[kciray]

Fluorine-19, the only stable isotope of fluorine. Its abundance is 100%. Why it has amu 18.99?

As example, deuterium has exactly 2.00 amu.

[Borek]

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.

[DrCMS]

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

[Enthalpy]

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

[AdiDex]

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 .

[Enthalpy]

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

[AdiDex]

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

Agreed .

[AdiDex]

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