The "plum pudding model" was an early model of what structure?

In the early 20th century a model for the structure of the atom was proposed by British scientist J.J. Thomson. At the time electrons were known, but other subatomic particles (protons and neutrons) had not yet been discovered. Thompson suggested a structure for the atom where the negatively charged electrons occupied a region that was a uniform positive charge. Sometimes considered a "soup" or "cloud" of positive charge.

The nickname of plum pudding was attributed to Thompson's model since it reminded many scientists of the structure of plum pudding.

The plum pudding model guided one of Thompson's students, Ernest Rutherford, to develop experiments to explore the composition of atoms. In 1909 two of Rutherford's students, Hans Geiger and Ernest Marsden, conducted experiments to examine scattering of radioactive particles sent through thin sheets of gold. It was found that more scattering was present than what would account for a distribution of electrons within the atom. Their findings pointed to the presence of a small center that caused deflection of the particles.

This experiment led to the rejection of the Thompson model of the atom. Refined models of the atom included the Bohr-Rutherford model that structured the atom with a small central nucleus with electrons orbiting around in circular orbitals.