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Isotopes and Isobars

Atoms of the same element having different masses are called isotopes. The difference in the mass results from the fact that the isotopes of some elements have the same number of protons but different numbers of neutrons. These isotopes have the same number of electrons. The concept of isotopes has led to the definition of mass number.

The mass number of an element is defined as the total number of protons and neutrons present in the nucleus of an atom of the element.

The mass number is the atomic weight of the element, corrected to the nearest integer.
Mass number (A) = number of protons + number of neutrons.



No. of Protons

No. of Neutrons






Hydrogen (Protium)


Hydrogen (Tritium)












1, 2 and 3 are isotopes; 3 is an isobar.
The composition of a nucleus can now be indicated by its nuclide symbol. This consists of the symbol for the element E with the atomic number Z written as a subscript at the lower left and the mass number A at the upper right, as a superscript. Thus the ordinary isotope of carbon is represented as 6C12.

This symbol indicates that there are six protons and six neutrons in the carbon nucleus. Therefore, its mass number is 6 + 6 = 12.

Isobars are atoms of different elements having the same mass number but different atomic numbers 1H3 and 2He3. Isotopes are nuclei having the same number of protons, but different numbers of neutrons, hence different mass number: 1H1 and 1H2.

Discovery of Isotopes of an Element
In 1919, Aston developed the mass spectrometer in which the particles of the same charge/mass ratio were focused together on a line. Using extremely pure neon, Aston showed that neon exists in two chemically identical forms having relative atomic masses of 20 and 22, respectively. 

These were called the two isotopes of neon. By definition isotopes mean substances having the same atomic number but different masses. The atomic mass of a naturally occurring element is the average atomic masses of its isotopes.

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