Worksheet Solutions: Radioacitivity | Physics Class 10 ICSE PDF Download

Part A — Multiple Choice Questions

Q1. The maximum number of electrons that can be accommodated in a shell with number n is given by:

(a) n²

(b) 2n

(c) 2n²

(d) n³
Answer: (c) 2n²
Explanation: The notes state shell capacity follows 2, 8, 18, 32… and the general rule 2n² (for n = 1, 2, 3, 4…). For example, n = 4 gives 2×4² = 32 electrons.

Q2. Two atoms have the same mass number but different atomic numbers. They are called:

(a) Isotopes

(b) Isobars

(c) Isotones

(d) Isomers
Answer: (b) Isobars
Explanation: Isobars share the same total nucleons (same A) but have different protons (different Z). Isotopes share Z, isotones share neutrons (A − Z).

Q3. In a magnetic or electric field, which radiation from a radioactive source goes undeflected?

(a) Alpha (α)

(b) Beta (β)

(c) Gamma (γ)

(d) All are deflected
Answer: (c) Gamma (γ)
Explanation: Gamma rays are electromagnetic waves with no charge, so they are not deflected. Alpha (positive) and beta (negative) are charged and get deflected.

Q4. Which statement correctly compares penetrating power and ionising power?

(a) α highest penetration, lowest ionising

(b) β highest penetration, moderate ionising

(c) γ highest penetration, lowest ionising

(d) γ lowest penetration, highest ionising
Answer: (c) γ highest penetration, lowest ionising
Explanation: Alpha: very strong ionising, very low penetration. Beta: moderate ionising, moderate penetration. Gamma: weak ionising, very high penetration.

Q5. During beta (β) emission from a nucleus:

(a) A increases by 4, Z decreases by 2

(b) A decreases by 4, Z increases by 2

(c) A unchanged, Z increases by 1

(d) A unchanged, Z unchanged
Answer: (c) A unchanged, Z increases by 1
Explanation: In β-emission a neutron effectively turns into a proton and an electron (the electron is emitted). Mass number A stays same; atomic number Z increases by 1.

Part B — Short Answer Questions

Q6. Define atomic number and mass number. Give one quick example.
Answer:

• Atomic number (Z): Number of protons in the nucleus (also equals number of electrons in a neutral atom).

• Mass number (A): Total nucleons = protons + neutrons.
  Example: For sodium-23 (written as 23Na), Z = 11 (protons), A = 23, so neutrons = 23 − 11 = 12.
  Why this helps: Knowing Z and A lets you find the particle counts and identify isotopes/isobars.

Q7. How are isotopes, isobars, and isotones different? Give one example of each.
Answer:

• Isotopes: Same Z, different A (same element). Example: Hydrogen isotopes — protium (A=1), deuterium (A=2), tritium (A=3).

• Isobars: Same A, different Z (different elements). Example: 23Na (Z=11) and 23Mg (Z=12).

• Isotones: Same neutrons (A − Z), different Z and A. Example: 23Na (neutrons 12) and 24Mg (neutrons 12).
  Why this matters: It explains why some atoms behave chemically alike (isotopes) or differ (isobars), and how nuclei can match in neutron count (isotones).

Q8. Why is an atom electrically neutral?
Answer: The number of protons (+ charge) in the nucleus equals the number of electrons (− charge) in the shells. Equal and opposite charges balance to give net zero charge.
Note: Neutrons are uncharged and add to mass only.

Q9. Explain why radioactivity is called a nuclear phenomenon.
Answer: Radioactivity arises from changes inside the nucleus (unstable nuclei emit α, β, γ). Orbital electrons are not responsible; their removal or addition does not create radioactivity.
Outcome: The element can transform into another (change in Z/A), or just lose energy (γ) without changing Z or A.

Q10. List two useful applications and two safety precautions for radioactive substances.
Answer:

Uses:

• Medicine: Cobalt-60 gamma rays for cancer therapy; radio-iodine for diagnosis (tracers).
• Industry/Agriculture: Thickness control using β-rays; tracers to study plant growth or detect leaks.

Safety:

• Use lead shielding, lead-lined aprons and gloves; handle with long tongs.

• Store in thick lead containers; never exceed exposure limits; dispose of waste in secure underground facilities.
  Why: Benefits are large, but ionising radiation can damage living cells.

