β’οΈ Chapter 20: Atomic & Nuclear Physics β Short Questions
Prepared by Muhammad Tayyab, Subject Specialist Physics, Govt Christian High School Daska. Based on PECTAA 2026 syllabus (National Curriculum 2023).
π What's Inside: This section covers all 58 short questions from Chapter 20 Atomic and Nuclear Physics including atomic structure, isotopes, radioactivity, alpha/beta/gamma radiation, nuclear reactions, fission, fusion, half-life, carbon dating, and applications of radiation. Each question is presented with a precise answer as per the official PECTAA 2026 Physics curriculum. Perfect for Punjab Boards (Lahore, Gujranwala, Multan, etc.) and all BISE boards across Pakistan.
π Related Resources β Chapter 20
π Exercise Short Qs
βοΈ Constructed Response
π Long Questions
π Numerical Problems
β
MCQs
Atomic and Nuclear Physics covers atomic structure, isotopes, radioactivity, alpha/beta/gamma decay, nuclear fission, fusion, half-life, and carbon dating.
π Quick Jump to Questions
π Short Questions with Answers
1 What are atomic and nuclear physics?
Answer: Atomic and nuclear physics are branches of science that help us to understand the smallest building blocks of matter. Atomic physics deals with the study of the structure of atoms, while nuclear physics focuses on the nucleus of the atom, which contains protons and neutrons.
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Atomic physics: structure of atoms. Nuclear physics: nucleus (protons & neutrons).
2 What is an atom?
Answer: An atom is a tiny particle of matter. Atoms contain positively charged protons, negatively charged electrons, and neutrons.
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Atom: protons (+), electrons (-), neutrons (neutral).
3 What did Rutherford's alpha-particle scattering experiment show?
Answer: It showed that the atom is mostly empty space and that most of its mass and positive charge are concentrated in a tiny central nucleus.
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Atom is mostly empty space; positive charge concentrated in nucleus.
4 What are the findings of Rutherford's experiment? OR What are the main characteristics of the nucleus?
Answer: i. Atoms have a small, dense, and positively charged nucleus. ii. The nucleus contains most of the mass of the atom. iii. Electrons orbit the nucleus at large distances, so the atom is mostly empty space. iv. The nucleus is extremely small, about 10,000 times smaller than the entire atom.
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Small, dense, positive nucleus; contains most mass; mostly empty space.
5 How do electrons exist around the nucleus?
Answer: Later studies revealed that electrons do not follow fixed orbits like planets around the Sun. Instead, they exist in regions called electron clouds, where their exact position cannot be known.
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Electrons exist in electron clouds (probability regions).
6 What is meant by an electron cloud?
Answer: This "fuzzy cloud" represents the probability of finding an electron in a certain area around the nucleus. The cloud is denser where the electron is more likely to be and thinner where it is less likely.
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Probability region for finding an electron.
7 What is the nucleus of an atom?
Answer: The nucleus is the central part of an atom which holds most of its mass. It is made up of protons and neutrons. Protons and neutrons are held together by the strong nuclear force.
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Central part: protons + neutrons held by strong nuclear force.
8 What is the charge number Z? OR What is atomic number (Z)?
Answer: The charge number Z tells us how many protons an atom has, which is unique for each element. It is also called atomic number.
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Atomic number = number of protons (Z).
9 What is the mass number A?
Answer: The mass number A is the total count of protons and neutrons in an atom's nucleus. Since electrons have very little mass, they are not included in the mass number.
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Mass number = protons + neutrons (A).
10 What is a nuclide? Explain its notation.
Answer: A nuclide is represented by \(_{Z}^{A}\mathrm{X}\), where X is its chemical symbol, the superscript is mass number A and subscript is charge number Z.
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\(_{Z}^{A}\mathrm{X}\): Z = protons, A = mass number.
11 How is the number of neutrons calculated?
Answer: The mass number A is the total count of protons and neutrons in an atom's nucleus. Therefore, the number of neutrons is: \(N = A - Z\), where A is the mass number and Z is the charge number.
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Neutrons (N) = A β Z.
