Here's why:

* Boron (B) is a metalloid in group 13 of the periodic table.

* Fluorine (F) is a halogen in group 17 of the periodic table.

* Boron has a valency of 3 (it can form three bonds).

* Fluorine has a valency of 1 (it can form one bond).

Therefore, to satisfy their valencies, one boron atom will bond with three fluorine atoms, resulting in the compound BF₃.