# What is the full name of this element?

## "_9^19 X

The $9$ subscript is the atomic number for ""_9^19X, and such an atomic number, $Z$, specfies the element fluorine. Most fluorine nuclei contain 10 neutrons, and thus the ""^19F isotope predominates.
As the element, fluorine largely exists as the ${F}_{2}$ molecule, which is the most reactive element on the Periodic Table.