What is the difference b/w isotopes , allotropes and isomers??

Asked by Palak Diwan | 4th Dec, 2013, 05:03: PM

Expert Answer:



  • Isotope can be defined as each of two or more forms of the same element that contain equal numbers of protons but differs in the numbers of neutrons in their nuclei, and therefore differ in relative atomic mass but not in chemical properties; in particular, a radioactive form of an element.
  • The identity of the isotope is the number of neutrons in the nucleus.
  • The atomic number remains the same while the mass number gets changed for the different isotopes of the same atom.

For example,

The three naturally-occurring isotopes of hydrogen are protium, deuterium, tritium. Each isotope has one proton hence makes them all variants of hydrogen. The symbol for these three isotopes can also be given as 1H, 2H, 3H respectively for protium, deuterium and tritium.



  • They are different the structural forms of the same element but can exhibit quite different physical and chemical properties.
  • This is a property of an element to exist in two or more different forms. It is because the atoms of that element are bonded together in different manner with each other.

For example, the allotropes of carbon as follows:

  1. Diamond ( in this, the carbon atoms are bonded together in a tetrahedral lattice arrangement),
  2. Graphite (in this, the carbon atoms are bonded together in sheets of a hexagonal lattice),
  3. Graphene (in this, single sheets of graphite are present),
  4. Fullerenes (in this, the carbon atoms are bonded together in tubular, spherical or ellipsoidal formations)



  • They are molecules with the same molecular fornula but different chemical structures.
  • Unlike isotopes and allotropes which correspond to the different forms of one element, isomers are molecules containing different elements.
  • The number of atoms of each element is same in each isomer but have different arrangement of these atoms in space.
  • Two main forms of isomerisms are structural isomerism and stereoisomerism.


For example,

 2-bromopropane and 1-bromopropane have same molecular formula (C3H7Br), but the position of bromine atom is different in these two. Hence, these two are structural isomers of bromopropane.  


Another example is propyl alcohol. The two structural isomers of propyl alcohol are n-propyl alcohol and isopropyl alcohol. In n- propyl alcohol, -OH is attached to first carbon atom of propyl chain while in isopropyl alcohol, -OH is attached to second number or middle carbon atom of propyl chain. But the molecular formula is same for both that is C3H8O.





Answered by Prachi Sawant | 5th Dec, 2013, 12:21: PM

Queries asked on Sunday & after 7pm from Monday to Saturday will be answered after 12pm the next working day.