Radioactivity

ICSE Class 10 · Physics · Chapter 12

What this chapter covers

Structure of the nucleus
Radioactivity as spontaneous disintegration
Alpha, beta and gamma radiations: nature and properties
Changes within the nucleus: alpha, beta and gamma decay
Uses of radioactivity: radioisotopes
Harmful effects of radiation and safety precautions
Background radiation
Nuclear energy: safety measures and safe disposal of waste
Nuclear fission
Nuclear fusion

Key definitions

Protons are positively charged particles found in the nucleus, each carrying a charge equal in size to the charge of an electron.

Neutrons are electrically neutral particles found in the nucleus, with a mass almost equal to that of a proton.

Isotopes are atoms of the same element that have the same atomic number but different mass numbers.

Radioactivity is the spontaneous emission of radiation from the nucleus of an unstable atom, as the nucleus disintegrates to become more stable.

Radioactive decay is the process in which an unstable nucleus changes into a different nucleus, or into a less energetic state of the same nucleus, by emitting radiation.

Transmutation is the change of the nucleus of one element into the nucleus of a different element during radioactive decay.

Alpha particles are positively charged particles emitted from a radioactive nucleus, each made of two protons and two neutrons bound together.

Beta particles are fast-moving electrons emitted from the nucleus of a radioactive atom.

Gamma rays are electromagnetic waves of very short wavelength and very high frequency emitted from the nucleus of a radioactive atom.

Alpha decay is the radioactive decay in which a nucleus emits an alpha particle, so that its atomic number falls by 2 and its mass number falls by 4.

Beta decay is the radioactive decay in which a nucleus emits a beta particle, so that its atomic number rises by 1 while its mass number stays the same.

Radioactive tracers are small amounts of a radioisotope added to a substance so that its path through a system can be followed by detecting the radiation.

Key points

The nucleus contains protons and neutrons; Z is the number of protons, A the number of nucleons, and N = A - Z.
Isotopes have the same Z but different A; radioactive isotopes are called radioisotopes.
Radioactivity is the spontaneous disintegration of unstable nuclei and is not affected by temperature, pressure or chemical state.
Alpha particles are helium nuclei, beta particles are fast electrons from the nucleus, and gamma rays are electromagnetic waves.
Ionising power: alpha > beta > gamma; penetrating power: gamma > beta > alpha.
Alpha decay lowers Z by 2 and A by 4; beta decay raises Z by 1 and leaves A unchanged; gamma emission changes neither.
In every nuclear equation, the totals of Z and of A are the same on both sides.
Radioisotopes are used in cancer treatment, tracers, sterilisation, food preservation, leak detection, thickness gauges and carbon dating.
Radiation damages cells, causing radiation sickness, cancer and genetic mutations; time, distance and shielding are the main protections.
Background radiation comes from cosmic rays, rocks, radon and the body, and from X-rays, fallout and other man-made sources.
Nuclear waste is sorted by level; high-level waste is cooled, vitrified, sealed and stored in shielded facilities.
Fission splits a heavy nucleus and fusion joins light nuclei; both release energy from a small loss of mass, E = mc².

Questions answered

Define atomic number and mass number. How many neutrons are there in ⁴⁰₁₉K?

The atomic number is the number of protons in the nucleus; the mass number is the total number of protons and neutrons in it. Neutrons: 40 - 19 = 21.

What are isotopes? Give one example.

Isotopes are atoms of the same element with the same atomic number but different mass numbers, for example carbon-12 and carbon-14.

Why is radioactivity called a nuclear phenomenon?

It starts in the nucleus and is not affected by temperature, pressure or chemical combination, which only affect the outer electrons.

Which radiation has (a) the greatest ionising power, (b) the greatest penetrating power, (c) no charge?

(a) Alpha particles. (b) Gamma rays. (c) Gamma rays.

Give the reason why beta particles are deflected more than alpha particles in a magnetic field.

A beta particle is thousands of times lighter than an alpha particle, so although its charge is half as large, it is bent far more easily.

A beta particle is an electron, yet the nucleus contains no electrons. Explain where it comes from.

A neutron in the nucleus changes into a proton and an electron; the proton stays and the electron is emitted as the beta particle.

Polonium-210, ²¹⁰₈₄Po, emits an alpha particle. Write the equation, taking the daughter as lead (Pb).

²¹⁰₈₄Po → ²⁰⁶₈₂Pb + ⁴₂He.

A nucleus ^A_ZX emits a gamma ray. What happens to A and Z, and why?

Neither changes, because a gamma ray has no charge and no mass; the nucleus only loses energy.

