Ammonium (NH₄⁺) is tetrahedral shaped due to the following reasons:

1. Hybridization: The nitrogen atom in ammonium undergoes sp³ hybridization. This means that the 2s orbital and three 2p orbitals of nitrogen mix to form four equivalent sp³ hybrid orbitals. These orbitals are arranged tetrahedrally around the nitrogen atom.

2. Lone Pair: Nitrogen in its ground state has five valence electrons. In ammonium, nitrogen forms four covalent bonds with four hydrogen atoms, using all its valence electrons. However, nitrogen still has a lone pair of electrons, but it is not present in the bonding structure and thus doesn't influence the shape.

3. Repulsion: The four sp³ hybrid orbitals, each containing a lone pair, experience maximum repulsion when arranged in a tetrahedral shape. This is because this arrangement minimizes the electron-electron repulsion between the bonding pairs, resulting in a stable and low-energy configuration.

4. VSEPR Theory: The Valence Shell Electron Pair Repulsion (VSEPR) theory predicts the geometry of a molecule based on the number of electron pairs around the central atom. In the case of ammonium, there are four electron pairs around the nitrogen atom (four bonding pairs), leading to a tetrahedral arrangement.

Therefore, due to the hybridization of nitrogen, the lone pair, and the repulsion between electron pairs, ammonium exhibits a tetrahedral shape.