Radioactivity Class 10 ICSE — Selina Chapter 12
Radioactivity class 10 ICSE is the subject of this Selina chapter. Chapter 12 moves from the atom’s structure to nuclear fission and fusion. You will learn why unstable nuclei emit alpha, beta and gamma radiation, how each type behaves, and how nuclear energy is released. The official CISCE website hosts the current syllabus for this chapter.
Download Radioactivity Class 10 ICSE Physics Chapter 12 PDF
Download the Radioactivity Class 10 ICSE Physics chapter PDF from the Selina Concise textbook to revise offline or print for practice. It is the full chapter as printed in the book — 24 pages, 23 sections, 74 questions.
What is inside Chapter 12
24 pages · 23 sections · 1 worked examples · 74 exercise questions
| Section | Title |
|---|---|
| 12.1 | STRUCTURE OF THE ATOM AND NUCLEUS |
| 12.2 | ATOMIC MODEL |
| 12.3 | ISOTOPES |
| 12.4 | ISOBARS |
| 13.5 | ISOTONES |
| 12.6 | RADIOACTIVITY |
| 12.7 | RADIOACTIVITY AS EMISSION OF ALPHA (alpha), BETA (beta) AND GAMMA (gamma) RADIATIONS |
| 12.8 | PROPERTIES OF ALPHA PARTICLES |
| 12.9 | PROPERTIES OF BETA PARTICLES |
| 12.10 | PROPERTIES OF GAMMA RADIATIONS |
| 12.11 | DISTINCTION BETWEEN THE PROPERTIES OF alpha, beta AND gamma RADIATIONS |
| 12.12 | CHANGES WITHIN THE NUCLEUS IN ALPHA, BETA AND GAMMA EMISSION |
| 12.13 | USES OF RADIOACTIVITY – RADIO ISOTOPES |
| 12.14 | SOURCES OF HARMFUL RADIATIONS |
| 12.15 | HARMFUL EFFECTS OF RADIATION |
| 12.16 | SAFETY PRECEDURE WHILE USING NUCLEAR ENERGY |
| 12.17 | BACKGROUND RADIATIONS |
| 12.18 | NUCLEAR ENERGY |
| 12.19 | NUCLEAR FISSION |
| 12.20 | DISTINCTION BETWEEN THE RADIOACTIVE DECAY AND NUCLEAR FISSION |
| 12.21 | NUCLEAR FUSION |
| 18.3 | MeV energy is released in this process. The nuclear reactions are |
| 12.22 | DISTINCTION BETWEEN THE NUCLEAR FISSION AND NUCLEAR FUSION |
- EXERCISE-12(A): 47 questions
- MULTIPLE CHOICE TYPE: 4 questions
- EXERCISE-12(B): 19 questions
- MULTIPLE CHOICE TYPE: 4 questions
12.1 STRUCTURE OF THE ATOM AND NUCLEUS
An atom holds electrons, protons and neutrons. Protons and neutrons sit in the nucleus at the centre. Electrons revolve in stationary shells and do not radiate energy. The maximum electrons a shell can hold is 2n^2. The K, L, M and N shells hold 2, 8, 18 and 32 electrons respectively.
The atomic number Z counts the protons in the nucleus. The mass number A counts all nucleons, meaning protons plus neutrons. A proton has charge +1.6 \times 10^{-19}\ \text{C} and mass 1.67 \times 10^{-27}\ \text{kg}. A neutron has the same mass with zero charge. The nucleus is 10^{-5} to 10^{-4} times the atom’s size. For nitrogen with 7 protons and 7 neutrons, Z = 7 and A = 14.
12.2 ATOMIC MODEL
An atom is neutral, so protons equal electrons. For atomic number Z and mass number A, electrons = Z, protons = Z, neutrons = A - Z. The symbol {}^A_Z\mathrm{X} specifies the atom, where \mathrm{X} is the element’s chemical symbol.
A chemical change alters only orbital electrons. A nuclear change alters the nucleons inside the nucleus. Nuclear changes need a few MeV of energy, about 10^6 times that of chemical changes. For oxygen with A = 16 and Z = 8, neutrons = 16 - 8 = 8. Its K shell holds 2 electrons and its L shell holds 6.
12.3 ISOTOPES
Isotopes are atoms of the same element with the same atomic number Z but different mass number A. They share the same number of protons and electrons, so their chemical properties are identical. Only the neutron count differs.
Oxygen-16 (^{16}_8\mathrm{O}) has 8 neutrons, while oxygen-18 (^{18}_8\mathrm{O}) has 10 neutrons. Stable isotopes have neutrons nearly equal to protons. Radio isotopes have many more neutrons than protons and decay. Carbon-14 (^{14}_6\mathrm{C}) is a radio isotope used for dating.
