Since time immemorial, man has pondered about the nature of things and structure of matter at a deeper level. The question of what makes up an atom, was explored by many scientists at the turn of the 19th century. The 'Plum Pudding Model' is one of the many theories that were hypothesized to explain atomic structure, in the beginning of the 20th century. J.J. Thomson is known for his discovery of the electron. This was the first of the subatomic particles of an atom to be discovered. It is this discovery that led him to hypothesize this model for atomic structure.
To describe a phenomenon in nature, scientists hypothesize a causal model, which may become an established theory, if it passes the test of experiment. For every established theory, there are hundreds of other hypotheses that fail. Same is the case with atomic theory. Scientists went down a lot of blind alleys before they came up with the correct understanding of the atomic structure.
Introduction to the Model
What a delicious name for a hypothesis to have, isn't it? While it may seem that this name was picked from the menu of an English restaurant, the name serves to visualize the structure of the atom. J.J. Thomson proposed the Plum Pudding model in 1904. He visualized an atom with negatively-charged electrons (which he referred to as corpuscles), to be embedded in a sphere of positive charge.
The electrons, like 'plums' in a pudding, would be placed within and free to move in the sphere of positive charge. Though the electrons could move in the confines of the sphere, they could not leave it due to Coulomb's law of attraction. Thus, the electrons embedded in the positively-charged sphere would constitute a stable and neutral atom. The model failed to explain the atomic spectra that were known at the time.
Why Did it Fail?
Rutherford set out to verify the model in 1907. When he bombarded a gold foil with alpha particles, he expected that most of the particles will pass through the sphere of the atom with minor deflections. What he noticed was that almost all passed through and only a few deflected back by 180 degrees. This meant that the picture sketched by this model of the atom was inaccurate.
Thus, the model failed, as its basic premise of the atom being a positive sphere, failed to be verified by experiment. The results meant that physics needed a new theory, that could account for the results of this experiment.
After Rutherford's experimentation, the true nature of the atom came to be known in actuality. Quite unexpectedly, almost all the mass of an atom was discovered to be concentrated in the nucleus, which occupies very less space. If you would enlarge an atom to the size of a big room, the nucleus would be a mere speck, hard to see with the naked eye.
As the experiment revealed, most of the atom was empty with electrons revolving around a positively-charged nucleus. Later, Bohr came up with a more accurate quantum theory of the atom. This theory had its imperfections, which were further solved through the establishment of a concrete theory of quantum physics. Ultimately, the correct model for the hydrogen atom was established through a solution of the Schrödinger equation.
The model failed magnanimously in describing atomic structure but gave an impetus to experimentalists to explore subatomic structure in greater detail.