What distinguishes conjugate acid-base pairs?

Conjugate acid-base pairs are primarily distinguished by their difference in protons. When an acid donates a proton (H⁺), it transforms into its conjugate base, while the base that accepts the proton is converted into its conjugate acid. This fundamental relationship is rooted in the Brønsted-Lowry theory of acids and bases, which defines acids as proton donors and bases as proton acceptors.

For example, when hydrochloric acid (HCl) donates a proton, it becomes its conjugate base, chloride ion (Cl⁻). Conversely, when ammonia (NH₃) accepts a proton, it turns into its conjugate acid, ammonium ion (NH₄⁺). The critical aspect that defines the relationship within conjugate acid-base pairs is therefore the transfer of a proton.

The other options do not accurately reflect the primary characteristic of conjugate acid-base pairs. For instance, they do not involve a hydroxide ion, an electron, or a lone pair as the distinguishing factor. Therefore, the focus on the transfer of a proton is what distinguishes these pairs in acid-base chemistry.