Which molecule produces hydronian atoms in water?

Answer 1

Any reasonably strong acid produces hydronium ions when dissolved in water. Sulfuric acid, with formula H2SO4, is a common example.

Answer 2

In water, the molecule that produces hydronium ions (H3O+) is the hydronium ion itself. This forms when a water molecule (H2O) accepts a proton (H+) from another water molecule.