Ammonium, or NH₄⁺, is not a substance that exists independently. It's a positively charged polyatomic ion, a cation. Therefore, it does not have a melting point or boiling point.

Here's why:

* Ionic compounds: Ammonium exists as part of ionic compounds, such as ammonium chloride (NH₄Cl) or ammonium nitrate (NH₄NO₃). These compounds have their own specific melting and boiling points.

* Not a standalone molecule: Ammonium ions are always found bonded to other negatively charged ions or molecules. They don't exist on their own, so they don't have independent physical properties like melting or boiling.

To get the melting and boiling points, you need to specify the specific ammonium compound you are interested in.