PhysicsICSE Class 10 · Chapter 20

Radioactivity ICSE Class 10: Nucleus, α, β and γ Radiations and Their Uses

Updated 14 October 2026 · Fact-checked

Radioactivity is the spontaneous disintegration of unstable nuclei, giving out α, β or γ radiation. In ICSE Class 10 you learn the structure of the nucleus (Z and A), the nature and properties of the three radiations, how α decay lowers A by 4 and Z by 2 while β decay raises Z by 1, and uses and safety.

What Radioactivity is about

Some nuclei are unstable. Without any outside cause, they break down and give out radiation, turning into nuclei of other elements. This is radioactivity, discovered by Henri Becquerel and studied by Marie and Pierre Curie.

The CISCE 2027 syllabus covers: a qualitative introduction to the nucleus, nuclear structure, atomic number $Z$ and mass number $A$ ; radioactivity as spontaneous disintegration; the nature and properties of α, β and γ radiations; changes within the nucleus, with one example each of α and β decay showing changes in $Z$ and $A$ ; uses of radioisotopes; harmful effects; safety precautions; background radiation; X-rays; radioactive fallout; and the safe disposal of nuclear waste.

The CISCE examiners' comments for 2024 and 2025 show errors in counting nucleons after β decay, thinking the rate of decay depends on surroundings, finding the new position in the periodic table after α emission, completing chained nuclear equations, and naming the radiation with the lowest ionising power.

Weightage: CISCE publishes no chapter or unit weightage for ICSE Physics. Radioactivity is in the 6. Modern Physics (i) part of the 2027 syllabus and can be asked in both Section A and Section B of the 80-mark paper.Source: CISCE ICSE Physics (Science Paper 1) syllabus, examination year 2027

Key concepts

The nucleus: Z, A and isotopes

The nucleus contains protons (positive) and neutrons (neutral), together called nucleons.

Atomic number $Z$ = number of protons (decides the element).
Mass number $A$ = number of protons + neutrons.
Number of neutrons $N = A - Z$ .

A nucleus is written $_{Z}^{A} X$ , e.g. $_{92}^{238} U$ .

Isotopes: same $Z$ , different $A$ (e.g. $_{6}^{12} C$ and $_{6}^{14} C$ ). Isobars: same $A$ , different $Z$ (e.g. $_{6}^{14} C$ and $_{7}^{14} N$ ). Unstable isotopes are radioisotopes.

Worked example

Find the numbers of protons, neutrons and nucleons in $_{92}^{238} U$ .

Try it first, then show the solutionSolution

Protons $= Z = 92$ .
Nucleons $= A = 238$ .
Neutrons $= 238 - 92 = 146$ .

Answer: 92 protons, 146 neutrons, 238 nucleons

Radioactivity: spontaneous disintegration

Radioactivity is the spontaneous emission of radiation (α, β or γ) from unstable nuclei, changing them into other nuclei (except for γ).

It is a nuclear process, so it is not affected by physical or chemical conditions: temperature, pressure, vacuum, chemical combination or physical state make no difference to the rate. Naturally radioactive elements include uranium, thorium, radium and polonium.

Worked example

A radioactive sample is moved from a room into a vacuum chamber at a much lower temperature. What happens to its rate of decay?

Try it first, then show the solutionSolution

Radioactive decay happens inside the nucleus.
It does not depend on pressure or temperature, so the rate stays the same.

Answer: Unchanged

Alpha (α) particles

An α particle is a helium nucleus, $_{2}^{4} He$ : 2 protons + 2 neutrons, charge $+ 2 e$ , mass about 4 u.

Ionising power: highest (heavy and doubly charged).
Penetrating power: lowest; stopped by a sheet of paper or a few cm of air.
Speed: about one-tenth of the speed of light.
Deflected by electric and magnetic fields (towards the negative plate), but less than β because they are heavy.
Affect photographic plates; cause fluorescence.

Worked example

Why can α particles not pass through a sheet of paper, though they ionise air strongly?

Try it first, then show the solutionSolution

α particles are heavy and doubly charged.
They interact strongly with atoms in their path, ionising many of them and losing energy quickly, so they stop within a short distance.

