Dalton's Atomic Theory - Basic

Chemistry

definition

Dalton's atomic theory

According to Daltons atomic theory, all matter, whether an element, a compound or a mixture is composed of small particles called atoms. The postulates of this theory may be stated as follows:
(i) All matter is made of very tiny particles called atoms.
(ii) Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction.
(iii) Atoms of a given element are identical in mass and chemical properties.
(iv) Atoms of different elements have different masses and chemical properties.
(v) Atoms combine in the ratio of small whole numbers to form compounds.
(vi) The relative number and kinds of atoms are constant in a given compound.

Limitations of Dalton's atomic theory : 
  •  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.
  •  With the discovery of sub-atomic particles, e.g.,electrons, neutrons and protons, the atom can no longer be considered indivisible.