It also tells us that chlorine has 18 neutrons (35 - 17) and, as the number of electrons and protons are equal in a neutral atom ... isotopes are forms of an element that have the same number ...

Everyone at that time imagined the atom as a "plum pudding ... emitted by the radioactive element radium) at a sheet of gold foil only 1/3000 of an inch thick, and tracing the particles' paths.

He showed that every radioactive element decreases in radioactivity ... He discovered the atomic nucleus and developed a model of the atom that was similar to the solar system.