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Introduction and Definitions:
At the simplest definition, an acid is any compound that yields
hydrogen ions (H+) or hydronium ions (H3O+)
when dissolved in water. Hydronium ions are really the combination
of H+ ions and H2O molecules, i.e.
H+ + H2O === H3O+.
The hydrogen ion with no electrons bonds to water through
what is called a coordinate covalent bond where the oxygen donates
both electrons for the bond. The molecule is still short an electron
so the positive charge is retained. This is reaction is shown
on the left graphic. The hydronium ion also has a trigonal pyramid
geometry.
Hydronium ion - Chime
in new window
Even though in solution a "bare" hydrogen ion is
surrounded by up to 20 water molecules and is the best representation,
most of the time just a simple hydrogen ion is used to represent
the acid. In the chime model, notice the close contact of the
water molecules as they engage in hydrogen
bonding.
Hydronium ion cluster - Chime
in new window
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