Dalton’s Atomic Theory (Postulates)

 Dalton’s Atomic Theory Postulates

The various postulates of this primitive theory were:
(i) Atom was considered as a hard, dense and smallest indivisible particle of matter
(ii) Each element consists of a particular kind of atoms
(iii) Atoms of different elements are different and that gives each element its own properties
(iv) Atoms combine to form molecules of various compounds and they do so in a simple whole number ratio
(v) Atoms can neither be created nor destroyed: law of conservation of atoms.
(vi) Therefore it successfully explained all the laws of conservation – Constant, Multiple, Reciprocal Proportions

Various drawbacks of this theory 

(i) It fails to explain why atoms of different kinds should differ in mass and valency.
(ii) The discovery of isotopes and isobars were against its postulates which stated that atoms of same element are same and that of different elements are different.
(iii) The atom was found to be divisible when sub-atomic particles were discovered

 Thomson’s Model

This was a very preliminary model according to which the atom is considered to be a large sphere of positive charge with electrons being embedded on it. This is also called “Plum-Pudding Model” or the “Watermelon Model”. Its only achievement was to explain the overall neutrality of the atom. After Rutherford’s findings this model was completely rejected.

Fig: Thomson’s atomic model

Limitations of Thomson’s atomic model

1.   This model of atom failed to explain how a positive charge holds the negatively charged electrons in an atom.Therefore, it failed to explain the stability of an atom.
2.   This theory also failed to account for the position of the nucleus in an atom.
3.   Thomson’s model failed to explain the scattering of alpha particles.

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