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 ...

An element's atomic number defines it. An element with 17 protons will always be chlorine. However an element's mass numbers can vary, which means that it can have different numbers of neutrons ...

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.