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Chapter 15: Problem 117

(a) Use VSEPR to predict the geometry of the hydronium ion, \(\mathrm{H}_{3} \mathrm{O}^{+}\). (b) The \(\mathrm{O}\) atom in \(\mathrm{H}_{2} \mathrm{O}\) has two lone pairs and in principle can accept two \(\mathrm{H}^{+}\) ions. Explain why the species \(\mathrm{H}_{4} \mathrm{O}^{2+}\) does not exist. What would be its geometry if it did exist?

Short Answer

Expert verified
The geometry of the hydronium ion (H3O+) is trigonal pyramidal. H4O^2+ doesn't exist due to excessive positive charge that would destabilize the molecule. If it did exist, its geometry would be tetrahedral.

Step by step solution

01

Determine the Geometry of Hydronium Ion (H3O+)

The hydronium ion (H3O+) can be viewed as a molecule of water (H2O) with an extra hydrogen ion (H+) attached to it. The central Oxygen atom is bonded to three hydrogen atoms and has one lone electron pair, giving it a total of 4 regions of electron density. According to VSEPR theory, these regions will arrange themselves around the oxygen atom in a way that minimizes electron-electron repulsion. This leads to a tetrahedral electron-pair geometry. However, the molecular geometry, which describes the spatial arrangement of the chemical bonds, is trigonal pyramidal, a shape derived from the tetrahedral arrangement with one vertex missing.
02

Explain Why H4O^2+ Doesn't Exist

Water, or H2O, indeed has two lone pairs of electrons and can accept two protons (H+ ions), theoretically leading to the formation of H4O^2+. However, an H4O^2+ ion does not exist in reality, because the ion would be too positively charged. Compounds strive for stability, and having too many positive charges in close proximity without enough electrons to neutralize them destabilizes the molecule.
03

Predict the Geometry of Hypothetical H4O^2+

If such a species did exist, the oxygen atom would be central and bonded to four hydrogen atoms, with no lone pairs of electrons. This would again give 4 regions of electron density, which according to VSEPR theory, would arrange themselves in a tetrahedral pattern to minimize electron-electron repulsion. So, the geometry would be tetrahedral.

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