Here's why:

* Hybridization: The nitrogen atom in ammonium is sp³ hybridized. This means that its 2s orbital and three 2p orbitals combine to form four equivalent sp³ hybrid orbitals.

* Tetrahedral Geometry: These four sp³ hybrid orbitals are arranged in a tetrahedral geometry, with the nitrogen atom at the center and the four hydrogen atoms at the corners of the tetrahedron.

* Bond Angle: In a perfect tetrahedron, the angle between any two bonds is 109.5 degrees.

Therefore, the angle around nitrogen in ammonium is approximately 109.5 degrees.