Saturday, 26 November 2016


Isotopes, their Relative Abundance and Occurrence

     Atoms of the same element which have the same atomic number but different mass numbers due to difference in the number of neutrons are called isotopes of that element. 
(i) Isotopes of Hydrogen: Hydrogen consists of three isotopes which are protium(¹H), deuterium(²H) and tritium(³H). All these isotopes have the same atomic number i.e. one, but they have different mass numbers 1,2, and 3 respectively.
(ii) Isotopes of Carbon: Carbon also consists of three isotopes which are C-12, C-13 and C-14. All these isotopes have the same atomic number i.e. six, but they have different mass numbers 12, 13 and 14 respectively. 
Relative Abundance of Isotopes
  • The percentage of each isotope in a mixture of isotopes of an element is called relative abundance. 
  • Different isotopes have their own natural abundance.
  • The relative abundance of isotopes is measured by mass spectrometery.
  • The properties of a particular element mostly correspond to the most abundant isotope of that element. 
  • Natural abundance of some common isotopes is given below;
  • The distribution of isotopes among the elements is varied and complex as it is evident from the above table. 
Occurrence of Isotopes
  • At present more than 280 different isotopes occur in nature. 
  • 40 radioactive isotopes are also included in this number (280)
  • Almost 300 unstable radioactive isotopes of different elements have been produced by the artificial disintegration.
  • The elements with odd atomic number almost never posses more than two stable isotopes.
  • The elements with even atomic number usually have larger number of isotopes. 
  • The isotopes whose mass numbers are multiple of four are particularly abundant. 
  • Out of 280 natural isotopes, 154 isotopes have even atomic number and even mass number. 

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