According to the Lewis concept, a Lewis acid is a species that can accept an electron pair. Molecules that are electron-deficient typically act as Lewis acids.

• $NH_3$ (ammonia) has a lone pair of electrons on the nitrogen atom, so it acts as a Lewis base.
• $H_2O$ (water) has two lone pairs of electrons on the oxygen atom, so it acts as a Lewis base.
• $CH_4$ (methane) is an electron-precise hydride with a complete octet and no lone pairs, so it does not act as a Lewis acid or base.
• $B_2H_6$ (diborane) is an electron-deficient hydride. The boron atoms lack a complete octet of electrons, making it capable of accepting an electron pair. Therefore, it acts as a Lewis acid.