Answer: They lose energy fast by strong ionisation.

Beta (β) particles and gamma (γ) rays

β particle: a fast electron ( $_{- 1}^{0} e$ ) emitted from the nucleus when a neutron changes into a proton and an electron. Charge $- e$ ; very small mass.

Ionising power: less than α; penetrating power: more than α (stopped by a few mm of aluminium).
Speed up to about 0.9 c; deflected more than α and in the opposite direction.

γ rays: electromagnetic waves of very high frequency (shorter wavelength than X-rays). No charge, no mass, speed $c$ .

Ionising power: lowest; penetrating power: highest (reduced by several cm of lead).
Not deflected by electric or magnetic fields.

Worked example

Arrange α, β and γ in increasing order of (a) ionising power, (b) penetrating power. Which is not deflected by a magnetic field?

Try it first, then show the solutionSolution

(a) γ < β < α.
(b) α < β < γ.
γ rays carry no charge, so they are not deflected.

Answer: (a) γ, β, α (b) α, β, γ; γ

Changes in the nucleus: α and β decay

α decay: the nucleus loses 2 protons and 2 neutrons:
$A \to A - 4$ , $Z \to Z - 2$ . The new element is two places to the left in the periodic table.
Example: $_{88}^{226} Ra \to_{86}^{222} Rn +_{2}^{4} He$ .

β decay: a neutron becomes a proton (and the electron is emitted):
$A$ unchanged, $Z \to Z + 1$ . The new element is one place to the right. The number of nucleons stays the same.
Example: $_{6}^{14} C \to_{7}^{14} N +_{- 1}^{0} e$ .

γ emission: no change in $A$ or $Z$ ; the nucleus only loses energy (it usually follows α or β decay).

In every nuclear equation, the totals of $A$ (top numbers) and of $Z$ (bottom numbers) must balance on both sides.

Worked example

$_{92}^{238} U$ emits an α particle, and the product then emits a β particle. Write the two equations.

Try it first, then show the solutionSolution

α: $_{92}^{238} U \to_{90}^{234} Th +_{2}^{4} He$ .
β: $_{90}^{234} Th \to_{91}^{234} Pa +_{- 1}^{0} e$ .
Check: top and bottom numbers balance in each.

Answer: Th-234 (Z = 90), then Pa-234 (Z = 91)

Uses of radioisotopes

Medicine: cobalt-60 (γ rays) to treat cancer; iodine-131 to diagnose and treat thyroid problems; sterilising surgical instruments and bandages.
Carbon dating: carbon-14 in dead plant and animal remains is used to estimate their age.
Tracers: radioactive phosphorus in agriculture to study how plants take up fertiliser; tracers to detect leaks in underground pipes.
Industry: checking the thickness of sheets; finding flaws in welds.
Food preservation: γ irradiation kills microbes.

Worked example

From $_{27}^{59} Co$ , $_{27}^{60} Co$ , $_{53}^{127} I$ and $_{53}^{131} I$ , pick the two radioisotopes and give one medical use of each.

Try it first, then show the solutionSolution

Cobalt-60 and iodine-131 are radioactive; cobalt-59 and iodine-127 are the stable isotopes.
Co-60: its γ rays treat cancer (radiotherapy). I-131: diagnosing and treating thyroid disorders.

Answer: Co-60 (cancer therapy) and I-131 (thyroid)

Harmful effects, background radiation and safety

Harmful effects: radiation ionises cells and can damage tissues and DNA, causing burns, cancer (e.g. leukaemia) and genetic (inherited) defects. Damage depends on the dose and type of radiation.

Background radiation: low-level radiation always present, from cosmic rays, radioactive rocks and soil (e.g. radon gas), potassium-40 in our bodies and food, medical X-rays, and fallout from nuclear tests or accidents.

Safety precautions: handle sources with long tongs; store them in thick lead containers; wear lead aprons and film badges (dosimeters); keep exposure time short and distance large; dispose of nuclear waste safely by sealing it in thick containers and burying it deep underground in stable rock, far from water; reinforce safety measures at nuclear plants strictly.

Worked example

Give two sources of background radiation and two safety precautions for a worker handling radioactive materials.

