What is the difference between ammonium and ammonia?

Ammonia and ammonium differ by one hydrogen ion (H+). Ammonia is the common name for the compound nitrogen trihydride, which has the formula . Ammonium is the name of the ion . An ion is an atom or molecule that has a net charge. 

Ammonia is a base, meaning it produces in solution. When ammonia reacts with water, an H+ ion is transferred from water to ammonia, forming ammonium ion, according to this equation:

 

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 accepts a proton (H+ ion), as seen in the equation above. When the ion donates a proton, its conjugate base is formed. This occurs in the reverse reaction: