Answer:
The number of electrons in its outermost shell.
Step-by-step explanation:
Atoms react to form bonds when the number of electron in the outermost shell is not maximum i. e the shell can still accommodate electrons. A fluorine atom combines with itself to form a fluorine molecule because the number of electrons in its outermost shell is not complete ( which is seven instead of the supposed eight). So, both atoms end up sharing an electron pair form a covalent bond.