# How can I classify each species as either a Lewis acid or a Lewis base for: Fe_3^+, BH_3, NH_3 and F^-?

May 2, 2015

Lewis acids:
$F {e}^{3 +}$
$B {H}_{2}$

Lewis bases:
$N {H}_{3}$
${F}^{-}$

Lewis acids(or electrophiles) are electron-deficient species. We can also say that they have a net positive charge.

$B {H}_{3}$ is a lewis acid because Boron only has 3 valence electrons and so it forms only three covalent bonds $\text{H}$, instead of four. So we say it lacks 2 more electrons to gain a complete octet!

Lewis bases(or nucleophiles) are electron-rich species. They have a net negative charge.

$N {H}_{3}$ is a lewis base because $\text{Nitrogen}$ forms only three bond with $\text{H}$ and so has a lone pair of electrons, and so it has a net negative charge.