Explain the structures of (a) Hydronium ion, and (b) Ammonium ion.

(a) Hydronium ion (H₃O⁺) is formed when an acid is added to water. Acid in water ionises to release hydrogen ion (H⁺) or proton, which is taken up by water molecules to form a hydronium ion. It contains a coordinate bond between oxygen of water and hydrogen ion as shown below:
${\left[\mathrm{H}-\underset{\stackrel{}{}}{\overset{\underset{\overline{)}}{\mathrm{H}}}{\mathrm{O}}}\to \mathrm{H}\right]}^{+}$

(b) Ammonium ion (${\mathrm{NH}}_4^{+}$): In ammonium ion, nitrogen atom has a lone pair of electrons to donate to a hydrogen ion (H⁺) that needs two electrons to complete its valence shell and acquire noble gas configuration. Hence, nitrogen of ammonia donates its lone pair of electrons to hydrogen ion to form a coordinate bond between them. The structure of ammonium ion is given below:
${\left[\mathrm{H}-\underset{\stackrel{\overline{)}}{\mathrm{H}}}{\overset{\underset{\overline{)}}{\mathrm{H}}}{\mathrm{N}}}\to \mathrm{H}\right]}^{+}$