Question

the atomic mass of fluorine is 18.998 amu and its mass spectrum shows a large peak at this mass. the atomic mass of chlorine is 35.45 amu, yet the mass spectrum of chlorine does not show peak at this mass. Explain the difference.

Answer 1

There is only one isotope of Fluorine so all atoms have said mass of 18.998 amu

Chlorine exists as isotopes, so chances are the atomic mass (which is really an average) doesn't resemble the weight of a true chlorine atom.