12.4 ISOBARS
Isobars are atoms of different elements with the same mass number A but different atomic number Z. They have the same total nucleons but different protons and neutrons. Since electrons equal protons, isobars differ in electron count too.
Argon-40 (^{40}_{18}\mathrm{Ar}) and calcium-40 (^{40}_{20}\mathrm{Ca}) are isobars. Each has 40 nucleons. Argon has 18 protons and 22 neutrons. Calcium has 20 protons and 20 neutrons. When the proton and neutron counts are swapped, the pair is called mirror isobars.
13.5 ISOTONES
Isotones are atoms with the same number of neutrons but different atomic number Z and mass number A. They share the same value of A - Z.
Carbon-14 (^{14}_6\mathrm{C}) has 6 protons and 8 neutrons. Nitrogen-15 (^{15}_7\mathrm{N}) has 7 protons and 8 neutrons. Both hold 8 neutrons, so they are isotones. Their different proton counts make them different elements.
12.6 RADIOACTIVITY
Becquerel discovered radioactivity in 1896. Uranium salts affected a wrapped photographic plate in a dark room. The rays passed through paper, glass and wood. Three types emerged: positively charged alpha, negatively charged beta, and uncharged gamma radiation.
Radioactivity is a nuclear phenomenon. It is the spontaneous emission of \alpha, \beta or \gamma radiation during nuclear decay. Temperature, pressure and chemical changes do not affect it. Individual decays are random and unpredictable. This ICSE chapter belongs to the Class 10 ICSE Physics collection.
12.7 RADIOACTIVITY AS EMISSION OF ALPHA (alpha), BETA (beta) AND GAMMA (gamma) RADIATIONS
Rutherford studied these radiations in 1903. In a perpendicular magnetic field, they split into three parts. Alpha particles deflect one way, beta particles the other, and gamma rays pass straight. Fleming’s left-hand rule gives the direction.
Beta particles deflect more than alpha because they are much lighter. In an electric field, alpha moves toward the negative plate, beta toward the positive plate, and gamma stays undeviated. ICSE questions often ask you to identify radiations from such deflection diagrams.
12.8 PROPERTIES OF ALPHA PARTICLES
An alpha particle is a helium nucleus with two protons and two neutrons, written ^{4}_{2}\mathrm{He} or \mathrm{He}^{2+}. Its mass is 6.68 \times 10^{-27}\ \text{kg}, about four times a proton’s mass. Its charge is +3.2 \times 10^{-19}\ \text{C}, twice a proton’s charge.
Alpha particles have the highest ionising power, roughly 100 times that of beta. Their penetrating power is small; a few centimetres of air or a sheet of paper stops them. They deflect less than beta because they are heavier. They cause fluorescence and affect photographic plates.
12.9 PROPERTIES OF BETA PARTICLES
Beta particles are fast electrons emitted from the nucleus, written ^{0}_{-1}\mathrm{e}. They form when a neutron inside the nucleus changes into a proton: ^{1}_{0}\mathrm{n} \rightarrow ^{1}_{1}\mathrm{p} + ^{0}_{-1}\mathrm{e}. They do not come from the electron shells.
Their mass is 9.1 \times 10^{-31}\ \text{kg}, equal to an electron’s. Their charge is -1.6 \times 10^{-19}\ \text{C}. Beta ionises less than alpha but about 100 times more than gamma. They pass through thin aluminium, but a 5 mm sheet stops them. Stopped by heavy metals, they produce X-rays.
12.10 PROPERTIES OF GAMMA RADIATIONS
Gamma radiations are electromagnetic waves with a wavelength near 10^{-13}\ \text{m}. They travel at 3 \times 10^{8}\ \text{m s}^{-1} in vacuum. They carry no charge and no rest mass, so fields do not deflect them.
Their ionising power is the lowest, 10^{-4} times that of alpha. Their penetrating power is the highest, passing through 30 cm of iron. Gamma rays come from the nucleus, while X-rays come from inner electron orbits. In the ICSE Class 10 syllabus, gamma is described as the most energetic of the three radiations.
12.11 DISTINCTION BETWEEN THE PROPERTIES OF alpha, beta AND gamma RADIATIONS
This section consolidates the differences in one comparative table. The key idea is the inverse relation between ionising and penetrating power. The Class 10 ICSE syllabus tests this table heavily in MCQs.
| Property | Alpha | Beta | Gamma |
|---|---|---|---|
| Speed | 10^7\ \text{m s}^{-1} | 2.7 \times 10^8\ \text{m s}^{-1} | 3 \times 10^8\ \text{m s}^{-1} |
| Charge | +3.2 \times 10^{-19}\ \text{C} | -1.6 \times 10^{-19}\ \text{C} | zero |
| Ionising power | maximum | moderate | minimum |
| Penetrating power | least | large | very large |
| Stopped by | thin paper | 5 mm aluminium | 30 cm iron |
Alpha does the least biological damage, gamma the most. The table is a frequent source of exam questions.
