Physics · Class 12 Science
Ch 13Nuclei — Class 12 Physics, concept-first.
Every atom has, at its centre, a tiny, extremely dense core called the nucleus, in which almost the whole mass of the atom -- but only a positive charge, never the atom's electrons -- is concentrated.
Key concepts
Hover a concept to preview it and jump to its most relevant Q&A.
Composition and Size of the Nucleus
A nucleus is made of protons and neutrons, together called nucleons; mass number = total nucleons, atomic number = proton count (fixing the element).
Most relevant Q&A
- A gold nucleus is written as $^{197}_{79}\text{Au}$. Find the number of protons and neutrons in this nucleus, and state its mass number and…Free
- Using the empirical formula $R = R_0 A^{1/3}$ with $R_0 = 1.2\ \text{fm}$, find the radius of the nucleus of $^{27}_{13}\text{Al}$.Free
- Explain, using the relation between nuclear radius and mass number, why the density of nuclear matter is nearly the same for every nucleus,…Free
In previous exams
How often this chapter’s concepts have been examined — real appearance data, never estimated.
Chapter contents
The NCERT structure, section by section. Open a section to see its questions, then read the concept-first solution.
Introduction
Every atom has, at its centre, a tiny, extremely dense core called the nucleus, in which almost the whole mass of the atom -- but only a positive charge, never the atom's electrons -- is concentrated.…
Composition and Size of the Nucleus
The nucleus of an atom is made up of two kinds of particles, collectively called NUCLEONS: PROTONS, which carry a positive charge of exactly each, and NEUTRONS, which carry no electric charge at all.
Atomic Mass Unit; Isotopes, Isobars and Isotones
Because atoms and nuclei are so light, their masses are almost never expressed in kilograms directly; instead, physics and chemistry use a convenient unit of their own, the ATOMIC MASS UNIT (symbol u)…
Radioactivity: Alpha, Beta and Gamma Rays and Their Properties
RADIOACTIVITY is the spontaneous emission of particles or high-energy radiation from the nucleus of an unstable atom, first discovered by Henri Becquerel in uranium salts and studied in detail by Mari…
The Radioactive Decay Law
Radioactive decay is a purely random, statistical process: it is impossible to say exactly WHEN any one particular unstable nucleus will decay, but every nucleus of a given radioactive species has exa…
Mass-Energy Relation and Mass Defect
Einstein's special theory of relativity established that mass and energy are not two separate, independently conserved quantities as classical physics had assumed, but are, in fact, two different form…
Nuclear Binding Energy and Its Variation with Mass Number
The BINDING ENERGY of a nucleus, , is defined as the energy equivalent of its mass defect (Section 8.6), found using Einstein's relation:
Nuclear Fission
NUCLEAR FISSION is the process in which a heavy nucleus splits into two lighter nuclei of roughly comparable size, usually accompanied by the release of a few free neutrons and a very large amount of…
Nuclear Fusion
NUCLEAR FUSION is the process, in a sense the mirror image of fission, in which two light nuclei combine (fuse) together to form a single heavier nucleus, again releasing a very large amount of energy…
Summary
Composition and size: a nucleus contains protons and neutrons (together, nucleons); nuclear radius , , so nuclear volume and nuclear density is nearly the same () for every nucleus.
Sample & Board Papers
Sample papers and previous-year board questions for this subject.
