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CBSE Class 12th Physics Notes: Nuclei (Part – II)

Jan 31, 2017 15:00 IST

    CBSE chapter wise notes based on chapter 13, Nuclei of Class 12 Physics NCERT textbook are available in this article. These notes are continuation of CBSE Class 12th Physics Notes: Nuclei (Part – I)

    In part I, we have studied about following topics: Atomic Masses, Discovery of Neutron, Basic Properties of Neutron, Composition of Nucleus, Size of the Nucleus, Nuclear Density, Mass Defect, Binding Energy, Binding Energy per Nucleon, Binding Energy Curve and its Features etc.

    Now, in part II we will study about the topics given below


    Alpha Particles and Alpha Decay

    Beta Particles and Beta Decay

    Gamma radiations and Gamma decay

    Law of Radioactive Decay

    Radioactive Decay Constant

    Half Life: Radioactive Substance

    Relation between Half Life and Decay constant

    Activity of Radioactive Constant

    Mean Life (or Average Life) of  a Radioactive Substance

    Units of Radioactive Decay

    Nuclear Fission

    Nuclear Fusion

    NCERT Solutions for Class 12 Physics

    The notes are given below:


    Radioactivity was discovered by A. H. Becquerel (1986) accidently.

    Radioactivity is defined as the spontaneous and continuous disintegration of a nucleus of a heavy element on its own with the emission of certain type of radiations is known as natural radioactivity.

    Alpha Particles and Alpha Decay

    Alpha particles are helium nuclei of nuclear origin. It carries 2 unit positive charge and its mass is about four times the mass of hydrogen atom.

    Phenomenon of emission of an α particles from a nucleus is called alpha decay

    Example: 92U23890Th234 + 2He4 + Q

    Here, Q = (mXmYmHe) c2


    ZXAz ‒ 2YA ‒ 4 + 2He4 + Q

    Beta Particles and Beta Decay

    Beta particles are fast moving electrons of nuclear origin. A nucleus that decays spontaneously by emitting an electron or a positron is said to undergo beta decay.

    In βdecay, an electron an electron is emitted by the nucleus.

    Example: 15P3216S32 + e + antineutrino

    In β+ decay, a positron is emitted by the nucleus.

    Example: 11Na22e + Neutrino

    In beta-minus decay, a neutron transforms into a proton within the nucleus according to, np + e + antineutrino

    Whereas in beta-plus decay, a proton transforms into neutron (inside the nucleus) by, pp + e+ + neutrino

    The above processes show that mass number A of a nuclide undergoing beta decay does not change.

    NCERT Exemplar Questions & Solutions: CBSE Class 12 Physics – All Chapters

    Gamma Radiations and Gamma decay

    There are energy levels in a nucleus, just like there are energy levels in atoms. When a nucleus is in an excited state, it can make a transition to a lower energy state by the emission of electromagnetic radiation. As the energy differences between levels in a nucleus are of the order of MeV, the photons emitted by the nuclei have MeV energies and are called gamma rays.

    Energy Level Diagram Showing Emission of Gamma Rays

    Law of Radioactive Decay

    In any radioactive sample, which undergoes α, β or γ-decay, it is found that the number of nuclei undergoing the decay per unit time is proportional to the total number of nuclei in the sample. If N is the number of nuclei in the sample and ΔN undergo decay in time Δt then,

    Nt) ∝ N or (ΔNt) = λ N

    Where, λ is known as radioactive decay constant or disintegration constant.

    Also, N = No eλt, here No is the number of radioactive nuclei present initially.

    Radioactive Decay Constant

    As, N = No eλt, now, if we put t = 1/λ, we have, N = No e‒1

    N = No/e

    N = No/2.718

    N =0.368 No

    The radioactive decay constant can may be defined as the reciprocal of the time during which the number of atoms is a radioactive substance reduces to 36.8 % of their initial number.

    CBSE Class 12 Video Tutorials

    Half Life: Radioactive Substance

    It is the time interval in which the mass of a radioactive substance or the number of its atoms is reduced to half of its initial value.

    Relation between Half Life and Decay constant

    Half life (T1/2) and decay constant (λ) are related as, T1/2 = 0.693/λ

    Activity of Radioactive Constant

    Rate of disintegration or count rate of a sample of radioactive material is called activity and is directly proportional to the number of atoms of left undecayed in the sample

    Activity A, = |dN/dt| = λ N

    Activity of Radioactive Constant

    Mean Life (or Average Life) of a Radioactive Substance

    It is the average of the lives of all the atoms in a radioactive substance is called the ‘mean life’ or ‘average life’ of that substance.

    The mean life (τ) of a radioactive substance is equal to reciprocal of decay constant.

    It means, τ = 1/ λ. Also, τ = 1.443 T1/2

    Units of Radioactive Decay

    Becquerel (Bq) = 1 disintegration per second

    Rutherford (rd) = 106 disintegration per second

    Curie (Ci) = 3.7 × 1010 disintegration per second

    Nuclear Fission

    It is the process in which a heavy nucleus splits up into two nuclei of nearly comparable masses.

    Example: 92U235 + 1n92U23636Ba141 + 36Kr92 + 31n +Energy

    Nuclear Fusion

    It is the process in which two or more small nuclei fuse together to form a single heavy nucleus.

    The mass of the single heavy nucleus formed is less than the total initial mass of the mass of the parent nuclei.

    This difference in mass appears in the form of energy (as per, E = mc2).

    Example: 1H2 + 1H22He4 + Enormous amount of energy

    Enormous amount of energy produced by the sun is due to the phenomenon of nuclear fusion.

    CBSE Class 12 Chapter Wise Notes

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