Bronsted base. neither donates a proton nor accepts a proton and cannot be classified as either a Bronsted acid or a Bronsted base.
Example:
Classify HCl, and as either a Lewis acids or bases.
The proton donated by HCl (as in the Bronsted definition) is an electron pair acceptor. HCl is therefore a Lewis acid.
has an extra pair of electrons in its outer shell and is therefore an electron pair donor, or Lewis base. has only six electrons in its outer shell and can therefore accept an electron pair. Although it does not donate protons, as a Bronsted acid does, it can accept an electron pair and is therefore a Lewis acid.
Although the Lewis definition is broader, the Bronsted definition deals specifically with the presence of hydrogen ions. Since many reactions depend more on the motion and properties of hydrogen ions than on the motion of electron pairs, the Bronsted definition is commonly used to describe acid-base reactions.
After a Bronsted acid donates a proton, an anion remains. This anion is now able to accept a proton. The anion is therefore a Bronsted base and is called the conjugate
base of the original acid. Likewise, the cation that remains when a Bronsted base has accepted a proton is the conjugate acid of the original base.
