Thomson originally believed that the hydrogen atom must be made up of more than two thousand electrons, to account for its mass.
An atom made of thousands of electrons would have a very high, negative electric charge. This was not observed, as atoms are usually uncharged. In 1906 Thomson suggested that atoms contained far fewer electrons, a number roughly equal to the atomic number. This is only one electron in the case of hydrogen, far fewer than the thousands originally suggested.
These electrons must have been balanced by some sort of positive charge. The distribution of charge and mass in the atom was unknown. Thomson proposed a 'plum pudding' model, with positive and negative charge filling a sphere only one ten billionth of a metre across.
This plum pudding model was generally accepted. Even Thomson's student Rutherford, who would later prove the model incorrect, believed in it at the time.