Dalton’ s Atomic Theory
All elements are consists of very small invisible particles, called atoms. Atoms of same element are exactly same and atoms of different element are different.
Thomson’ s Atomic Model
Every atom is uniformly positive charged sphere of radius of the order of 10-10 m, in which entire mass is uniformly distributed and negative charged electrons are embedded randomly. The atom as a whole is neutral.
Limitations of Thomson’ s Atomic Model
1. It could not explain the origin of spectral series of hydrogen and other atoms. 2. It could not explain large angle scattering of α – particles.
Rutherford’ s Atomic Model
On the basis of this experiment, Rutherford made following observations
( i) The entire positive charge and almost entire mass of the atom is concentrated at its centre in a very tiny region of the order of 10-15 m, called nucleus.
( ii) The negatively charged electrons revolve around the nucleus in different orbits.
( iii) The total positive charge 011 nucleus is equal to the total negative charge on electron. Therefore atom as a overall is neutral.
( iv) The centripetal force required by electron for revolution is provided by the electrostatic force of attraction between the electrons and the nucleus.