Ammonium Ion : Tetrahedral Molecular Geometry 

In this example, NH₄⁺, the Lewis diagram shows N at the center with no lone electron pairs. Contrast this with ammonia, NH₃, which does have a lone pair. The fourth hydrogen bonds to the ammonia molecule as a hydrogen ion (no electrons) bonding to the lone pair on the nitrogen.

This shows tetrahedral geometry for both the electron pair geometry and the molecular geometry. Ammonium ions are found in solutions of household ammonia or window cleaner. It is also found as part of a solid as ammonium nitrate in fertilizer.