ²H + ³H → ⁴He + n.

G. HOTS (higher-order thinking)

1. Why can't iron nuclei release energy by fission or fusion?
2. Why is fusion harder to achieve than fission?
3. Why is the binding energy of ²H so small?
4. A heavy nucleus of BE/A 7.6 MeV splits into two of 8.4 MeV. Estimate the energy for A = 230.

20 MCQs

1. 1 u in MeV/c²
   (a) 9.31 (b) 93.1 (c) 931.5 (d) 9315
2. R₀ is about
   (a) 1.2 fm (b) 12 fm (c) 0.12 fm (d) 120 fm
3. Same neutron number
   (a) isotopes (b) isomers (c) isobars (d) isotones
4. Peak of BE/A curve
   (a) He-4 (b) Fe-56 (c) U-238 (d) C-12
5. Nuclear density depends on A as
   (a) A (b) independent (c) A^⅓ (d) 1/A
6. Nuclear force is
   (a) long-range (b) charge dependent (c) inverse square (d) short-range
7. Radius ratio of A = 8 and A = 64
   (a) 1 : 2 (b) 1 : 8 (c) 1 : 4 (d) 2 : 1
8. Energy per U-235 fission
   (a) 2 MeV (b) 20 MeV (c) 200 MeV (d) 2000 MeV
9. Sun's energy source
   (a) fission (b) radioactivity (c) chemical (d) fusion
10. BE of deuteron
    (a) 0.22 MeV (b) 2.22 MeV (c) 22.2 MeV (d) 222 MeV
11. ¹⁴C and ¹⁴N are
    (a) isotopes (b) isotones (c) isobars (d) isomers
12. Below 0.8 fm the nuclear force is
    (a) repulsive (b) attractive (c) zero (d) weak
13. Energy equivalent of 1 g
    (a) 9 × 10¹⁶ J (b) 9 × 10¹³ J (c) 3 × 10⁸ J (d) 9 × 10¹⁰ J
14. BE/A for medium nuclei
    (a) 1 MeV (b) 80 MeV (c) 8 MeV (d) 0.8 MeV
15. Exothermic reaction has
    (a) Q < 0 (b) Q = 0 (c) any Q (d) Q > 0
16. F_pp, F_nn, F_pn
    (a) all about equal (b) F_pp largest (c) F_nn largest (d) F_pn zero
17. Neutron discovered by
    (a) Chadwick (b) Rutherford (c) Bohr (d) Becquerel
18. Radioactivity discovered by
    (a) Curie (b) Einstein (c) Rutherford (d) Becquerel
19. Mass of nucleus compared with its nucleons
    (a) more (b) less (c) equal (d) double
20. Controlled fission is used in
    (a) H-bomb (b) stars (c) reactors (d) the Sun

1 (c) · 2 (a) · 3 (d) · 4 (b) · 5 (b) · 6 (d) · 7 (a) · 8 (c) · 9 (d) · 10 (b) · 11 (c) · 12 (a) · 13 (b) · 14 (c) · 15 (d) · 16 (a) · 17 (a) · 18 (d) · 19 (b) · 20 (c)

Assertion–Reason (6)

(a) Both true, R explains A. (b) Both true, R does not explain A. (c) A true, R false. (d) A false, R true.

1. A: Nuclear density is the same for all nuclei. R: Nuclear volume is proportional to A.
2. A: Fe-56 is very stable. R: It has the highest BE per nucleon.
3. A: Fusion requires very high temperature. R: Nuclei must overcome Coulomb repulsion.
4. A: Nuclear force is long-range. R: It is charge independent.
5. A: The mass of a nucleus is more than that of its nucleons. R: Energy is released when a nucleus forms.
6. A: Heavy nuclei release energy on fission. R: The fragments have higher BE per nucleon.

Key: 1 (a) · 2 (a) · 3 (a) · 4 (d) · 5 (d) · 6 (a)

Answers to the practice set