Part C — Long Answer Questions

Q11: Consider these neutral atoms: ¹H, ²H, ³H (hydrogen isotopes), ²³Na, ²³Mg, ²⁴Mg.
(a) Group them as isotopes, isobars, and isotones.
(b) For each, state protons (Z), neutrons (A − Z), and electrons.

Stepwise Solution:
Step 1: Write Z and A, then neutrons = A − Z.

• ¹H: Z=1, A=1 → neutrons=0, electrons=1

• ²H: Z=1, A=2 → neutrons=1, electrons=1

• ³H: Z=1, A=3 → neutrons=2, electrons=1

• ²³Na: Z=11, A=23 → neutrons=12, electrons=11

• ²³Mg: Z=12, A=23 → neutrons=11, electrons=12

• ²⁴Mg: Z=12, A=24 → neutrons=12, electrons=12

Step 2: Classify.

• Isotopes (same Z, different A): ¹H, ²H, ³H (all Z = 1).

• Isobars (same A, different Z): ²³Na (A=23) and ²³Mg (A=23).

• Isotones (same neutrons): ²³Na (12 n) and ²⁴Mg (12 n).

Step 3: Summarise particle counts clearly.

• Hydrogen set: (1p,0n,1e), (1p,1n,1e), (1p,2n,1e) → isotopes

• ²³Na: (11p,12n,11e) and ²³Mg: (12p,11n,12e) → isobars

• ²³Na (12n) and ²⁴Mg (12n) → isotones

Answer: Grouping and counts as above; this shows how Z and A define families (isotopes/isobars/isotones).

Q12. Explain the harmful effects of nuclear radiations and the safety precautions needed while handling radioactive materials.

Stepwise Solution:

Step 1: Harmful Effects of Nuclear Radiations

1. Short-term effects – Exposure can cause nausea, diarrhoea, sore throat, headaches, and hair loss.

2. Long-term effects – Prolonged exposure can lead to blood cancer (leukaemia) and other cancers.

3. Genetic effects – Radiations can damage genes and cause birth defects in future generations.

4. Environmental effects – Radioactive waste from nuclear power plants can contaminate soil, water, and air.

Step 2: Safety Precautions

1. Workers should wear lead-lined aprons and gloves to protect themselves.

2. Radioactive substances should be handled with long tongs to avoid direct contact.

3. Materials must be kept in thick lead containers with narrow openings to minimise radiation leakage.

4. Exposure limits must never exceed the safety limit set by authorities.

5. Nuclear power plants must have shielded reactors and emergency cooling systems to prevent accidents.

Q13.  Explain, step by step, the difference between nuclear fission and nuclear fusion, what a chain reaction is, and how energy is released in these processes. Include controlled vs uncontrolled reactions.

Stepwise Solution:
Step 1: What is fission?

• A heavy nucleus (like uranium-235) splits into two lighter nuclei when hit by a slow neutron, releasing more neutrons and a large amount of energy.

• Energy source: A small loss of mass converts to energy (E = Δm c²).

Step 2: What is fusion?

• Two light nuclei (like forms of hydrogen) combine to make a heavier nucleus (helium), releasing energy because the final mass is a bit less than the total starting mass.

• Needs very high temperature and pressure to overcome repulsion between positively charged nuclei.

Step 3: Chain reaction in fission.

• The neutrons released in one fission can trigger more fissions, forming a chain.

  • Uncontrolled: Neutrons are not absorbed → explosive energy release (nuclear bomb).

  • Controlled: Some neutrons are absorbed by materials (moderators/absorbers), keeping the rate steady (nuclear reactor for electricity).

Step 4: Why is energy so large?

• In both fission and fusion, a tiny mass defect becomes energy via E = Δm c². Even a very small Δm gives a big E because c² is huge.

Step 5: Uses and safety.

• Constructive: Controlled fission in reactors generates electricity; fusion powers the Sun (future reactors aim to harness it).

• Safety: Reactors use shielding (lead/steel), strong containment buildings, and backup cooling; radioactive waste is stored in secure, remote facilities.

Answer: Fission splits heavy nuclei; fusion joins light nuclei. Both release energy from a small loss of mass. Keeping fission controlled allows safe power generation; lack of control leads to dangerous explosions.