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

Because atomic mass is calculated by averageing the whole spectrum.
Chlorine probably has many different atomic masses that average out to 35.45.

Answer 2

As given that there is only one isotope of Fluorine so all atoms of it have said mass of 18.998 amu.and the Chlorine exists as isotopes, so chances are the atomic mass doesn't resemble the true weight of a chlorine atom.
so yet the massspectrum of chlorine does not show peak at this mass
hope it helps