Question

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an early model for an atom considered it to have a positively charged point Nucleus of charge Ze, surrounded by a uniform density of negative charge upto radius R. the atom as a whole is neutral. for this model, what is the electric field at a distance r from the nucleus?

Answer 1

When r <=R, E(r) = 0 
When r >R, E(r) = Ze/4π£° (1/r^2 - r/R^3)

STEPS AND Understanding :
1) We know, that size of Nucleus in negligible as compared to Nucleus. 
So, At any distance r, we have to understand there exists nucleus with charge( +Ze. )

2) As atom is Neutral, the total Negative Charge in a sphere of Radius R is (-Ze. )


3) We will find Negative charge density to calculate charge at any distance r. 

4) We will. apply Gauss law to find Electric Field at less than equal to R distance and more than that. 

Answer 2

In Thomson's model, the atom is composed of electrons surrounded by a soup of positive charge to balance the electrons' negative charges, like negatively charged “plums” surrounded by positively charged “pudding”.