Dalton's Atomic Theory

In 1808, an English chemist by the name John Dalton published 'A New System of Chemical Philosophy' , in which he proposed the following postulates :

  1. Matter consists of small indivisible particles called atoms.

  2. All atoms of a given element have the same properties and have the same mass. Atoms of different elements have different mass values.

  3. Atoms of different elements combine in a fixed ratio to form a compound.

  4. Chemical reactions involve reorganisation of atoms. Atoms can neither be created nor be destroyed in a chemical reaction.

  5. Atoms of same element can combine in more than one ratio to form two or more compounds.

Although Dalton's Atomic Theory was successful in explaining the laws of Chemical Combination, it also had certain drawbacks. Some of those drawbacks are :

  1. The indivisibility of atom was proved wrong as it was discovered that an atom could be further subdivided into electrons, protons and neutrons.

  2. It could not justify the presence of isotopes which atoms of the same element which had different masses. For example - Carbon has 2 isotopes with mass numbers 12 and 14 respectively.

  3. It could also not justify the presence of isobars which were atoms of different elements which had the same masses. For example - Calcium and Potassium have the same atomic mass of 40 amu.

In spite of these drawbacks, Dalton's Atomic Theory forms the framework of Modern Chemistry and has also served as the basis of several studies and experiments.

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