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Dalton Atomic Theory

Dalton Atomic Theory



John Dalton was an English chemist, physicist, and meteorologist. He developed modern atomic theory.

Postulates of Dalton's Atomic Theory

The main points of Dalton's atomic theory were explained below-
  • Elements are made-up of very small particles known as atoms.
  • Atoms of an element are identical in mass, size and many other chemical or physical properties, but atoms of two-different elements differ in mass, size and many other chemical or physical properties.
  • Atoms cannot be created, subdivided, or destroyed.
  • Chemical compounds are formed, when atoms of different elements (atoms of two or more elements) combine in simple whole number ratios.
  • Chemical reactions occurs, when atoms are combined, separated, or rearranged.

Drawbacks of Dalton's Atomic Theory of Matter

  • Atom is indivisible was proved wrong. As an atom can be subdivided into electrons, protons and neutrons. But remember that atom is the tiniest particle that takes part in a chemical reactions.
  • According to Dalton Atomic Theory, Atoms of an element are identical in mass, size and many other chemical or physical properties. But practically we observe that atoms of several elements differ in their densities and masses. These atoms with the different masses are known as isotopes. For example, Chlorine (Cl) has 2 isotopes with the mass numbers of 35 and 37.
  • Also According to Dalton Atomic Theory, Atoms of two-different elements differ in mass, size and many other chemical or physical properties. But practically we observe that this is not true all times. For example, Argon (Ar) and Calcium (Ca) atoms, as in this case each have an atomic mass of 40 amu. These atoms with similar atomic masses are known as isobars.
  • According to Dalton Atomic Theory, Chemical compounds are formed, when atoms of different elements (atoms of two or more elements) combine in simple whole number ratios. But this is not true in case of complex organic compounds.
  • Dalton Atomic Theory do not explain the existence of allotropes, means this Dalton atomic theory fails to explain the differences in properties of charcoal, graphite and diamond (allotropes of carbon).

 Supporting Laws for Dalton's Theory

John Dalton based his theory on two laws which are explained below-

Law of Conservation of Mass

According to law of conservation of mass, Matter is neither created nor destroyed. Which means in a chemical reaction, amount of elements remains same in starting when only reactants there and at the completion of the reaction when product formed. We always use the “Law of conservation of mass” when we balance chemical equations.

Law of Constant Composition

According to law of constant composition, A pure compound will always have the same proportion of the same elements. For example, table salt with molecular formula of NaCl, holds the same proportions of the elements Na (sodium) and Cl (chlorine); no matter from where the salt came from and how much salt one should have.


According to Dalton Atomic theory atom cannot be divided, but after some years it is proved that atom can be divided further by many scientists like J.J.Thomson, L.Rutherford, N.Bohr, Chadwick etc.

Constituting particles of atom

Atom made up from 3 main particles-
1. Electron 2. Proton 3. Neutron
 for more chemistry terminology visit our page on Chemistry-Terminology at http://chemistrynotesinfo.blogspot.in/p/chemistry-terms.html

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