If water Formula is H2O , what is H3O?

it called Hydronium ion , that resulted from adding an acid , so acid ionized into H+ cation (=proton) and an anion A- , partially negative charged Oxygen atom attract H+ to form Hydronium ion :

HA = H+ + A- ( dissociation of an acid )

H2O + H+ = H3O+ ( protonation of H2O molecule )

and I'd like to correct to Luay .... it's not heavy water ....heavy water is consist of Deuterium atom instead of Hydrogen atom , which is ( chemically ) the same , but Deut. is a Hydrogen atom , with an extra neutron =D2O .

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RE:

If water Formula is H2O , what is H3O?

Hydronium ion. Written H3O+

H3O+ is hydronium. It's what you get when your water is acidic. Acids can be thought of as just solutions with extra H+'s floating around. Those H+'s attached themselves to any water that is around to form H30+. This then floats around and kind of carries along extra water molecules to spread the charge and you get things like H30[H20]6+

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it is hydronium ion. it is H3O+ and has a unit positive charge. it is formed due to protonation of water ie., when a H+ ion accepts one of the lone pair of electrons on oxygen. as H+ is only given out by acids when they are dissolved in water, H3O+ is sometimes reffered to as H+ itself.

also the pH of a solution is the negative log of the H+ concentration in it. another definition of pH is the negative log of the H3O+ concentration.

It's called Hydronium, also hydroxomium or oxonium. It's an unstable molecule with strong acidic properties that occurs if water is spiked with hydrogen atoms.

What is the formal charge on H3O

it's hydronium, H3O+

there is another: H2O2 ( hydrogen peroxide)

by the way, O2 is oxygen, while O3 is ozone

also, there are ions: H+(hydrogen ions) and OH-(hydroxyl ions)

that's all I know, hope this will help you

Ammonia