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In this example, H3O+, the Lewis diagram
shows O at the center with one lone electron pair and three hydrogen
atoms attached. Compare this with ammonia, NH3, which
also has a lone pair.
Compare it to the water molecule
which has 2 hydrogen atoms and 2 lone electron pairs.. The third
hydrogen bonds to the water molecule as a hydrogen ion (no electrons)
bonding to the lone pair on the oxygen.
This shows tetrahedral geometry for the electron pair geometry
and and trigonal pyramid the molecular geometry.
Hydronium ion is a more accurate method to depict the hydrogen
ion associated with acid properties of some molecules in water