Why Fluorine mass is 18.99, not 19 exactly?

https://en.wikipedia.org/wiki/Nuclear_binding_energy#Binding_energy_for_atoms

Why should it be exactly 19? The only isotope with an exact mass is C-12, and that's because we defined it to be so.

also you are wrong about the atomic mass of deuterium it is 2.01.

There are two reasons !!
1. Mass Defect (Search nuclear binding energy )
    Even you don't have to go through this reason to disprove that F-19 Can have whole no. value of atomic mass. There is much simpler reason which is 2nd one .

2. Proton and neutron don't have exactly the same mass .  since  F-19 doesn't contain protons and neutrons in the multiples of the as it contain in case of C-12 .

In reason 2, you should compare neutron with proton+electron.

by the way second reason has it's drawback too.
Eg. mass of Mg-24 is 23.985 amu .

Quote from:

In reason 2, you should compare neutron with proton+electron.

Agreed .