Past board exam questions

ICSE 2027 sample paper · 1 mark

A radioactive nucleus has 90 protons and 140 neutrons. It undergoes two successive alpha decays. What are the mass number and atomic number of the final nucleus?

Mass number = 222, atomic number = 86
Mass number = 226, atomic number = 88
Mass number = 224, atomic number = 86
Mass number = 222, atomic number = 88

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Mass number = 222, atomic number = 86

ICSE 2027 sample paper · 2 marks

Answer the following questions according to the instructions given: (a) When ⁶Li₃ is bombarded with a neutron, it produces ⁴He₂ and an isotope of hydrogen. Write the notation for the hydrogen isotope formed. (b) Fill in the blank: The mass of the reactants in a fission reaction is __________ (greater / less) than the mass of the products.

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(a) ⁶₃Li + ¹₀n → ⁴₂He + ³₁H; the isotope is tritium, ³₁H (written ³H₁ in the notation of the question). (b) greater (the lost mass is released as energy).

ICSE 2026 board paper · 1 mark

An atom of lithium contains 3 electrons, 3 protons and 4 neutrons. Its mass number is:

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ICSE 2026 sample paper · 2 marks

(a) Name the radiation that is emitted during the decay of a nucleus that has the highest penetrating power. (b) Does the emission of the above-mentioned radiation result in a change in the mass number?

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(a) Gamma (γ) radiation. (b) No. Gamma rays are electromagnetic waves with no mass and no charge, so neither the mass number nor the atomic number changes; only the energy of the nucleus decreases.

ICSE 2026 board paper · 2 marks

Give two differences between nuclear fission and nuclear fusion.

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1. In nuclear fission a heavy nucleus (e.g. U-235) splits into two lighter nuclei; in nuclear fusion two light nuclei (e.g. of hydrogen) combine to form a heavier nucleus. 2. Fission can take place at ordinary temperature (when the nucleus is bombarded with slow neutrons); fusion needs extremely high temperature and pressure (about 10⁷ K). (Also: the products of fission are radioactive, while the products of fusion are generally not.)

ICSE 2026 sample paper · 4 marks

The graph (Fig. A) illustrates the correlation between the number of protons (x-axis) and the number of neutrons (y-axis) for elements A, B, C, D, and E in the periodic table. These elements are denoted by the letters rather than their conventional symbols. The points plotted in Fig. A are: | Element | A | B | C | D | E | |---|---|---|---|---|---| | Number of protons | 88 | 90 | 92 | 93 | 93 | | Number of neutrons | 144 | 144 | 146 | 146 | 145 | (a) Identify the radioactive radiation emitted when element C decays into element E. Represent this using a nuclear reaction. (b) What is the special name given to elements D and E? (c) If element C transforms into element B by emitting a radioactive ray, how will this ray behave in an electric field?

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(a) C has 92 protons and 146 neutrons (mass number 238); E has 93 protons and 145 neutrons (mass number 238). The atomic number increases by 1 and the mass number is unchanged, so a beta (β) particle is emitted: ²³⁸₉₂C → ²³⁸₉₃E + ⁰_-1β (b) Isotopes (same number of protons, 93, but different numbers of neutrons — mass numbers 239 and 238). (c) C (Z = 92, A = 238) → B (Z = 90, A = 234): an alpha particle is emitted. Being positively charged, it is deflected towards the negative plate in an electric field.

ICSE 2026 board paper · 4 marks

(a) 1. Which element is used in the lining of the special aprons worn by workers in nuclear power plants? 2. Why is this element preferred? (b) ²⁴₁₁Na emits a nuclear radiation which does not alter the mass number but is deflected by a magnetic field. 1. Name the type of nuclear radiation emitted by ²⁴₁₁Na. 2. Write the equation for this radioactive decay.

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(a) 1. Lead. 2. Lead is very dense with a high atomic number, so it absorbs harmful radiations (gamma rays, X-rays) effectively and stops them from reaching the body. (b) 1. Beta (β⁻) radiation — it is charged (so deflected by a magnetic field) and leaves the mass number unchanged. 2. ²⁴₁₁Na → ²⁴₁₂Mg + ⁰_-1e (+ antineutrino)

ICSE 2025 sample paper · 1 mark

A nucleus of an atom consists of 146 neutrons and 95 protons. It decays after emitting an alpha particle. How many protons and neutrons are left in the nucleus after an alpha emission?

protons = 93 , neutrons = 142
protons = 95 , neutrons = 144
protons = 93 , neutrons = 144
protons = 95 , neutrons = 142

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protons = 93 , neutrons = 144