Table of Contents
Class 12 Physics Revision Notes Chapter 13 Nuclei
- Atomic Number: The number of protons in the nucleus is called the atomic number. It is denoted by Z.
- Mass number: The total number of protons and neutrons present in a nucleus is called the mass number of the element. It is denoted by A.
- No. of Protons, Electrons, Nucleons, and Neutrons in an Atom:
- Number of protons in an atom = Z
- Number of electrons in an atom = Z
- Number of nucleons in an atom = A
- Number of neutrons in an atom = N = A – Z.
- Nuclear Mass: The total mass of the protons and neutrons present in a nucleus is called the nuclear mass.
- Nuclide: A nuclide is a specific nucleus of an atom characterized by its atomic number Z and mass number A. It is represented as,
Where X = chemical symbol of the element, Z = atomic number and A = mass number
- Isotopes:
- The atoms of an element which have the same atomic number but different mass number are called isotopes.
- Isotopes have similar chemical properties but different physical properties.
- Isobars: The atoms having the same mass number but different atomic number are called isobars.
- Isotones: The nuclides having the same number of neutrons are called isotones.
- Isomers: These are nuclei with same atomic number and same mass number but in different energy states.
- Electron Volt: It is defined as the energy acquired by an electron when it is accelerated through a potential difference of 1 volt and is denoted by eV.
- Atomic Mass Unit:
- It is of the actual mass of a carbon atom of isotope . It is denoted by amu or just by u.
- 1 amu = kg
- The energy equivalence of 1 amu is 1 amu = 931 MeV
- Discovery of Neutrons:
- Neutrons were discovered by Chadwick in 1932.
- When beryllium nuclei are bombarded by alpha-particles, highly penetrating radiations are emitted, which consists of neutral particles, each having mass nearly that of a proton. These particles were called neutrons.
- A free neutron decays spontaneously, with a half- life of about 900 s, into a proton, electron and an antineutrino.
- Size of the Nucleus:
- It is found that a nucleus of mass number A has a radius
- This implies that the volume of the nucleus, which is proportional to R3 is proportional to A.
- Density of the Nucleus: Density of nucleus is constant; independent of A, for all nuclei and density of nuclear matter is approximately
which is very large as compared to ordinary matter, say water which is 103 kg m-3.
- Mass-Energy equivalence: Einstein proved that it is necessary to treat mass as another form of energy. He gave the mass-energy equivalence relation as, E = mc2 Where m is the mass and c is the velocity of light in vacuum.
- Mass Defect: The difference between the rest mass of a nucleus and the sum of the rest masses of its constituent nucleons is called its mass defect. It is given by-
- Binding Energy:
- It may be defined as the energy required to break a nucleus into its constituent protons and neutrons and to separate them to such a large distance that they may not interact with each other.
- It may also be defined as the surplus energy which the nucleus gives up by virtue of their attractions which they become bound together to form a nucleus.
- The binding energy of a nucleus is-
- Binding Energy per Nucleon: It is average energy required to extract one nucleon from the nucleus.
It is obtained by dividing the binding energy of a nucleus by its mass number.
- Nuclear Forces:
- These are the strong in attractive forces which hold protons and neutrons together in a tiny nucleus.
- These are short range forces which operate over very short distance of about 2 – 3 fm of separation between any two nucleons.
- The nuclear force does not depend on the charge of the nucleon.
- Nuclear Density: The density of a nucleus is independent of the size of the nucleus and is given by-
- Radioactivity:
- It is the phenomenon of spontaneous disintegration of the nucleus of an atom with the emission of one or more radiations like -particles, -particles or -rays.
- The substances which spontaneously emit penetrating radiation are called radioactive substances.
- Radioactivity Displacement Law: It states that-
- When a radioactive nucleus emits an -particle, atomic number decreases by 2 and mass number decreases by 4.
- When a radioactive nucleus emits -particle, its atomic number increases by 1 but mass number remains same.
- The emission of a -particle does not change the mass number or the atomic number of the radioactive nucleus. The -particle emission by a radioactive nucleus lowers its energy state.
- Alpha Decay: It is the process of emission of an -particle from a radioactive nucleus. It may be represented as,
- Beta Decay: It is the process of emission of an electron from a radioactive nucleus. It may be represented as,
- Gamma Decay: It is the process of emission of a -ray photon during the radioactive disintegration of a nucleus. It can be represented as,
- Radioactive Decay Law: It states that the number of nuclei disintegrated of undecayed radioactive nuclei present at that instant. It may be written as-
WhereN(0) is the number of nuclei at t = 0 and is disintegration constant.
- Decay or disintegration Constant: It may be defined as the reciprocal or the time interval in which the number of active nuclei in a given radioactive sample reduces to 36.8% of its initial value.
- Half-life: The half-life of a radioactive substance is the time in which one-half of its nuclei will disintegrate. It is inversely proportional to the decay constant of the radioactive substance.
- Mean Life: The mean-life of a radioactive sample is defined as the ratio of the combined age of all the atoms and the total number of atoms in the given sample. It is given by,
- Rate of Decay or Activity of a Radioactive Sample: It is defined as the number of radioactive disintegrations taking place per second in a given sample. It is expressed as-
- Curie:
- It is the SI unit of decay.
- One curie is the decay rate of 3.7 X 1010 disintegrations per second.
- Rutherford: One Rutherford is the decay rate of 106 disintegrations per second.
- Natural Radioactivity: It is the phenomenon of the spontaneous emission of , and radiations from the nuclei of naturally occurring isotopes.
- Artificial or Induced Radioactivity: It is the phenomenon of inducing radioactivity in certain stable nuclei by bombarding them by suitable high energy sub atomic particles.
- Nuclear Reaction: It is a reaction which involves the change of stable nuclei of one element into the nucleus of another element.
- Nuclear Fission: It is the process in which a heavy nucleus when excited gets split into two smaller nuclei of nearly comparable masses. For example-
- Nuclear Reactor: It is a device in which a nuclear chain reaction is initiated, maintained and controlled.
- Nuclear Fusion: It is the process of fusion of two smaller nuclei into a heavier nucleus with the liberation of large amount of energy.
- Critical size and Critical Mass:
- The size of the fissionable material for which reproduction factor is unity is called critical size and its mass is called critical mass of the material.
- The chain reaction in this case remains steady or sustained.
- Moderator:
- Any substance which is used to slow down fast moving neutrons to thermal energies is called a moderator.
- The commonly used moderators are water, heavy water (D2O) and graphite.
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