Question

What is the polyatomic ion ${{H}_{3}}{{O}^{+}}$ called?

Answer 1

Polyatomic ions consist of more than 2 atoms and a charge, which is positive in case of a cation, and negative in case of anion. For naming any species the cation, or the positive ion is named first. ${{H}_{3}}{{O}^{+}}$ion is associated with acids, as acids when liberate hydrogen ion in water, forms this polyatomic ion ${{H}_{3}}{{O}^{+}}$.

Complete answer:
A polyatomic ion has more than 2 atoms with a positive or a negative charge. We have been given a polyatomic ion that consists of 3 hydrogen atoms and one oxygen with the formula, ${{H}_{3}}{{O}^{+}}$. This ion is formed when any acid is dissolved in water. The acid liberates the hydrogen ion that gets bonded to the water molecule to form${{H}_{3}}{{O}^{+}}$. The reaction is,
$HCl(aq)+{{H}_{2}}O\to {{H}_{3}}{{O}^{+}}(aq)+C{{l}^{-}}(aq)$
As the hydrogen oxide, which is water gets protonated with the hydrogen liberated by acids, the oxide ion gets the name oxonium and with hydrogen the ${{H}_{3}}{{O}^{+}}$ion is called as hydronium ion. This ions makes any solution acidic.
Also, water dissociates to form hydronium ion, and hydroxide ion as,
$2{{H}_{2}}O\rightleftharpoons {{H}_{3}}{{O}^{+}}(aq)+O{{H}^{-}}(aq)$
Hence, the name of ${{H}_{3}}{{O}^{+}}$ion is hydronium ion.

Note:
The pH of any acidic solution depends on the concentration of hydronium ions. The more the hydronium ions, the less is the pH of any solution, and so more acidic is the nature of that solution. Hydronium ion in IUPAC naming is called oxonium ion, and also referred to as simply oxonium ion.