According to the Lewis concept, a Lewis acid is a species that can accept an electron pair, while a Lewis base is a species that can donate an electron pair.

• In $(CH_3)_3B$ (trimethylborane), the central boron atom has only six electrons in its valence shell (three from boron and three shared from the methyl groups). Because it is an electron-deficient molecule and needs an electron pair to complete its octet, it acts as a Lewis acid.
• In $(CH_3)_2O$, $(CH_3)_3P$, and $(CH_3)_3N$, the central atoms (oxygen, phosphorus, and nitrogen, respectively) possess at least one lone pair of electrons. This makes them capable of donating an electron pair, so they act as Lewis bases.