Draw the structure of H3O+ and NH4+.

HI,

Hydronium ion: The compound giving hydronium ion is water. One lone pair on the oxygen atom of water molecule is shared with the hydrogen ion. In this combination, the hydrogen ion accepts a pair of electrons and distributes the charge over the entire hydronium radical.

Ammonium ion: One lone pair on the nitrogen atom of ammonia molecule is shared with the hydrogen ion. In this combination, the hydrogen ion accepts a pair of electrons and distributes the charge across the entire ammonium radical.