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PCAT Chemical Processes – Trigonal Planar Molecular Geometry

Which of the following exhibits trigonal planar molecular geometry?

A. Ammonia (NH3)
B. Methanol (CH3OH)
C. Aluminum chloride (AlCl3)
D. Acetylene (C2H2)

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C is correct. Aluminum is an exception to the octet rule. It exists in a stable state with an open-shell configuration. Therefore, the central aluminum atom in aluminum chloride does not have a lone pair of electrons, and VSEPR theory dictates it will have a trigonal planar molecular geometry. In contrast, nitrogen cannot violate the octet rule. The central nitrogen atom in ammonia has a lone pair of electrons and exhibits a trigonal pyramidal molecular geometry (eliminate Choice A). Methanol has a tetrahedral molecular geometry around the carbon and a bent molecular geometry around the oxygen (eliminate Choice B). Acetylene has a linear molecular geometry (eliminate Choice D). Recall that molecular geometry considers the arrangements of atoms alone, whereas electronic geometry considers the arrangements of atoms and lone pairs of electrons around the central atom.

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