Final answer:
'True'. The mass of the atom 199F is indeed 18.99840 amu.
Step-by-step explanation:
The statement is true.
The mass of the atom 199F is indeed 18.99840 amu, which represents the average mass of the atom based on the relative abundance of different isotopes of fluorine in nature.
Isotopes are atoms of the same element that have different numbers of neutrons. The atomic mass takes into account the mass of each isotope, weighted by its abundance.