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.