What are the postulates and limitation of Dalton’s atomic theory ?

Some of the important postulates and limitation of Dalton’s atomic theory is given below :

Major postulates of Dalton’s atomic theory are:

(i) All forms of matter are made up of very small particles called atoms ( from the Greek word “Atom” meaning “indivisible”).

(ii) Atoms cannot be created, divided or destroyed as a result of a chemical change.

(iii) All atoms of an element are identical, and different from those of other elements.

(iv) Atoms of elements combine in the ratio of whole numbers to produce a large number of compound-atoms of a new substance. The compound-atoms of a particular substance are identical in all properties, and differ from those of other substances.

Thus, according to Dalton’s atomic theory, hydrogen was considered to be composed of only the atoms of hydrogen, while oxygen contained only the atoms of oxygen.

Limitations of Dalton’s atomic theory

According to Dalton’s atomic theory, an atom is the ultimate, discrete and indivisible particle of matter. Later researches proved that Dalton’s atomic theory was not wholly correct.

Dalton’s atomic theory suffered from the following drawbacks:

i) Atoms of the same or different types have a strong tendency to combine together to form a new ‘group of atoms’. For example, hydrogen, nitrogen, oxygen gases exist in nature as ‘group of two atoms’. This indicates that the smallest unit capable of independent existence is not an atom, but a ‘group of atoms’.

ii) With the discovery of sub-atomic particles, e.g., electrons, neutrons and protons, the atom can no longer be considered indivisible.

iii) Discovery of isotopes indicated that all atoms of the same element are not perfectly identical. At least, they differ in their masses. Atoms of the same element having different masses are called isotopes.

Dalton’s atomic theory could not explain why certain substances, all containing atoms of the same element, should differ in their properties. For example, charcoal, graphite and diamond all are made up of only Can-atoms, but still their properties are quite different.