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 .