Why did John Dalton made atomic theory?and how?

Dear Student,

Democritus first suggested the existence of the atom but it took almost two millennia before the atom was placed on a solid foothold as a fundamental chemical object by John Dalton (1766-1844). The basis of John Dalton' s atomic theory was that he accepted the idea of divisibility of matter and accepted the name atoms . His theory was based on the two laws of chemical combination , which were proposed by Antoine Lavoisier and Joseph L Proust .

On the basis of following observations, Dalton proposed his postulates

(i)Atoms combine in a ratio of small numbers to form compounds.

it means atoms do not combine in fractions.

for e.g in CaCO3 there is one atom of  ca, one atom of C and 3 atoms of O.

(ii)Atoms of different elements have different masses and chemical properties.For example Carbon has atomic mass 12u

and Mg has 24u.

and both have different physical and chemical properties.

Carbon is a non-metal whereas Mg is a metal.

(iii) The relative number and kinds of atoms are constant in a given compound.For example in case of H2O there will always be 2 atoms of H and 1 atom of O irrespective of it's source.

Dalton’s Atomic Theory

  • Postulates:
  • All matter is made of very tiny indivisible particles called atoms.
  • All the atoms of a given element are identical in mass and chemical properties whereas those of different elements have different masses and chemical properties.
  • Atoms of different elements combine in a fixed whole number ratio to form compounds.
  • Chemical reactions involve reorganisation of atoms. Atoms are neither created nor destroyed in a chemical reaction.
  • The laws of chemical combination could be explained by Dalton’s atomic theory.

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