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 ...

Since a nucleus contains protons and neutrons, most of the mass of an atom is concentrated in its nucleus.

Understanding the origin of the mass of subatomic particles like protons and neutrons is a major challenge in modern physics.