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, chlorine also has 17 electrons. The relative mass of a nucleus is ...

Everyone at that time imagined the atom as a "plum pudding ... He realized that Rutherford's model wasn't quite right. By all rules of classical physics, it should be very unstable.