Question

In terms of orbitals and electron arrangements, what must be present for a molecule or an ion to act as a Lewis acid (use \(\mathrm{H}^{+}\) and \(\mathrm{BF}_{3}\) as examples)? What must be present for a molecule or ion to act as a Lewis base (use \(\mathrm{OH}^{-}\) and \(\mathrm{NH}_{3}\) as examples)?

Short answer

A Lewis acid must have an empty orbital that can accept a pair of electrons, like \(\mathrm{H}^{+}\) and \(\mathrm{BF}_{3}\). A Lewis base must have a spare pair of electrons in its outer energy level that it can donate, like \(\mathrm{OH}^{-}\) and \(\mathrm{NH}_{3}\).

Step-by-step solution

Define Lewis Acids and Lewis Bases

A Lewis acid is a substance that can accept a pair of electrons to form a covalent bond. A Lewis base, on the other hand, is a substance that can donate a pair of electrons to form a covalent bond.

Identify Characteristics of a Lewis Acid

In terms of orbitals and electron arrangements, a Lewis acid must have an empty orbital or orbitals that can accept a pair of electrons. The \(\mathrm{H}^{+}\) ion is a Lewis acid because it has no electrons and can easily accept a pair of electrons. The \(\mathrm{BF}_{3}\) molecule is also a Lewis acid because the boron atom has an empty orbital that can accept a pair of electrons.

Identify Characteristics of a Lewis Base

A Lewis base must have an unshared pair of electrons in its outer energy level that it can donate. The \(\mathrm{OH}^{-}\) ion is a Lewis base because it has a spare pair of electrons that it can donate. Similarly, the \(\mathrm{NH}_{3}\) molecule is a Lewis base because the nitrogen atom has an unshared pair of electrons that it can donate.