What's a hydronium atom and what role does it play in Acids and Bases?

Answer 1

Hydronium is the H3O + ion

It is formed by the protonation of water

H-A + H2O <=> A-(aq) + H3O+(aq)


Water protonates itself
H-OH + H2O <=> OH-(aq) + H3O+(aq)


pH is -log (to the base 10) of the concentration of H3O+ ion in the aqueous solution

This gives an indication of the strength of the acid (or base) and hence it can be seen that the more H3O+ an acid generates, the stronger an acid it is.

See the wiki page

Answer 2

It's not an atom, it's an ion. Basically protons cannot exist in solution so when a Bronsted-Lowry acid dissolves in water, the proton is transferred to a water molecule (i.e. the water acts as a base). With a strong acid, almost all protons will dissolve so you'll have as many moles of hydronium ions as you do of acid. Hope that helps!