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- Q1How many α and β-particles are emitted when U^238 changes to Pb^206 due to radioactivity? Atomic number of U^238 and Pb^206 are 92 and 82 re…Preview
- Q2A radioactive substance decays to 1/32 th its initial activity in 30 days. Calculate its half-life.Preview
- Q3Deduce the expression N = N₀ e⁻λt, for the law of radioactive decay.Preview
- Q4Derive the relation between half-life and decay constant for a radioactive element. **OR** The energy of an excited hydrogen atom is –3.4 eV…Preview
- Q5State the exponential law of radioactive decay. Establish the relation between half-life and decay constant of a radioactive element.Preview
- Q6i) Draw a diagram to show the variation of binding energy per nucleon with mass number of nucleus. ii) Use this graph to explain why light n…Preview
More questions
26 Q+−Show 9 questionsHide questions9 questions
- Example 1A gold nucleus is written as $^{197}_{79}\text{Au}$. Find the number of protons and neutrons in this nucleus, and state its mass number and…Free
- Example 2Using the empirical formula $R = R_0 A^{1/3}$ with $R_0 = 1.2\ \text{fm}$, find the radius of the nucleus of $^{27}_{13}\text{Al}$.Free
- Example 3Define the atomic mass unit (u). Given that $1\ \text{u} = 1.660539\times10^{-27}\ \text{kg}$, show that its energy equivalent is close to $…Free
- Example 4Classify each of the following pairs of nuclides as isotopes, isobars, or isotones: (a) $^{12}_{6}\text{C}$ and $^{14}_{6}\text{C}$; (b) $^{…Preview
- Example 5Write the balanced nuclear equation for the alpha decay of $^{238}_{92}\text{U}$, and identify the daughter nucleus formed.Preview
- Example 6Write the balanced nuclear equation for the beta-minus decay of $^{14}_{6}\text{C}$, and identify the daughter nucleus formed.Preview
- Example 7Starting from the radioactive decay law $\dfrac{dN}{dt} = -\lambda N$, derive the expression $N(t) = N_0 e^{-\lambda t}$ for the number of u…Preview
- Example 8A radioactive sample has decay constant $\lambda = 0.0231\ \text{per year}$. Find its half-life.Preview
- Example 9Calculate the mass defect and the binding energy of the alpha particle $^{4}_{2}\text{He}$, given: mass of hydrogen atom $m_H = 1.007825\ \t…Preview
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- Q10Define isotopes, isobars and isotones. Illustrate each definition with one example, using the three naturally occurring isotopes of hydrogen…Free
- Q11Explain, using the relation between nuclear radius and mass number, why the density of nuclear matter is nearly the same for every nucleus,…Free
- Q12Compare alpha particles, beta particles and gamma rays with respect to their charge, mass, speed and penetrating power.Free
- Q13State the radioactive decay law in words and write it as a differential equation. What is the physical meaning of the decay constant $\lambd…Preview
- Q14Define the mean life of a radioactive sample and derive its relation with the decay constant $\lambda$ and with the half-life $T_{1/2}$.Preview
- Q15Define the activity of a radioactive sample. State its SI unit and its relation to the older unit, the curie.Preview
- Q16State Einstein's mass-energy relation. Explain why a nuclear reaction releases far more energy per reacting particle than an ordinary chemic…Preview
- Q17Explain the process of nuclear fission with reference to the fission of $^{235}_{92}\text{U}$ by a slow neutron. How does a self-sustaining…Preview
- Q18Explain the process of nuclear fusion. Why does fusion require an extremely high temperature to occur, and where does it happen naturally?Preview
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- Q19A radioactive sample has decay constant $\lambda = 0.1\ \text{per day}$. Find the half-life of the sample and the time taken for its activit…Free
- Q20A radioactive sample initially contains $N_0 = 8\times10^{10}$ atoms and has a half-life of $20$ days. Find the number of atoms remaining af…Free
- Q21A radioactive sample contains $N_0 = 3.2\times10^{18}$ atoms of an isotope with half-life $T_{1/2} = 8$ days. Find (a) the initial activity…Free
- Q22A radioactive isotope has a half-life of $140$ days. Calculate its mean life.Preview
- Q23Calculate the mass defect, the total binding energy, and the binding energy per nucleon of $^{16}_{8}\text{O}$, given: mass of $^{16}_{8}\te…Preview
- Q24The binding energy per nucleon of $^{56}_{26}\text{Fe}$ is about $8.8\ \text{MeV}$. Calculate its total binding energy and the corresponding…Preview
- Q25In a fission reaction of $^{235}_{92}\text{U}$ induced by a slow neutron, the total mass defect is found to be $0.215\ \text{u}$. Calculate…Preview
- Q26In the deuterium-tritium fusion reaction $^{2}_{1}\text{H} + {}^{3}_{1}\text{H} \rightarrow {}^{4}_{2}\text{He} + {}^{1}_{0}n$, the atomic m…Preview