According to Dalton‟s 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.
Drawbacks of Dalton’s Atomic Theory:
 The atom is further subdivided into protons, neutrons and electrons.
 The atoms of same elements are not similar in all respect. They may vary in mass and density. These are known as isotopes. For example: chlorine has two isotopes having mass numbers 35 a.m.u and 37 a.m.u.
 Atoms of different elements are not different in all respects. Atoms of different elements that have the same atomic mass are called isobar.
 According to Dalton atoms of different elements combine in simple whole number ratio to form compounds. This is not seen in complex organic compounds like sugar C12H22O11.
 The theory fails to explain the existence of allotropes like Diamond and Graphite which having different properties even these are made up of same kind ot atom namely Carbon.
The introduction of matter wave concept by de Broglie, the principle of uncertainty by Heisenberg etc., paved the way for modern atomic theory [Modification in Dalton‟s atomic theory]
Modifications in Dalton’s atomic theory - Modern atomic theory are as follows. 
 Atom is considered to be a divisible particle
. Atoms of the same element may not be similar in all respects. eg: Isotopes (17Cl35,17Cl37 ) 
 Atoms of different elements may be similar in some respects eg. Isobars (18Ar 40 , 20Ca 40 )
  Atom is the smallest particle which takes part in chemical reactions. 
The ratio of atoms in a molecule may be fixed and integral but may not be simple e.g., C12H22O11 is not a simple ratio (Sucrose)
 Atoms of one element can be changed into atoms of other element by transmutation.
 The mass of an atom can be converted into energy. This is in accordance with Einstein‟s equation E = mc2