According to early model of an atom,the atom is considered it to have a positively charged point nucleus of charge `Ze` surrounded by a uniform density of negative charge up to a radius `R`. The atom as a whole is neutral. The electric field at a distance`r` from the nucleus is `(r lt R)`

Explanation

For `r leR , E = E_("nucleus")+ E_("electron destribution") = ( Ze)/( 4pi epsilon_(0)r^(2))-(Zer)/(4piepsilon_(0r)R^(3))= (Ze)/( 4piepsilon_(0))[(1)/(r ^(2)) - ( r )/(R^(3))]`
For `r ge R , E =0`
( Net charge enclosed by Gaussian surface = 0)