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Nucleus and Its Composition

It is a well known fact that the atoms contain three fundamental subatomic particles namely: electrons, protons and neutrons. Protons and neutrons constitute nucleus whereas electrons are present in the extranuclear region of the atom. Chemical transformations involve the changes in the valence electrons of the participating atoms while their atomic nuclei remain

unaffected. In the beginning of 20th century, another kind of transformation, which involve

changes in the atomic nucleus, were observed by the scientists. These transformations are called nuclear reactions. The nuclear reactions have become tremendous source of energy all over the world both for peaceful and destructive purposes. The study of nuclear reactions led to the development of a new branch of chemistry called nuclear chemistry. The nuclear chemistry may, therefore, be defined as the branch of chemistry which deals with the study of atomic nucleus and nuclear changes.





Nucleus is a small positively charged part of an atom at the centre where the entire mass of the atom is concentrated. The existence of nucleus was first of all proposed in 1911 on the basis of series of scattering experiments conducted by Rutherford and his co-workers. These experiments were conducted to understand the arrangement of sub-atomic particles (electrons, protons and neutrons) in an atom. Experimental observations led to the conclusion that

(i)                 the protons and neutrons are present in the nucleus.

(ii)               the electrons are present in empty space around the nucleus.

(iii)             the number of protons in the nucleus or electrons present outside the nucleus is known as atomic number.

(iv)             the sub-atomic particles (protons and neutrons) of nucleus are collectively called nucleons. The total number of nucleons is denoted by A and is called mass number of the nucleus Mass Number (A) = No. of protons (p) +No. of neutrons (n)

Representation of nuclide

The nucleus of any atom may be represented by specifying the atomic number as a subscript at the left hand bottom of the atomic symbol and the mass number as a superscript at the left hand top of the symbol. For example, the nucleus of most common isotope of carbon is designated as 12/6  C. Such symbols are referred to as nuclides.

Nuclear Dimensions

The radius of nucleus is of the order of 10- 12 – 10-13 cm as compared to that atom which is of the order of w-8 cm. The radii of various nuclei (assuming them to be spheres) can be calculated from the following empirical relation

r = A0A1/3

where, r is the radius of the nucleus ; A is the mass number and R0 is constant whose value has been found to be 1.4 x 10-13 cm.


Isotopes. These are the atoms having same atomic number but different mass numbers. They contain same number of protons but different number of nucleons. Some examples are 1/1H, 2/1H, 3/1H are isotopes of hydrogen. Similarly, 215/84Po and 211/84Po are isotopes of polonium.

Isobars. These are the atoms having different atomic numbers but same mass number. They contain same number of nucleons but different number of protons.

Some examples are 40/20Ca and 40 /20Ar 211/82Pb and 211/83Bi 20 20 ‘ 82 83 .

Isotones. These are the atoms having different number of nucleons but same number of neutrons. Some example  30/14si 3l/15p 32/15S.