CBSE Class 12 Physics Nuclei Notes Set B

Class-XII: Physics

Chapter : Nuclei

TOP Concepts

1 Atomic Number. The number of protons in the nucleus is called the atomic number. It is denoted by Z.

2 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.

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.

3 Nuclear Mass. The total mass of the protons and neutrons present in

a nucleus is called the nuclear mass.

4 Nuclide. A nuclide is a specific nucleus of an atom characterized by its atomic number Z and mass number A. It is represented as zXA where X = chemical symbol of the element.

Z = atomic number, and

A = mass number.

5 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.

6 Isobars. The atoms having the same mass number but different atomic number are called isobars.

7 Isotones. The nuclides having the same number of neutrons are called isotones.

8 Isomers. These are nuclei with same atomic number and same mass number but in different energy states.

9 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.

1 eV = 1.602 Χ 10^-19 J

1 MeV = 10⁶ eV = 1.602  Χ 10^-13 J

10 Atomic Mass Unit:. It is 1/12 th of the actual mass of a carbon atom of isotope ₆C¹². It is denoted by amu or just by u. 1 amu = 1.660565 Χ 10^-27 kg The energy equivalence of 1 amu is 1 amu = 931 MeV

11 Discovery of Neutrons. Neutrons were discovered by Chadwick in 1932. When beryllium nuclei are bombarded by α-particles, highly penetrating radiations are emitted, which consists of neutral particles, each having mass nearly that of a proton. These particles were called neutrons.

⁴₂He+⁹₄Be→¹₀n+¹²₁C 

A free neutron decays spontaneously, with a half life of about 900 s, into a proton, electron and an antineutrino.

¹₀n®¹₁H+ ⁰_-1e+ n

12 Size of the Nucleus. It is found that a nucleus of mass number A has a radius
R = R₀A^1/3 where R₀ = 1.2 ´ 10^-15 m. This implies that the volume of the nucleus, which is proportional to R³ is proportional A. Density of nucleus is constant; independent of A, for all nuclei and density of nuclear matter is approximately 2.3 ´ 10¹⁷ kg m^-3 which is very large as compared to ordinary matter, say water which is 10³ kg m^-3.

13 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.

14 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 Dm = [ Zm_p + (A - Z) m_n ] – m

15 Binding Energy. It may be defined as the energy required to break up 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 _zX^A is given by B.E. = [Zm_p + (A-Z) m_n – m]c²

16 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.

B=B.E/A= [Zm_p+(A-Z)m_n- m]c²/A

17 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. 

18 Nuclear Density. The density of a nucleus is independent of the size of the nucleus and is given by
þ_v =Nuclearmass/ Nuclear volume 

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