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Atoms and Nuclei Practice Problems with Solutions - CBSE Grade 12

CBSE Grade 12 | Physics | Atoms and Nuclei
Name: _________________________ Date: _________________________ Score: _______ / 40

Inside an atom the nucleus holds protons and neutrons, and the mass number A equals the count of protons plus neutrons, so this Class 12 nuclear sheet begins with what the nucleus actually contains. The key later records that iron is the most stable nucleus and that after one half-life half the nuclei decay.

Key Takeaways

Section A: Multiple Choice Questions (1 Mark Each)

Choose the correct option for each question.
1. The nucleus of an atom contains
(a) Electrons only(b) Protons and neutrons(c) Protons and electrons(d) Neutrons only
2. The mass number A of a nucleus is equal to the
(a) Number of protons(b) Number of protons plus neutrons(c) Number of neutrons(d) Number of electrons
3. The radius of a nucleus is proportional to
(a) A(b) A²(c) A^(1/3)(d) 1/A
4. The binding energy per nucleon is maximum for nuclei around
(a) Hydrogen(b) Iron(c) Uranium(d) Helium
5. During a radioactive decay, the total number of nucleons in a nucleus
(a) Increases(b) Decreases(c) Remains constant(d) Varies randomly
6. The half-life of a radioactive substance is the time in which
(a) All the nuclei decay(b) Half the nuclei decay(c) One-quarter of the nuclei decay(d) No nuclei decay

Section B: Short Answer Type Questions (2 Marks Each)

Show all steps clearly.
7. Write the constituents of an atomic nucleus.
8. Define the mass number of a nucleus.
9. Write the relation between the nuclear radius and the mass number.
10. Define the half-life of a radioactive substance.
11. Write the expression for the activity of a radioactive sample.
12. Write the SI unit of radioactivity.
13. Define binding energy.
14. Write the symbol of an alpha particle.

Section C: Numericals & Word Problems (3 Marks Each)

Apply the concepts to solve the problems. Show all working.
15. A radioactive sample has a half-life of 4 days. Find the fraction of the sample remaining after 12 days.
16. The half-life of a radioactive substance is 2 days. How many half-lives are needed to reduce it to 1/16 of its initial amount?
17. A sample initially contains 8000 radioactive nuclei. How many nuclei remain after three half-lives?
18. Write the nuclear equation for the alpha decay of uranium-238 (²³⁸U → ...).
19. A nucleus has 92 protons and 146 neutrons. Find its mass number.
20. Why is the binding energy per nucleon highest for iron?

Answer Key

1. b) Protons and neutrons
2. b) Number of protons plus neutrons
3. c) A^(1/3)
4. b) Iron
5. c) Remains constant
6. b) Half the nuclei decay
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End of Worksheet

People Also Ask

What does the nucleus of an atom contain?

Protons and neutrons, option (b) in this worksheet (Q1).

What is the mass number A equal to?

The number of protons plus neutrons, option (b) in this worksheet (Q2).

What happens after one half-life?

Half the nuclei decay, from the answer key on this page.

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