According to the Lewis concept of acids and bases, a Lewis acid is a species that accepts an electron pair, while a Lewis base is a species that donates an electron pair .

• $\text{BF}_3$ (Boron trifluoride) is an electron-deficient molecule. The central boron atom has only six valence electrons and does not have a complete octet. Therefore, it can accept a lone pair of electrons (for example, from $\text{NH}_3$), acting as a Lewis acid .
• On the other hand, $\text{OH}^-$, $\text{H}_2\text{O}$, and $\text{NH}_3$ all possess at least one unshared (lone) pair of electrons which they can donate. Thus, they act as Lewis bases .