According to the Lewis concept, an acid is a chemical species that can accept an electron pair. Therefore, electron-deficient molecules act as Lewis acids.

• $NH_3$ and $H_2O$ possess lone pairs of electrons on their central atoms (nitrogen and oxygen, respectively) which they can donate, so they act as Lewis bases.
• $CH_4$ is an electron-precise molecule with a complete octet and no lone pairs, meaning it generally neither acts as a Lewis acid nor a Lewis base.
• $B_2H_6$ (diborane) is an electron-deficient hydride because it does not have enough electrons to form conventional 2-center-2-electron bonds between all its atoms. It accepts electron pairs to complete its octet, thus acting as a Lewis acid.