The nuclei have been divided on the basis of the number of protons (atomic number) or the total number of nucleons (mass number) as follows

(i) Isotopes: The atoms of element having same atomic number but different mass number are known as isotopes. All isotopes have the similar chemical properties. The isotopes of similar components are the following

(ii) Isobars: The nuclei which have the same mass number (A) but different atomic number (Z) are called isobars. Isobars occupy different positions in periodic table so all isobars have different chemical characteristics. Some of the types of isobars are

(iii) Isotones: The nuclei having equal number of neutrons are called isotones. For them both the atomic number (Z) and mass number (A) are different, but the value of (A - Z) is same. Some examples are

(iv) Mirror nuclei: Nuclei having the same mass number A but with the proton number (Z) and neutron number (A - Z) interchanged (or whose atomic number differ by 1) are called mirror nuclei for example.

Size of nucleus:

(i) Nuclear radius: Experimental results indicate that the nuclear radius is related to A^1/3, where A is the volume number of nucleus i.e.     

(ii) Nuclear volume: The mass of nucleus is shown by 

(iii) Nuclear density: Volume per unit mass of a nucleus is called nuclear density.

where m = Average of mass of a nucleon (= mass of proton + mass of neutron = 1.66 * 10^-27 kg)  and mA = Mass of nucleus

Pair production and pair-annihilation:

The rest-mass power of every of positron and electron is

            E₀ = m₀c² = (9.1 * 10^-31 kg) * (3.0 * 10⁸ m/s)²

                 = 8.2 * 10^-14 J = 0.51 MeV

Hence, for pair-production it is essential that the energy of g-photon must be at least 2 ´ 0.51 = 1.02 MeV. If the energy of g-photon is less than this, it would create problem photo-electric effect or Compton Effect on striking the matter.

The converse phenomenon pair-annihilation is also possible. Whenever an electron and a positron get very close to one another, they annihilate one another by combining together and two g-photons (energy) are given. This process is known as pair annihilation and is represented by the following equation.

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