12.12 CHANGES WITHIN THE NUCLEUS IN ALPHA, BETA AND GAMMA EMISSION
Alpha emission removes 2 protons and 2 neutrons. Atomic number Z drops by 2 and mass number A drops by 4. Beta emission converts a neutron into a proton, so Z rises by 1 while A stays the same. Gamma emission changes no Z or A; it only releases excess energy.
For polonium-210 decaying by alpha: ^{210}_{84}\mathrm{Po} \rightarrow ^{206}_{82}\mathrm{Pb} + ^{4}_{2}\mathrm{He}. In a single decay, alpha and beta are never emitted together. Gamma may accompany either. The anti-neutrino \bar{\nu} accompanies beta emission.
12.13 USES OF RADIOACTIVITY – RADIO ISOTOPES
Radio isotopes serve medical, scientific and industrial fields. Cobalt-60 emits gamma rays used to treat cancer. Iodine-131 and other salts act as tracers to detect brain tumours and study blood circulation. Carbon-14 dating estimates the age of archaeological finds.
Industry uses beta penetration to control the thickness of paper and metal sheets. Uranium-235 fuels atomic reactors. The tracer principle is simple: emitted radiation reveals where the isotope has gone. This ICSE chapter links these uses directly to the properties of each radiation.
12.14 SOURCES OF HARMFUL RADIATIONS
Three main sources produce harmful radiation. Nuclear fall-out escapes when reactor accidents release radioactive material into the atmosphere. Nuclear waste consists of spent fuel rods that stay radioactive and can contaminate soil and water. Cosmic rays and X-rays from accelerators add further exposure.
Three major reactor accidents occurred in 1979, 1986 and 2011. Waste must be sealed because it remains dangerous for long periods. Cosmic charged particles are partly deflected by Earth’s magnetic field.
12.15 HARMFUL EFFECTS OF RADIATION
Radiation interacts with living tissue within 10^{-14}\ \text{s} and causes biological damage. Effects fall into three types. Short-term effects like nausea, diarrhoea and hair loss are recoverable. Long-term effects like leukemia and cancer are irrecoverable. Genetic effects appear in later generations through mutated genes.
Pregnant women face higher risks from long X-ray exposure. Gamma rays damage blood cells, the gastrointestinal tract and DNA. The section keeps the facts clinical, not alarmist.
12.16 SAFETY PRECEDURE WHILE USING NUCLEAR ENERGY
Safety covers three areas: plant construction, handling radioactive materials, and waste disposal. A reactor needs lead and steel shielding, an airtight concrete building, and backup cooling to prevent core melting.
Workers wear lead aprons, use long tongs, and carry film badges that record exposure. Waste is sealed in thick casks and buried deep underground, far from populated areas. Each measure exists because radiation cannot be seen or felt.
12.17 BACKGROUND RADIATIONS
Background radiation is the radiation we all receive even without a radioactive source nearby. Internal sources include potassium-40, carbon-14 and radium inside our bodies. External sources include cosmic rays, radon-222 and solar radiation.
Total background exposure stays below the maximum permissible dose. It poses no health concern. We cannot avoid it, but we need not worry about it.
12.18 NUCLEAR ENERGY
Einstein’s mass-energy equivalence gives E = (\Delta m)c^2. Here \Delta m is the mass lost in kilograms, c is the speed of light, and E is the energy in joules. One atomic mass unit equals 1.66 \times 10^{-27}\ \text{kg}. Losing 1 a.m.u. releases 931 MeV.
If a reaction loses 0.15 a.m.u., the energy released is 0.15 \times 931 = 140\ \text{MeV}. This relation explains why nuclear changes release far more energy than chemical ones. For more chapters, browse the Selina Class 10 books page.
12.19 NUCLEAR FISSION
Fission splits a heavy nucleus into two lighter fragments when a slow neutron strikes it. The general pattern for uranium-235 is ^{235}_{92}\mathrm{U} + ^{1}_{0}\mathrm{n} \rightarrow ^{144}_{56}\mathrm{Ba} + ^{89}_{36}\mathrm{Kr} + 3^{1}_{0}\mathrm{n} + \text{energy}. Each fission releases about 190 MeV.
The three released neutrons can trigger more fissions, forming a chain reaction. Moderators like graphite or heavy water absorb excess neutrons to control it. A nuclear reactor runs a controlled chain reaction. An atomic bomb relies on an uncontrolled one.