Try it first, then show the solutionSolution

Sources: cosmic rays; radioactive rocks and radon gas.
Precautions: use long tongs and lead shielding; wear a lead apron and a film badge to monitor the dose.

Answer: As listed

Physics in real life: Radioactivity

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Carbon dating

Suppose archaeologists find an old wooden boat and want to know its age.

Which radioisotope helps, and why does it work only for once-living material?

Try it first, then show the solutionSolution

Carbon-14 is used.
Living things take in carbon (including a tiny fixed fraction of C-14) while alive; after death no new carbon enters, and the C-14 slowly decays, so measuring what remains gives the age.

Answer: Carbon-14

Memory hook: Dead wood stops topping up its C-14.

Radiotherapy

Imagine a hospital uses a cobalt-60 source to treat a tumour.

Which radiation is used and why, and how is the medical staff protected?

Try it first, then show the solutionSolution

γ rays: highly penetrating, so they reach deep tumours.
Staff stay behind thick lead or concrete shielding, operate the source remotely and wear dose badges.

Answer: γ rays; lead shielding and remote operation

Memory hook: Penetrating rays need heavy shields.

Detecting a pipe leak

Suppose an underground water pipe is leaking and the ground cannot be dug up everywhere.

How can a radioactive tracer find the leak?

Try it first, then show the solutionSolution

A small amount of a short-lived radioisotope is added to the water.
A detector moved along the surface finds a higher count where the water (and tracer) collects at the leak.

Answer: Trace the extra radiation where the water escapes

Memory hook: Follow the glow to the leak.

Topics in this chapter

Radioactivity formula sheet

Nucleus: $N = A - Z$; Neutrons = mass number − atomic number.
α decay: $_{Z}^{A} X \to_{Z - 2}^{A - 4} Y +_{2}^{4} He$; Two places left in the periodic table.
β decay: $_{Z}^{A} X \to_{Z + 1}^{A} Y +_{- 1}^{0} e$; One place right; nucleons unchanged.
γ emission: $_{Z}^{A} X^{*} \to_{Z}^{A} X + γ$; No change in A or Z.
Counting decays: $n_{α} = \frac{Δ A}{4}, n_{β} = 2 n_{α} - Δ Z$; ΔA, ΔZ = decreases from parent to final nucleus.

Radioactivity: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

Deflection of α, β and γ in an electric field

Show how the three radiations behave between charged plates.

Draw

A radioactive source in a lead block with a narrow opening.
Two parallel plates, + on one side and − on the other.
α: a slightly curved path bending towards the − plate.
β: a more sharply curved path bending towards the + plate.
γ: a straight, undeviated path.

Label

Source in lead block
+ and − plates
α (small deflection to −)
β (large deflection to +)
γ (undeflected)

Watch out: β is deflected more than α because it is much lighter.

Penetrating power of α, β and γ

Compare penetration with absorbers.

Draw

A source on the left, with three arrows going right.
A sheet of paper, then a few mm of aluminium, then several cm of lead.
α stopped at the paper; β stopped by the aluminium; γ reduced (greatly weakened) by the lead.

Label

Paper
Aluminium (few mm)
Lead (several cm)
α, β, γ

Watch out: Most ionising (α) is least penetrating, and the reverse for γ.

How the board asks Radioactivity

The question patterns to expect, and the method that works for each.

Question type	How it is asked	Method
MCQ and fill-in-the-blanks	Nucleons after β decay; effect of surroundings on decay; lowest ionising radiation; position in the periodic table.	β: A same; α: A − 4, Z − 2 (two left); γ least ionising; decay independent of conditions.
Complete nuclear equations	Fill in missing A, Z or particle; chained decays.	Balance top and bottom numbers step by step; write each decay separately.
Properties of radiations	Nature, charge, ionising and penetrating power, deflection.	Use a comparison table.
Graph or data interpretation	A protons-vs-neutrons graph: locate parent and daughter after α and β decays.	Each α: protons −2, neutrons −2; each β: protons +1, neutrons −1.
Uses and safety	Identify a radioisotope and its use; background radiation; precautions; waste disposal.	Give specific examples (Co-60, I-131, C-14).

