Definition of Isotopes | Properties & Representation of Isotopes |

Isotopes Definition.

Definition of isotopes

The atoms of the same element which have the same atomic number and therefore similar chemical properties but different atomic masses are called isotopes and the phenomenon is called isotopy.


8O168O178O181H1,1H2,1H36C12,6C13,6C14, etc.  are isotopes of oxygen, hydrogen, and carbon, respectively.


According to Dalton’s atomic theory atoms of the same elements are similar in all respects including their masses. Later on, it was discovered by Soddy that atoms of the same elements could possess different masses but the same atomic number and are called isotopes.

Properties of isotopes:

  1. They have different physical properties.

  2. They have the same chemical properties.

  3. They have different masses.

  4. They have the same position in the periodic table.

  5. They have the same atomic number i.e., number of protons and electrons.

  6. They differ in the number of neutrons in the nucleus.

Representation of isotopes:

Isotopes are represented as:
The symbol of an element is written with atomic mass and atomic number.
The atomic mass is written at the top and the atomic number at the bottom, For example, carbon and hydrogen are represented as, 6C121H2
The symbol of the element is followed by atomic mass with a hyphen(-) in
between. For example, C-12, C-13, C-14.


  1. Carbon has three isotopes 6C126C13, and 6C14. These are expressed as C-12, C-13, and C-14. They all have six protons and six electrons but have 6,7 and 8 neutrons respectively.

  2. Hydrogen has three isotopes 1H11H2, and 1H3. They are named as protium, deuterium, and tritium, respectively. Each having the same number of electrons and protons but different numbers of neutrons, as 0, 1, and 2 respectively.

  3. Oxygen has three isotopes 8O168O17, and 8O18 expressed as 0-16, 0-17, O-18. They have 8, 9, and 10 neutrons respectively.

Separation of Isotopes:

The separation method depends upon the properties of isotopes. Various techniques are as follow:
  1. Gaseous diffusion

  2. Thermal diffusion 

  3. Distillation

  4. Ultracentrifuge

  5. Electromagnetic separation

  6. Laser separation

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