Back-donation, definition

When a base accepts a proton, it becomes an acid capable of returning that proton. When an acid donates its proton, it becomes a base capable of accepting that proton back. One of the most important principles of the Br0nsted-Lowry definition is this concept of conjugate acids and bases. For example, NH4 and NH3 are a conjugate acid-base pair. NH3 is the base when it accepts a proton, it is transformed into its conjugate acid, NH4. Many compounds (water, for instance) can react either as an acid or as a base. Here are some additional examples of conjugate acid-base pairs. 

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