12 If protons repel each other, how can so many of them stay packed in a tiny nucleus without flying apart? What holds them together? OR What holds protons and neutrons together in the nucleus?
Answer: Protons and neutrons are held together by the strong nuclear force.
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Strong nuclear force holds protons and neutrons together.
13 What are isotopes?
Answer: Isotopes are atoms of an element that have the same number of protons but different numbers of neutrons.
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Same protons, different neutrons.
14 What happens when an atom gains or loses electrons?
Answer: Atoms are electrically neutral, meaning they have equal number of protons and electrons. If an atom gains or loses electrons, it becomes an ion and carries an electric charge.
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Gains/loses electrons β becomes ion (charged).
15 How many isotopes does carbon have and how many neutrons do they contain?
Answer: Carbon has three isotopes \(_{6}^{12}\mathrm{C}\), \(_{6}^{13}\mathrm{C}\) and \(_{6}^{14}\mathrm{C}\) containing 6, 7 and 8 neutrons, respectively.
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Carbon-12 (6n), Carbon-13 (7n), Carbon-14 (8n).
16 What is radioactivity?
Answer: Radioactivity is the process in which unstable atoms release energy to become stable. This energy is given off as radiation, which can be in the form of particles or waves.
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Unstable atoms release energy (particles or waves).
17 What is radioactive decay?
Answer: Some nuclei become unstable due to an imbalance of protons and neutrons or excess energy inside. To become stable, the nucleus breaks down and gives off radiation in a process called radioactive decay.
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Unstable nucleus breaks down and emits radiation.
18 What are the three types of radiation released during radioactive decay?
Answer: Radioactive decay releases three types of radiation: Alpha (Ξ±), Beta (Ξ²), and Gamma (Ξ³) radiation, each with different properties.
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Alpha (Ξ±), Beta (Ξ²), Gamma (Ξ³).
19 What are alpha particles (Ξ±)?
Answer: Alpha particles (Ξ±) are helium nuclei, consisting of 2 protons and 2 neutrons each. They carry a +2e charge (e = 1.6 Γ 10β19 C), and are relatively large and heavy [4 atomic mass units, u].
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Helium nucleus: 2p + 2n, +2e charge.
20 What are the properties of alpha particles?
Answer: Due to their size, alpha particles have low penetration power and can be stopped by a sheet of paper or a few centimetres of air. However, they have high ionizing power, meaning they strongly affect other atoms they come in contact with.
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Low penetration, high ionizing power.
21 What are beta particles (Ξ²)?
Answer: Beta particles (Ξ²) are high-energy electrons (\(_{-1}^{0}\mathrm{e}\)) or positrons (\(_{+1}^{0}\mathrm{e}\)) emitted from the nucleus. They carry a β1e charge (electrons) or +1e charge (positrons) and have a very small mass.
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High-energy electrons/positrons, small mass.
22 What are the properties of beta particles?
Answer: Beta radiation has moderate penetration power. It can pass through paper but is stopped by a few millimetres of aluminium. Their ionizing power is moderate, meaning they cause less ionization than alpha particles but more than gamma rays.
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Moderate penetration, moderate ionizing power.
23 What are gamma rays (Ξ³)?
Answer: Gamma radiation (Ξ³) consists of high-energy electromagnetic waves (photons), making them massless and neutral (0 charge). Gamma rays are emitted out due to de-excitation of the nucleus.
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High-energy EM waves, massless, neutral.
24 What are the properties of gamma rays?
Answer: Gamma rays have very high penetration power, allowing them to pass through thick materials like lead or concrete. However, they have low ionizing power, meaning they cause much less ionization compared to alpha and beta particles.
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Very high penetration, low ionizing power.
25 What is ionization?
Answer: Ionization occurs when radiation removes electrons from atoms, turning them into ions. The ability of radiation to ionize atoms depends on its energy and charge.
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Radiation removes electrons β ions formed.
26 Compare the ionization of alpha, beta and gamma radiation.
Answer: Alpha (Ξ±) particles: Have strongest ionizing power due to their large size and charge. Beta (Ξ²) particles: Have moderate ionizing power, as they are smaller and faster than alpha particles. Gamma (Ξ³) rays: Have weakest ionizing power, as they have no charge and no mass.
