<P> The plum pudding model is one of several scientific models of the atom . First proposed by J.J. Thomson in 1904 soon after the discovery of the electron, but before the discovery of the atomic nucleus, the model represented an attempt to consolidate the known properties of atoms at the time: 1) electrons are negatively - charged particles and 2) atoms are neutrally - charged . </P> <P> In this model, atoms were known to consist of negatively charged electrons . Though Thomson called them "corpuscles", they were more commonly called "electrons" as G.J. Stoney proposed in 1894 . At the time, atoms were known to be neutrally charged . To account for this, Thomson knew atoms must also have a source of positive charge to balance the negative charge of the electrons . He considered three plausible models that would satisfy the known properties of atoms at the time: </P> <Ol> <Li> Each negatively - charged electron was paired with a positively - charged particle that followed it everywhere within the atom . </Li> <Li> Negatively - charged electrons orbited a central region of positive charge having the same magnitude as all the electrons . </Li> <Li> The negative electrons occupied a region of space that itself was a uniform positive charge (often considered as a kind of "soup" or "cloud" of positive charge). </Li> </Ol> <Li> Each negatively - charged electron was paired with a positively - charged particle that followed it everywhere within the atom . </Li>

Where is the mass of an atom found according to the plum-pudding model