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What is the Difference Between Ammonia and Ammonium?

Ammonia refers to the molecule with formula NH3, whilst ammonia refers to the cation with formula NH4+.  There are significant differences between the two chemical species.

Ammonia is a molecule that is made up of one nitrogen atom bonded to three hydrogen atoms, and has no overall charge.  Ammonia is gaseous at room temperature and pressure, and has RMM of 17. 

The nitrogen in ammonia has one lone pair of electrons which dominate its’ chemical properties and reactivity

Ammonium is an ion made up of one nitrogen atom bonded to four hydrogens.  Ammonium is a cation with an overall charge of 1+.  Ammonium must exist with a corresponding anion, and can’t be completely isolated.   The RFM of ammonium ion is 18.

Ammonium is formed from ammonia and a hydrogen cation H+.  A dative bond is formed between ammonia’s nitrogen atom and the H+ using the lone pair of electrons on the nitrogen atom. 

The nitrogen atom of ammonium does not have a lone pair, so ammonium’s chemical properties are significantly different from ammonia’s chemical properties.  Therefore it important to understand the difference between ammonia and ammonium, and between NH3 and NH4+.

Finally, the formula NH4 is often quoted confused students.  This formula does not refer to either ammonia nor ammonium, and is not a possible chemical formula.  Hence, we should really ask what is the difference between NH3, NH4+, and NH4.