Tricks to remember Radioactivity

Short memory aids. Say each one aloud once, then use it on a question the same day.

Alpha minus four-two
α: A − 4, Z − 2 (two steps left). β: A same, Z + 1 (one step right).
Use it when: Changes in the nucleus.
Paper, aluminium, lead
α stopped by paper, β by aluminium, γ needs lead.
Use it when: Penetrating power.
Ionising is the reverse
α ionises most, γ least: the order of penetration reversed.
Use it when: Ionising power.
Neutron → proton + electron
β decay makes a proton from a neutron, so nucleons stay the same.
Use it when: Counting nucleons after β decay.

Radioactivity: board-level solved questions

Ordered from board level to HOTS. Cover the solution, write your own, then compare step by step.

Example 1: Nucleons after β decay · 1 mark · MCQBoard level

A radioactive nucleus with 214 nucleons emits a β particle. The number of nucleons in the new nucleus is:
(a) 213
(b) 214
(c) 215
(d) 210

Try it first, then show the solutionSolution

A neutron becomes a proton: the total number of nucleons is unchanged.

Answer: (b)

Example 2: Decay and surroundings · 1 mark · Assertion-reasonBoard level

Assertion (A): Heating a radioactive substance does not change its rate of decay.
Reason (R): Radioactivity is a nuclear process, independent of physical and chemical conditions.
(a) (A) is true and (R) is false.
(b) (A) is false and (R) is true.
(c) Both (A) and (R) are true, and (R) is the correct explanation for (A).
(d) Both (A) and (R) are true, but (R) is not the correct explanation for (A).

Try it first, then show the solutionSolution

Decay is not affected by temperature.

Answer: (c)

Example 3: Protons and neutrons · 2 marks · Short answerBoard level

For $_{88}^{226} Ra$ , find (a) the number of protons, (b) the number of neutrons. (c) What is the new nucleus after it emits an α particle?

Try it first, then show the solutionSolution

(a) 88; (b) $226 - 88 = 138$ .
(c) $_{86}^{222} Rn$ (radon).

Answer: (a) 88 (b) 138 (c) ²²²₈₆Rn

Example 4: A chain of decays · 3 marks · Short answerHarder board level

A nucleus $_{88}^{228} X$ emits a β particle to form Y, and Y then emits an α particle to form Z. Find the mass number and atomic number of Y and Z, and the number of neutrons in Z.

Try it first, then show the solutionSolution

Y (after β): $A = 228$ , $Z = 88 + 1 = 89$ .
Z (after α): $A = 228 - 4 = 224$ , $Z = 89 - 2 = 87$ .
Neutrons in Z $= 224 - 87 = 137$ .

Answer: Y: ²²⁸₈₉; Z: ²²⁴₈₇; 137 neutrons

Example 5: Comparing the radiations · 3 marks · Short answerHarder board level

Compare α, β and γ radiations in terms of (a) nature, (b) charge, (c) penetrating power.

Try it first, then show the solutionSolution

(a) α: helium nuclei; β: fast electrons; γ: electromagnetic waves.
(b) α: +2e; β: −e; γ: none.
(c) α least (stopped by paper); β medium (few mm of aluminium); γ most (several cm of lead).

Answer: As tabulated

Example 6: Matching radioisotopes to uses · 2 marks · Long answerBoard level

Name a radioisotope used (a) to find the age of ancient wood, (b) to treat cancer, (c) to study how plants take up a fertiliser. Which radiation from the isotope in (b) is used, and why?

Try it first, then show the solutionSolution

(a) Carbon-14. (b) Cobalt-60. (c) Phosphorus-32 (as a tracer).
Cobalt-60 gives γ rays, which are highly penetrating and can reach a tumour deep inside the body.

Answer: (a) C-14 (b) Co-60, γ rays (c) P-32

Example 7: Position in the periodic table · 3 marks · Long answerHarder board level

An element P emits an α particle to form Q, which then emits two β particles to form R. (a) Where is Q relative to P in the periodic table? (b) What is the relation between P and R? (c) Compare their mass numbers.

Try it first, then show the solutionSolution

(a) α: Z decreases by 2, so Q is two places to the left of P.
(b) Two β: Z increases by 2, back to P's atomic number: R and P are isotopes.
(c) R's mass number is 4 less than P's.

Answer: (a) two places left (b) isotopes (c) R = P − 4

Example 8: How many α and β decays? · 4 marks · Long answerHOTS / competency

$_{90}^{232} Th$ decays through a series of α and β emissions to the stable $_{82}^{208} Pb$ . (a) How many α particles are emitted? (b) How many β particles? (c) How many neutrons are lost in total? (d) Why is γ radiation not counted?

Try it first, then show the solutionSolution

(a) Mass number falls by $232 - 208 = 24$ ; each α removes 4: 6 α particles.
(b) 6 α would lower Z by 12, to 78; the actual fall is $90 - 82 = 8$ , so Z must rise by 4: 4 β particles.
(c) Neutrons: $232 - 90 = 142$ before, $208 - 82 = 126$ after: 16 lost.
(d) γ emission changes neither A nor Z.

Answer: (a) 6 (b) 4 (c) 16 (d) γ does not change A or Z

Case-study practice (Section B style)

Read the situation, then answer every part. In the board paper this is a 10-mark question split into 3 + 3 + 4 marks.

Radiation in a hospital

Suppose a hospital uses cobalt-60 to treat tumours and iodine-131 to study the thyroid gland. Staff wear film badges.

(i) · 3 marks
Which radiation from cobalt-60 is used in therapy, and which property makes it suitable?
Answer
Answer: γ rays; high penetration
γ rays: they are highly penetrating and can reach tumours deep inside the body; their energy kills cancer cells.
(ii) · 3 marks
State two harmful effects of excessive exposure to radiation.
Answer
Answer: Tissue damage/burns; cancer or genetic defects
Radiation ionises cells: it can cause burns and tissue damage, cancers such as leukaemia, and genetic defects passed to offspring.
(iii) · 4 marks
Give four safety precautions the staff should follow, and the purpose of the film badge.
Answer
Answer: Tongs, lead, time, distance; dose monitoring
Handle sources with long tongs; keep them in lead containers; use lead aprons and screens; keep exposure time short and stay far away. The film badge records the total radiation dose each worker receives.

A decay series

Imagine uranium-238 ( $_{92}^{238} U$ ) in a rock decays by emitting an α particle, then two β particles in succession.

(i) · 3 marks
Write the equation for the α decay.
Answer
Answer: ²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He
$_{92}^{238} U \to_{90}^{234} Th +_{2}^{4} He$ : A falls by 4 and Z by 2.
(ii) · 3 marks
Find A and Z of the nucleus after the two β decays, and its relation to the starting nucleus.
Answer
Answer: A = 234, Z = 92; an isotope of uranium
Each β raises Z by 1 and leaves A unchanged: Z = 90 → 91 → 92, A = 234. Same Z as uranium: it is $_{92}^{234} U$ , an isotope of the parent.
(iii) · 4 marks
Does the rock's rate of decay change if it is heated strongly or kept in a vacuum? Name two sources of background radiation from rocks and space.
Answer
Answer: No; radon/rocks and cosmic rays
No: radioactive decay is a nuclear process not affected by temperature or pressure. Background sources: radioactive rocks and the radon gas they release; cosmic rays from space.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Complete: $_{6}^{14} C \to_{?}^{?} N +_{- 1}^{0} e$ . Which radiation is emitted? (2 marks)

Step	Marks
$_{7}^{14} N$ (A same, Z + 1)	1
β particle	1

Give the nature, charge and one property of γ radiation. (3 marks)

Step	Marks
Nature: electromagnetic waves of very high frequency	1
Charge: none	1
Property: highest penetrating power (or least ionising, or not deflected by fields)	1

A nucleus $_{84}^{218} X$ emits an α particle, then a β particle, then a γ ray. Find the final A, Z and the number of neutrons. (4 marks)

Step	Marks
After α: $A = 214$ , $Z = 82$	1
After β: $A = 214$ , $Z = 83$	1
After γ: no change	1
Neutrons $= 214 - 83 = 131$	1

Where students lose marks in Radioactivity

The slips examiners see most often, and what to write instead.

Mistake: Changing the nucleon count in β decay
CISCE examiners' comments, 2024: after β emission from a nucleus of 128 nucleons, candidates added 1 by rote or wrote 127.
Do this: In β decay a neutron becomes a proton: A (nucleons) is unchanged; only Z rises by 1.
Usually costs: 1 mark.
Mistake: Thinking decay depends on surroundings
CISCE examiners' comments, 2024: candidates linked radioactive decay to chemical reactions and thought a vacuum would change the rate.
Do this: Radioactivity is independent of all physical and chemical conditions.
Usually costs: 1 mark.
Mistake: Chained equations done in one go
CISCE examiners' comments, 2024 and 2025: candidates were confused by sequential decays, could not work backwards through more than one emission, and misread a protons-vs-neutrons graph.
Do this: Write each decay as a separate step and balance A and Z each time; for graphs, α: −2 protons, −2 neutrons; β: +1 proton, −1 neutron.
Usually costs: 1 to 2 marks.
Mistake: Wrong direction in the periodic table
CISCE examiners' comments, 2025: candidates could not relate the fall in atomic number after α emission to the position in the periodic table, or mixed up left and right.
Do this: Z − 2 → two places to the left; Z + 1 → one place to the right.
Usually costs: 1 mark.
Mistake: Wrong order of ionising power
CISCE examiners' comments, 2025: asked for the radiation with the lowest ionising power, candidates answered α or β.
Do this: Ionising: α > β > γ (γ lowest). Penetrating: γ > β > α.
Usually costs: 1 mark.
Mistake: Vague or extra answers on radioisotopes
CISCE examiners' comments, 2024: candidates listed stable isotopes along with C-14, and gave a general use of radioisotopes rather than one specific to C-14.
Do this: Pick only the radioactive one and give its own use (C-14: carbon dating).
Usually costs: 1 mark.

Practice questions on Radioactivity

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
An α particle is the same as:
(a) an electron
(b) a proton
(c) a helium nucleus
(d) a neutron
Hint
2p + 2n.
Answer
(c)
Q2 · 1 mark · MCQBoard level
The radiation that is NOT deflected by a magnetic field is:
(a) α
(b) β
(c) γ
(d) both α and β
Hint
No charge.
Answer
(c)
Q3 · 1 mark · MCQHarder board level
When a nucleus emits a β particle, its new position in the periodic table is:
(a) two places to the left
(b) one place to the left
(c) one place to the right
(d) unchanged
Hint
Z + 1.
Answer
(c)
Q4 · 1 mark · Assertion-reasonBoard level
Assertion (A): γ rays are the least ionising of the three radiations.
Reason (R): γ rays are uncharged electromagnetic waves.
(a) (A) is true and (R) is false.
(b) (A) is false and (R) is true.
(c) Both (A) and (R) are true, and (R) is the correct explanation for (A).
(d) Both (A) and (R) are true, but (R) is not the correct explanation for (A).
Hint
No charge, weak interaction.
Answer
(c)
Q5 · 2 marks · Very short answerHarder board level
Complete: $_{84}^{210} Po \to_{82}^{?} Pb +_{?}^{?} He$ . Name the decay.
Hint
Balance A and Z.
Answer
$_{82}^{206} Pb +_{2}^{4} He$ ; α decay.
Q6 · 2 marks · Short answerHarder board level
Give one use each of (a) cobalt-60, (b) iodine-131.
Hint
Medicine.
Answer
(a) Treating cancer (γ radiotherapy) or sterilising equipment. (b) Diagnosing and treating thyroid disorders.
Q7 · 3 marks · Long answerBoard level
(a) Define radioactivity. (b) State three properties of α particles.
Hint
Spontaneous; charge, ionising, penetration.
Answer
(a) Spontaneous emission of α, β or γ radiation from unstable nuclei. (b) Positively charged (+2e); highest ionising power; least penetrating (stopped by paper); deflected by fields.
Q8 · 3 marks · Long answerHarder board level
A nucleus $_{Z}^{A} X$ emits one α and one β particle. Write A and Z of the final nucleus and find how many neutrons it has fewer than X.
Hint
α: −4, −2; β: 0, +1.
Answer
Final: $A - 4$ , $Z - 1$ . Neutrons: X has $A - Z$ ; final has $(A - 4) - (Z - 1) = A - Z - 3$ : 3 fewer.
Q9 · 3 marks · Long answerHOTS / competency
$_{92}^{238} U$ decays to $_{82}^{206} Pb$ . Find the numbers of α and β particles emitted.
Hint
ΔA = 32.
Answer
α: $\frac{32}{4} = 8$ . 8 α lower Z by 16 (to 76); actual fall is 10, so Z must rise by 6: 6 β.
Q10 · 3 marks · Long answerHarder board level
State four safety precautions for handling radioactive materials and one method of disposing of nuclear waste.
Hint
Tongs, lead, time, distance.
Answer
Use long tongs; store in lead containers; wear lead aprons and film badges; minimise time near the source and keep distance. Waste: seal in thick containers and bury deep underground in stable rock.
Q11 · 1 mark · MCQHOTS / competency
Two nuclei with the same mass number but different atomic numbers are called:
(a) isotopes
(b) isobars
(c) isotones
(d) isomers
Hint
β decay produces one.
Answer
(b)
Q12 · 4 marks · Long answerHOTS / competency
A radioactive source is placed between two charged plates and in front of a detector. (a) Describe the paths of α, β and γ. (b) When a sheet of paper is placed in front of the detector, the count falls a little; with 5 mm of aluminium it falls much more; with 5 cm of lead it is close to the background level. Which radiations does the source emit? Explain.
Hint
Use penetration.
Answer
(a) α bends slightly towards the negative plate, β bends more towards the positive plate, γ goes straight. (b) Paper stops α: small fall, so a little α. Aluminium stops β: large fall, so mostly β. Some radiation passes aluminium but lead reduces it to background: γ is also present. The source emits α, β and γ.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.What is an α particle?Show answer
A helium nucleus, ⁴₂He (2 protons + 2 neutrons).
2.What is a β particle?Show answer
A fast electron emitted from the nucleus.
3.What are γ rays?Show answer
High-frequency electromagnetic waves.
4.Change in A and Z on α decay?Show answer
A − 4, Z − 2.
5.Change in A and Z on β decay?Show answer
A unchanged, Z + 1.
6.Most penetrating radiation?Show answer
γ rays.
7.Does temperature affect radioactive decay?Show answer
No.

Best order to study it

Nucleus, Z, A, isotopes.
Radioactivity as spontaneous disintegration.
Nature and properties of α, β and γ.
Changes in Z and A; balancing equations.
Uses of radioisotopes.
Harmful effects, background radiation, safety and waste disposal.

Quick revision checklist

The night before the exam: can you do each of these without looking?

Can you find protons and neutrons from ᴬ_Z X?
Can you complete α and β decay equations?
Can you order the radiations by ionising and penetrating power?
Can you count α and β decays between two nuclei?
Can you give three uses and four precautions?

Radioactivity: frequently asked questions

Do I need half-life calculations?

The CISCE 2027 syllabus does not list half-life; focus on the nature of radiations, changes in Z and A, uses and safety.

Where does the electron in β decay come from?

From the nucleus: a neutron changes into a proton and an electron, and the electron is emitted.

Are γ rays the same as X-rays?

Both are electromagnetic, but γ rays come from the nucleus and usually have shorter wavelengths (higher energy).

What is background radiation?

The low-level radiation always around us, from cosmic rays, rocks and soil, radon gas, our own bodies, medical uses and fallout.

How is nuclear waste disposed of safely?

It is sealed in thick, strong containers and buried deep underground in stable rock, away from water sources, and monitored.

Based on: CISCE ICSE 2027 Physics (Science Paper 1) syllabus; CISCE ICSE 2027 Physics (Science Paper 1) specimen question paper and marking scheme; CISCE Analysis of Pupil Performance, ICSE Physics (Science Paper 1), 2025; CISCE Analysis of Pupil Performance, ICSE Physics (Science Paper 1), 2024