The Nuclear Model of the Atom
 
We know that an atom consists of a tiny nucleus carrying a positive charge Ze, where Z id the atomic number of the atom. The nucleus is surrounded by a much larger volume containing Z electrons, each carrying a charge of –e, so that the atom as a whole is electrically neutral. 
 
According to a theory in the 20th century, the positive charge is distributed more or less uniformly throughout the entire spherical volume of the atom. This model of the structure of the atom is called Thomson model after J. J. Thomson who proposed it. 
 
One way of testing this model is to determine the electric field of the atom by probing it with a beam of positively charged projectiles that pass nearby. The particles in the beam are deflected or scattere by the electric field of the atom. We assume that the projectile is both much less massive than the atom and much more massive than an electron. In this way the electrons have a negligible effect on the scattering of the projectile, and the atom can be assumed to remain at rest while the projectile is deflected.