The dot and cross diagram for the ammonium ion (NH₄⁺) shows how the ammonium ion is formed from an ammonia molecule (NH₃) and a hydrogen ion (H⁺). It represents the valence electrons of atoms using dots and crosses to distinguish electrons from different atoms.

Formation of the Ammonium Ion (NH₄⁺)

1. Ammonia (NH₃) has a lone pair of electrons on the nitrogen atom.
2. A hydrogen ion (H⁺), which has no electrons, approaches the ammonia molecule.
3. The lone pair on nitrogen donates both electrons to form a dative covalent (coordinate) bond with H⁺.
4. This results in the formation of the NH₄⁺ ion, where nitrogen is covalently bonded to four hydrogen atoms.
5. The overall charge is +1 because an extra proton (H⁺) has been added without adding any extra electrons.

Diagram Representation

• Dots and crosses are used to represent the electrons from different atoms.
• The three original N-H bonds in ammonia are normal covalent bonds, where each hydrogen shares one electron with nitrogen.
• The additional N-H bond is a dative covalent bond, where both electrons come from nitrogen (often marked with an arrow pointing toward H⁺).
• The overall charge (+1) is placed outside the brackets to indicate the ionic nature of NH₄⁺.