Question

Why does boron triflouride behave as a Lewis acid ?

Answer 1

B atom in $B{F}_3$ has 6 valence electrons. To complete octet, it requires 2 electrons. It readily accepts an electron pair from Lewis bases such as fluoride ion, ethoxide ion and behaves as Lewis acid.

$B{F}_3+F^{-}\to B{F}_4^{-}$