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 ...
BBC Bitesize A simple model of an atom shows a nucleus orbited by a number of electrons In 1803, John Dalton's atomic theory brought them into scientific view, but the Manchester scientist was ...