A Lewis acid must contain an empty orbital (or be able to rearrange electrons to create one) that can accept an electron pair. A Lewis base needs a pair of electrons that can be donated.
Work Step by Step
According to Lewis' definition, an acid is a substance able to accept an electron pair. In order to accept it, the substance must have at least one empty orbital to receive the pair of electrons. In order to be an electron pair donor, which is the definition of a Lewis base, the substance must have at least one pair of electrons to donate.