12.20 DISTINCTION BETWEEN THE RADIOACTIVE DECAY AND NUCLEAR FISSION
Radioactive decay is a spontaneous self-process. Fission needs bombardment by neutrons. Decay releases modest energy as alpha or beta particles with gamma rays. Fission releases a tremendous burst and splits the nucleus into two nearly equal fragments.
Decay cannot be controlled. Fission can be controlled in a reactor. Think of decay as a watch spring breaking on its own, and fission as the same spring shattered by a hammer.
12.21 NUCLEAR FUSION
Fusion combines two light nuclei into a heavier one. Mass is lost, and energy appears according to E = (\Delta m)c^2. Fusion needs a temperature near 10^7\ \text{K} because the positive charges of the nuclei repel strongly.
At that temperature, thermal agitation gives the nuclei enough kinetic energy to overcome repulsion. Fusion powers the Sun and stars. An example is deuterium plus tritium: ^{2}_{1}\mathrm{H} + ^{3}_{1}\mathrm{H} \rightarrow ^{4}_{2}\mathrm{He} + ^{1}_{0}\mathrm{n} + 17.6\ \text{MeV}.
18.3 MeV energy is released in this process. The nuclear reactions are
This section continues the fusion chain from 12.21. Two deuterium nuclei fuse: ^{2}_{1}\mathrm{H} + ^{2}_{1}\mathrm{H} \rightarrow ^{3}_{2}\mathrm{He} + ^{1}_{0}\mathrm{n} + 3.3\ \text{MeV}.
Then helium-3 fuses with another deuterium: ^{3}_{2}\mathrm{He} + ^{2}_{1}\mathrm{H} \rightarrow ^{4}_{2}\mathrm{He} + ^{1}_{1}\mathrm{H} + 18.3\ \text{MeV}. Three deuterium nuclei in total release 21.6 MeV. Per unit mass, fusion yields far more energy than fission.
12.22 DISTINCTION BETWEEN THE NUCLEAR FISSION AND NUCLEAR FUSION
Fission splits a heavy nucleus; fusion joins two light nuclei. Fission works at ordinary temperature and pressure; fusion needs about 10^7\ \text{K}. One fission releases nearly 190 MeV, while one fusion releases about 24.7 MeV.
For the same mass, fusion releases much more energy than fission. Fission fuel is radioactive and its waste is hard to dispose. Fusion fuel is abundant and non-radioactive. Fission can be controlled in reactors; fusion cannot yet be controlled. This comparison is a common exam question.
What to practise from Radioactivity Class 10 ICSE
Work through the exercise blocks in the book’s order.
- EXERCISE-12(A): 47 questions
- MULTIPLE CHOICE TYPE: 4 questions
- EXERCISE-12(B): 19 questions
- NUMERICALS inside Exercise 12(B): 2 questions
- MULTIPLE CHOICE TYPE: 4 questions
For the current Class 10 ICSE Physics syllabus and exam format, check the official CISCE website.
Radioactivity Class 10 ICSE frequently asked questions
What is radioactivity Class 10 ICSE and why is it a nuclear phenomenon?
Radioactivity is the spontaneous emission of alpha, beta or gamma radiation from an unstable nucleus. It is a nuclear phenomenon because physical and chemical changes do not alter it. Temperature, pressure and chemical treatment leave the rate of decay unchanged.
Why is a beta particle emitted from the nucleus in ICSE Class 10 radioactivity?
A beta particle forms when a neutron inside the nucleus changes into a proton. The electron is created at that instant and emitted immediately. It does not come from the electron shells, which is why beta rays differ from cathode rays.
What is the difference between nuclear fission and nuclear fusion in this ICSE chapter?
Fission splits a heavy nucleus after neutron bombardment. Fusion joins two light nuclei at very high temperature. Fission releases about 190 MeV per reaction and can be controlled. Fusion releases more energy per unit mass but cannot yet be controlled.
How do I complete a nuclear equation for alpha or beta decay in Class 10 ICSE Physics?
For alpha decay, reduce the mass number by 4 and the atomic number by 2, and add a helium nucleus ^{4}_{2}\mathrm{He} as a product. For beta decay, keep the mass number the same, increase the atomic number by 1, and add an electron ^{0}_{-1}\mathrm{e}. Check that protons and mass numbers balance on both sides.
Why does gamma emission not change the atomic number or mass number in radioactivity?
Gamma radiation carries no mass and no charge. It removes only excess energy from an excited nucleus. No protons or neutrons are lost, so Z and A stay the same. The nucleus simply drops to its ground state.
Reference: ICSE Class 10 Physics textbook material, chapter 12: Radioactivity — CISCE-aligned.