In the early 20th century, scientists were struggling to understand the structure of atoms. They had parts of the answer. The electron, which has a negative electrical charge, had been discovered earlier. They knew that the basic atom had no overall charge. Together these pieces of information made it natural to assume that the atom also contained something that carried a positive charge. Scientists guessed that since electrons are extremely small, whatever this positive something was, it must be make up most of the mass of atoms, and be much larger.