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All the atoms of a particular element have same number of protons in their nuclei, however, the number of neutrons may be different. Such atoms have same atomic number but different mass numbers and are known as isotopes of the element. Thus Isotopes of an element are the atoms of the element with same atomic number but different mass numbers.

For example, hydrogen has three isotopes, protium (H), deuterium (D) and tritium (T). All the three isotopes have atomic number 1, however, their mass numbers are 1, 2 and 3 respectively. The isotopes of other elements do not have special names; they are indicated by giving mass number value on the symbol. Mass number and atomic number of an atom are generally indicated as shown below:

Representation of an isotope

Thus, three isotopes of hydrogen can be represented as

11 H 21 Hand 31 H

It may be noted that there is large difference between masses of various isotopes of hydrogen. Mass of deuterium is twice as that of protium.

The two isotopes of chlorine are represented as

Another way to describe an isotope is to cite its elemental name and mass number. The two isotopes of chlorine may be represented as chlorine-35 and chlorine-37.

Isotopes have similar chemical properties but different physical properties.

An atom which is characterized by a specified atomic number and mass number is called a nuclide. For example,  U is a nuclide of uranium.


Radioactive isotopes of some elements find many applications in the field of medicine, research, agriculture, etc. Some applications of these isotopes are listed below:

  1. An isotope of uranium (U-235) is used as a fuel in nuclear reactors.
  2. An isotope of cobalt (Co-60) is used in treatment of cancer by radiation therapy.
  3. An isotope of iodine is used in the treatment of goitre.
  4. Ages of old wooden articles are determined by observing the radioactivity of C-14 isotope of carbon.
  5. Ages of rocks are determined by determining the relative amounts of U-238 and Pb-206 isotopes in the rock.
  6. Uptake of phosphorus from fertilised soil by plants can be traced by using fertilisers containing P-32 isotope of phosphorus.