Question

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

Answer 1

Lewis acids:
`Fe^(3+)`
`BH_2`

Lewis bases:
`NH_3`
`F^-`

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

`BH_3` is a lewis acid because Boron only has 3 valence electrons and so it forms only three covalent bonds `"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.

`NH_3` is a lewis base because `"Nitrogen"` forms only three bond with `"H"` and so has a lone pair of electrons, and so it has a net negative charge.