According to the Dalton’s atomic theory, atom was indivisible and indestructible. The discovery of two fundamental particles (electrons and protons) inside the atom, led to the failure of this aspect of Dalton’s theory. .
To know the arrangement of electrons and protons with in an atom many scientists proposed various atomic models.
- Thomson’s Model of an atom :
Thomson’s model of an atom was compared to Christmas pudding. The electrons in a sphere of positive change were like currants (dry fruits) in a spherical chritmas pudding.
It can also be compared to a water melon, in which the positive change in an atom is spread all over like the red edible part, while the electrons studded in the +vely changed sphere, like seeds in the water melon.
The following are the postulates of this theory
(i) Electrons are embedded in the sphere of positive change.
(ii) Mass of an atom is due to these electrons.
(iii) The -ve and +ve charge balance each other. Therefore, atom as a whole is neutral. - Rutherford’s Model of an atom :
The postulates of this theory are
(i) Most of the space inside the atom is empty because most of the d-particles passed through the gold foil without getting deflected in an alpha - ray scattering experiment.
(ii) Very few particles were deflected from their path, indicating that the positive charge of the atom occupies a little space.
(iii) A very small fraction of a - particles were deflected by 180 (rebound), indicating that all the +ve charge and mass of an atom were concentrated in a very small volume with in atom called the nucleus. Nearly, whole mass of the atom reside in the nucleus.
(iv) The size of the nucleus (${{10}^{-15}}$m) is very small as compared to the size of the > atom (${{10}^{-10}}$m).
Limitations of Rutherford’s Model :
(i) Any charged particle when accelerated is expected to radiate energy. To remain in a circular orbit, the electron would meed to undergo acceleration. .-. it would radiate energy.
Thus, the revolving ${{e}^{-}}$ would lose energy and finally fall into the nucleus. If this were so, the atom should be highly unstanle. The matter would not exist, but we know that atoms are quite stable.
(ii) It could not explain the distribution of electrons in the extra nucleus portion of the atom.