Answer:
Due to incomplete outermost shell.
Step-by-step explanation:
Fluorine cannot exist monoatomic in nature because of its incomplete outermost shell. This incomplete outermost shell makes Fluorine more reactive and it can only be stable if it makes bond with other atoms through sharing of electron i. e. covalent bond or by complete transfer of electron i. e. ionic bond. This bond making completes its outermost shell and the Fluorine attain stability so that's why the Fluorine cannot exists in monoatomic in nature.