β’οΈ Chapter 20: Atomic & Nuclear Physics β Long 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 5 long questions from Chapter 20 Atomic and Nuclear Physics including atom structure & chemical properties, Einstein's mass-energy equation, radioactive decay processes (alpha, beta, gamma), isotopes properties, and carbon dating. Each question is presented with a detailed 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 Short Questions
π Exercise Short Qs
βοΈ Constructed Response
π Numerical Problems
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MCQs
Atomic and Nuclear Physics covers atomic structure, isotopes, radioactivity, alpha/beta/gamma decay, nuclear fission, fusion, half-life, and carbon dating.
π Long Questions with Answers
20.1 How does the structure of an atom determine its chemical properties? Explain with reference to the arrangement of protons, neutrons, and electrons?
Answer: The structure of an atom determines its chemical properties through the arrangement of its protons, neutrons, and electrons. The number of protons and electrons determines an atom's chemical properties. The charge number Z tells us how many protons an atom has, which is unique for each element. The number of protons in the nucleus is what defines an element.
Role of Electrons: Atoms are electrically neutral, meaning they have equal number of protons and electrons. Electrons exist in regions called electron clouds around the nucleus, and their arrangement determines the chemical behaviour of the atom.
Role of Neutrons: The number of neutrons affects an atom's nuclear properties, like stability and radioactivity, rather than its chemical properties.
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Protons define element; electrons determine chemical behaviour; neutrons affect nuclear properties.
20.2 Explain Einstein's equation; \(E = mc^2\), and its significance. How does this equation relate to nuclear reactions?
Answer: Einstein's equation \(E = mc^2\) states that a small amount of mass \((m)\) converts into a large amount of energy \((E)\), where \(c\) is the speed of light \((3 \times 10^{8} \text{m s}^{-1})\). This equation helps to calculate the energy released in nuclear reactions.
Mass Defect: 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 \((\Delta m)\). Using \(E = mc^2\), we can find the energy released.
Relation to Nuclear Reactions: The equation helps to calculate the energy released in nuclear reactions like fission and fusion.
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\(E = mc^2\): mass-energy equivalence; used to calculate energy from mass defect in fission/fusion.
20.3 Describe the process of radioactive decay and how it leads to the emission of alpha, beta, and gamma radiation?
Answer: Radioactive decay is the process in which unstable atoms release radiation to become stable. Some atoms have too many or too few neutrons in their nucleus, making them unstable. To become stable, they release energy in the form of radiation. This radiation can be in the form of particles (alpha and beta) or waves (gamma rays).
Alpha Decay: In alpha decay, an unstable nucleus emits an alpha particle (\(_{2}^{4}\mathrm{He}\)), reducing its atomic number by 2 and mass number by 4. General Equation: \(_{Z}^{A}\mathrm{X} \rightarrow _{Z-2}^{A-4}\mathrm{Y} + _{2}^{4}\mathrm{He}\). Example: \(_{92}^{238}\mathrm{U} \rightarrow _{90}^{234}\mathrm{Th} + _{2}^{4}\mathrm{He} + \text{Energy}\).
Beta Decay: In beta decay, a neutron converts into a proton and emits a beta particle (electron). General Equation: \(_{Z}^{A}\mathrm{X} \rightarrow _{Z+1}^{A}\mathrm{Y} + _{-1}^{0}\beta + \text{Energy}\). Example (Decay of Carbon-14): \(_{6}^{14}\mathrm{C} \rightarrow _{7}^{14}\mathrm{N} + _{-1}^{0}\beta + \text{Energy}\).
Gamma Decay: In gamma decay, the nucleus releases excess energy as a gamma ray (Ξ³) without changing the number of protons or neutrons. 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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Alpha: emits helium nucleus (Z-2, A-4). Beta: neutronβproton + electron. Gamma: emits energy (Ξ³) without changing Z or A.
20.4 Why do different isotopes of the same element have similar chemical properties but different physical properties? Explain.
Answer: Isotopes are atoms of an element that have the same number of protons but different numbers of neutrons.
Similar Chemical Properties: The number of protons and electrons determines an atom's chemical properties. Isotopes of the same element have the same number of protons and, in neutral atoms, the same number of electrons. Therefore, they have similar chemical properties. Even though hydrogen isotopes have different masses, they still behave chemically in the same way.
Different Physical Properties: Isotopes have different numbers of neutrons, which result in different masses. The number of neutrons also affects an atom's nuclear properties, like stability and radioactivity. Therefore, different isotopes can have different physical and nuclear properties. Example: The three isotopes of hydrogen have different numbers of neutrons and different masses, but they behave chemically in the same way because they have the same number of protons and electrons.
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Same protons/electrons β same chemical properties. Different neutrons β different mass & nuclear properties.
20.5 Explain how carbon dating uses carbon-14 to estimate the age of artifacts. Discuss its reliability, considering the influence of carbon-14's half-life?
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, a type of carbon found in the atmosphere.
How Carbon Dating Works: Living organisms constantly absorb carbon-14 while they are alive, maintaining a steady balance with carbon-12. However, when an organism dies, it stops absorbing carbon, and the carbon-14 starts to decay at a fixed rate. 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.
Half-Life of Carbon-14: Carbon-14 has a half-life of about 5,730 years. The half-life of a radioactive isotope is the time it takes for half of its radioactive atoms to decay into a more stable form. After each half-life, only half of the remaining carbon-14 is left. Thus, the fixed half-life allows scientists to predict how fast carbon-14 decays and estimate the age of the sample.
Reliability of Carbon Dating: Carbon dating is useful for dating organic materials up to about 50,000 years old. Its reliability depends on accurately measuring the amount of carbon-14 remaining in the sample. As carbon-14 decays at a fixed rate, the proportion of carbon-14 left in a specimen indicates its age.
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Based on C-14 decay (half-life 5730 years); measures remaining C-14 to estimate age up to 50,000 years; reliability depends on accurate measurement.
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