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Alpha: strongest, Beta: moderate, Gamma: weakest.
27 What is meant by penetration power?
Answer: Penetration power refers to how far radiation can travel through materials before being absorbed.
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How far radiation travels through materials.
28 Compare the penetrating powers of alpha, beta and gamma radiation.
Answer: Alpha (Ξ±) particles: Lowest penetration β stopped by a sheet of paper or a few centimetres of air. Beta (Ξ²) particles: Moderate penetration β can pass through a sheet of paper but are stopped by a few millimetres of aluminum. Gamma (Ξ³) rays: Highest penetration β can pass through paper, aluminum, and even thick lead or concrete slabs.
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Alpha: lowest, Beta: moderate, Gamma: highest.
29 Write the general equation and example of Ξ±-decay.
Answer: General Equation: \(_{Z}^{A}\mathrm{X} \rightarrow _{Z-2}^{A-4}\mathrm{Y} + _{2}^{4}\mathrm{He}\). Example: \(_{92}^{238}\mathrm{U} \rightarrow _{90}^{234}\mathrm{Th} + _{2}^{4}\mathrm{He} + \text{Energy}\).
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\(_{Z}^{A}\mathrm{X} \rightarrow _{Z-2}^{A-4}\mathrm{Y} + _{2}^{4}\mathrm{He}\).
30 Write the general equation and example of Ξ²-decay.
Answer: General Equation: \(_{Z}^{A}\mathrm{X} \rightarrow _{Z+1}^{A}\mathrm{Y} + _{-1}^{0}\beta + \text{Energy}\). Example (Decay of Carbon-14): \(_{6}^{14}\mathrm{C} \rightarrow _{7}^{14}\mathrm{N} + _{-1}^{0}\beta + \text{Energy}\).
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\(_{Z}^{A}\mathrm{X} \rightarrow _{Z+1}^{A}\mathrm{Y} + _{-1}^{0}\beta + \text{Energy}\).
31 Write the general equation and example of Ξ³-decay.
Answer: General Equation: \(_{Z}^{A}\mathrm{X}^{*} \rightarrow _{Z}^{A}\mathrm{Y} + \gamma\). Example: \(_{27}^{60}\mathrm{Co}^{*} \rightarrow _{27}^{60}\mathrm{Co} + \gamma\).
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\(_{Z}^{A}\mathrm{X}^{*} \rightarrow _{Z}^{A}\mathrm{Y} + \gamma\).
32 What are nuclear reactions? What are the two types of nuclear reactions?
Answer: Nuclear reactions involve changes in the nucleus of an atom, leading to the transformation of elements and the release or absorption of energy. These reactions are different from chemical reactions, which only involve electrons. The two types of nuclear reactions are: (i) Fission Reaction (ii) Fusion Reaction.
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Fission (splitting) and Fusion (combining).
33 What is nuclear fission? Write the equation of fission reaction.
Answer: In fission reaction, a heavy nucleus (like Uranium-235) splits into two smaller nuclei when bombarded by a neutron. This process releases a large amount of energy and additional neutrons, which can cause a chain reaction. Equation: \(_{0}^{1}\mathrm{n} + _{92}^{235}\mathrm{U} \rightarrow _{56}^{141}\mathrm{Ba} + _{36}^{92}\mathrm{Kr} + 3_{0}^{1}\mathrm{n} + \text{Energy}\).
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Heavy nucleus splits into smaller nuclei + neutrons + energy.
34 What happens during nuclear fission?
Answer: A uranium nucleus splits into two nearly equal fragments after absorbing a slow-moving neutron. This process also releases extra neutrons and a large amount of energy.
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Uranium splits into fragments + neutrons + energy.
35 How does nuclear fission differ from chemical reactions in energy release?
Answer: The energy released from nuclear fission is far greater than energy from chemical reactions. For instance, burning 1 ton of coal releases 3.6 Γ 10βΉ J, while fission of 1 kg of U-235 produces around 6.7 Γ 10ΒΉΒ³ J.
