The atomic mass of an element is the weighted average of the mass numbers of all of its isotopes. In general, we take it to be equal to the number of protons and neutrons (except when isotopes are present in significant proportion).
An ion is formed when an atom gains or loses electron/s. For example, the sodium ion loses an electron to form the sodium cation (Na^+). Similarly, when chlorine gains an electron, it forms the chloride ion (Cl^-).
Since the atomic mass does not include the mass of the electron, the atomic mass of an atom and its ion is the same. Thus, the chloride ion (Cl^-) has the same atomic mass as the chlorine atom, which is equal to about 35.45 atomic mass units (amu). We generally use an atomic mass of 35.5 g/mole for chlorine and chloride.
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