Ammonia and ammonium differ by one hydrogen ion (H+). Ammonia is the common name for the compound nitrogen trihydride, which has the formula `NH_3` . Ammonium is the name of the ion `NH_4^+` . An ion is an atom or molecule that has a net charge.
Ammonia is a base, meaning it produces `OH^- ` in solution. When ammonia reacts with water, an H+ ion is transferred from water to ammonia, forming ammonium ion, according to this equation:
`NH_3 + H_2O -> NH_4^+ + OH^-`
Ammonia and ammonium ion are a conjugate acid-base pair. Ammonium ion is the conjugate acid of ammonia because it forms when when the base ammonia `(NH_3)` accepts a proton (H+ ion), as seen in the equation above. When the `NH_4^+` ion donates a proton, its conjugate base `NH_3` is formed. This occurs in the reverse reaction:
`NH_4^+ + OH^(-) -> NH_3 + H_2O`