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Fission energy is millions of times greater than chemical reactions.
36 What is nuclear fusion? Under what conditions does nuclear fusion occur?
Answer: Nuclear fusion is a process where two or more light nuclei (like hydrogen isotopes) combine to form a heavier nucleus, releasing a huge amount of energy. This occurs at extremely high temperatures and speeds, where some mass is converted into energy.
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Light nuclei combine β heavier nucleus + huge energy (at high temperatures).
37 What happens when deuterium and tritium fuse?
Answer: When deuterium and tritium fuse, they form helium (alpha particle) and release energy. Equation: \(_{1}^{2}\mathrm{H} + _{1}^{3}\mathrm{H} \rightarrow _{2}^{4}\mathrm{He} + _{0}^{1}\mathrm{n} + \text{Energy}\).
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Deuterium + Tritium β Helium + neutron + energy.
38 Why do the Sun and stars shine?
Answer: The Sun and stars shine because of fusion reactions, where four hydrogen nuclei fuse into one helium nucleus, releasing about 25.7 MeV of energy.
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Fusion of hydrogen into helium releases energy.
39 What is meant by the interconversion of matter and energy?
Answer: Matter and energy can be converted into each other, a concept explained by Einstein's equation \(E = mc^2\). A small amount of mass can turn into a huge amount of energy, as seen in nuclear fission and fusion.
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Matter β Energy conversion (\(E = mc^2\)).
40 What is Einstein's equation?
Answer: Einstein's equation is \(E = mc^2\). It states that a small amount of mass \((m)\) converts into a large amount of energy \((E)\), where \(c = 3 \times 10^{8} \mathrm{ms}^{-1}\) is the speed of light.
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\(E = mc^2\): mass-energy equivalence.
41 What is mass defect?
Answer: In nuclear reactions, the total mass of the products is slightly less than the mass of the reactants. This missing mass is called mass defect \((\Delta m)\).
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Mass defect = missing mass in nuclear reactions.
42 How is energy released from mass defect?
Answer: Using Einstein's equation \(E = mc^2\), the energy released can be calculated from the mass defect. Here, \(E\) is energy, \(m\) is mass, and \(c\) is the speed of light.
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Energy released = mass defect Γ \(c^2\).
43 What is the uniform mass scale \((u)\)?
Answer: The term uniform mass scale \((u)\) refers to atomic mass unit \((u)\) being used as a standardized unit to measure the mass of atoms and molecules. \(1u = \frac{1}{12}\) of the mass of a carbon-12 atom, \(1u = 1.66605 \times 10^{-27} \mathrm{kg}\).
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Atomic mass unit (u) = 1/12 of carbon-12 mass.
44 What is the activity of a radioactive material?
Answer: The activity of a radioactive material is the rate at which its atoms decay, releasing radiation in the form of alpha, beta, or gamma particles. It tells us how many atoms break down in a given period.
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Rate of radioactive decay (atoms per second).
45 What is the unit of activity of a radioactive material?
Answer: The activity of a radioactive material is measured in Becquerels \((Bq)\), where \(1Bq = 1\) disintegration per second, or in Curies \((Ci)\), where \(1Ci = 3.7 \times 10^{10}\) disintegrations per second (dps).
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Becquerel (Bq) = 1 decay per second; Curie (Ci) = 3.7Γ10ΒΉβ° dps.
46 What is half-life?
Answer: The half-life of a radioactive isotope is the time it takes for half of the radioactive atoms in a sample to decay into a more stable form.
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Time for half of atoms to decay.
47 How does half-life relate to activity?
Answer: A material with a shorter half-life decays faster and has higher activity, while one with a longer half-life decays slower and has lower activity.
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Shorter half-life = higher activity; longer half-life = lower activity.
48 What is the half-life of a radioactive isotope?
Answer: The half-life of a radioactive isotope is the time it takes for half of its atoms to decay into a more stable form.
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Time for half of atoms to decay.
49 What is meant by the exponential pattern of radioactive decay?
Answer: Radioactive decay follows an exponential pattern, meaning that after each half-life, only half of the remaining material is left.
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After each half-life, half of the remaining material decays.
