Dalton’s Atomic Theory: Concept and Limitations

Physical Change:

  • A change in which only physical properties of a substance is changed is called physical change.
  • e.g. heating of platinum wire, melting of ice, vapourization of water due to boiling etc.
  • Characteristics of Physical Change:

    • In a physical change the only physical properties of substance change.
    • In physical change there is no change in the composition of the substance
    • In a physical change, no new substance is formed.


Chemical Change:

  • A change in which a substance is converted into another substance of different physical and chemical properties is called chemical change.
  • e.g. rusting of iron, iron and sulphur on heating reacts together and give iron sulphide etc.
  • Characteristics of Chemical Change:

    • In chemical change the physical and chemical properties of substance takes place
    • In chemical change, there is change in the composition of the substance
    • In chemical change new substance is formed.

Assumptions of Dalton’s Atomic Theory:

  • Every element is made up of extremely small particles called an atom.
  • The atoms are indivisible and they can neither be created nor be destroyed. Atoms of the same element resemble each other in all respects but differ from the atoms of other elements.
  • When chemical compounds are formed they do so by the combination of atoms of different elements in a simple proportion of whole numbers.
  • Atoms of different elements may combine in more than one proportion to form different compounds.
  • The combining weights of the elements represent the combining weights of the atoms.

Limitations of Dalton’s Atomic Theory:

  • Now it is proved that the atom consists of particles like electrons, protons, and neutrons. Thus the assumption of Dalton’s theory that the atom is not divisible is false.
  • Discovery of isotopes revealed that the atoms of the same element possess different weights. Thus the assumption of Dalton’s theory that the atoms of the same element are identical in all respect is false.

The Review of Dalton’s Atomic Theory:

  • Every element is made up of extremely small particles called an atom. The atoms are indivisible.
  • Modern Concept: Atoms are not the smallest particles. They can be further divided into subatomic particles like protons, neutrons, and electrons.
  • Atoms can neither be created nor be destroyed.
  • Modern Concept: With the advent of nuclear chemistry, atoms of one element can be converted into atom of another element by transmutation.
  • Atoms of the same element resemble each other in all respects;
  • Modern Concept: With the knowledge of isotopes we can have atoms of same elements having the same atomic number but different mass number. Thus atoms of the same element may not resemble each other in all respects;
  • Atoms of the different element differ from the atoms of other elements.
  • Modern Concept: There are certain elements; with the difference in their atomic number have the same mass number. Thus atoms of different elements may resemble in some properties
  • Dalton didn’t consider the mass-energy relation. According to Einstein’s theory in a chemical reaction, some mass is getting converted into energy.


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