50 What is carbon dating?
Answer: Carbon dating is a scientific method used to determine the age of ancient organic materials like wood, bones, and shells, up to 50,000 years old. This technique is based on the radioactive decay of carbon-14.
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Method to age ancient organic materials using C-14 decay.
51 How does carbon dating determine the age of ancient materials?
Answer: By measuring the remaining carbon-14 in a sample and comparing it to its original amount, scientists can estimate how long ago the organism died.
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Compare remaining C-14 to original amount β estimate age.
52 What are the effects of ionizing radiation on living organisms?
Answer: (i) Ionizing radiation can alter or damage atoms and molecules, including DNA. The impact depends on the dose, exposure time, and type of radiation. (ii) Radiation can cause DNA mutations, breaking chemical bonds and altering genetic codes, which may lead to genetic disorders or hereditary diseases. (iii) Long-term exposure to low doses increases the risk of cancer by mutating normal body cells. For example, too much UV radiation from the Sun can lead to skin cancer. (iv) Severe exposure over a short time can cause radiation sickness, leading to nausea, vomiting, organ failure, and even death in extreme cases.
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DNA damage, mutations, cancer, radiation sickness.
53 What are the medical uses of nuclear radiation?
Answer: Short-lived isotopes are used in radiotherapy and imaging to minimize long-term radiation risks. Cobalt-60 is used in radiotherapy, while Iodine-131 is used in thyroid diagnosis and treatment.
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Cobalt-60: radiotherapy; Iodine-131: thyroid diagnosis/treatment.
54 How are Cobalt-60 and Cesium-137 used in food preservation?
Answer: Gamma rays from Cobalt-60 and Cesium-137 kill bacteria in food, increasing shelf life without affecting quality. Cobalt-60, with a moderate half-life (5.27 years), is commonly used in the food industry.
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Gamma rays kill bacteria β increase shelf life.
55 How is Cobalt-60 used in sterilization?
Answer: Gamma radiation is used to sterilize medical equipment, killing bacteria and viruses without damaging the materials.
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Gamma radiation sterilizes medical equipment.
56 How is Americium-241 used in smoke detectors?
Answer: Americium-241 is used in fire alarms, where alpha particles ionize air molecules, creating an electric current. When smoke enters, the current is disrupted, triggering the alarm.
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Alpha particles ionize air β current disrupted by smoke β alarm.
57 How are Cobalt-60 and Iodine-131 used in medicine?
Answer: Gamma rays from Cobalt-60 are used in radiotherapy to destroy cancer cells, while Iodine-131 is used in thyroid diagnosis and treatment to minimize long-term radiation exposure.
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Cobalt-60: destroy cancer cells; Iodine-131: thyroid diagnosis/treatment.
58 What are the medical uses of nuclear radiation? (Alternative wording)
Answer: Short-lived isotopes are used in radiotherapy and imaging to minimize long-term radiation risks. Cobalt-60 is used in radiotherapy, while Iodine-131 is used in thyroid diagnosis and treatment.
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Cobalt-60: radiotherapy; Iodine-131: thyroid diagnosis/treatment.
π Key Formulas β Atomic & Nuclear Physics
\[ N = A - Z \]
Number of Neutrons
Number of Neutrons
\[ n = \frac{t}{T_{1/2}} \]
Number of Half-lives
Number of Half-lives
\[ N = \frac{1}{2^n} N_o \]
Remaining Quantity
Remaining Quantity
\[ E = mc^2 \]
Mass-Energy Equivalence
Mass-Energy Equivalence
Unit 10Thermal Physics
Unit 11Transfer of Thermal Energy
Unit 12Waves
Unit 13Sound
Unit 14Light
Unit 15Electrostatics
Unit 16Electricity
Unit 17Electromagnetism
Unit 18Electromagnetic Induction & EM Waves
Unit 19Electronics
Unit 20β¨ Atomic and Nuclear Physics
Unit 21Space and Environment
π Complete syllabus coverage for Class 10 Physics (PECTAA 2026) β Units 10 to 21
Created by Hira Science Academy | Aligned with PECTAA 2026 Syllabus