Question 7.EP.7: Determine the shape of each of the following molecules: (a) ...
Determine the shape of each of the following molecules: (a) CF_{4}, (b) PF_{5}
Strategy
Draw the Lewis structures. Each of these molecules has only single bonds around its central atom. So we can count the number of bonding pairs around the central atom and assign the geometry by consulting Table 7.3 as needed.
| Table 7.3 | ||||||||
| Each of the geometrical arrangements shown in the table minimizes the electron pair repulsions for the indicated number of electron pairs. To visualize the shapes of molecules, it is essential that you have a sound mental picture of each of these geometries. | ||||||||
| Number of Electron Pairs |
Geometric Name | Bond Angles | Diagram | |||||
| 2 | Linear | 180° |  |
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| 3 | Trigonal planar | 120° |  |
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| 4 | Tetrahedral | 109.5° |  |
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| 5 | Trigonal bipyramidal | 120°, 90° |  |
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| 6 | Octahedral | 90°, 180° |  |
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(a) The Lewis structure of CF_{4} is
\begin{matrix} & & \\ & \overset{..}{\underset{..}{:F}}-\overset{\underset{\mid}{\overset{..}{:F:} } }{\underset{\overset{\mid}{\overset{}{\underset{..}{:F:}}}}{C}}-\overset{..}{\underset{..}{F:}} & \\ & & \end{matrix}
The central carbon atom has a complete octet of electrons, contributed by four bonding electron pairs. The four electron pairs have minimal interaction when they point toward the corners of a tetrahedron. The shape of the molecule is tetrahedral (Table 7.3).
(b) The Lewis structure of PF_{5} is somewhat unusual because the central phosphorus atom has more than an octet around it. There are a number of molecules with this property and they serve as useful examples for structures in VSEPR.
The central phosphorus atom has an expanded octet with 10 electrons arranged in five bonding pairs. The five electron pairs occupy the vertices of a trigonal bipyramid (Table 7.3). Note that, unlike the fluorine atoms in the tetrahedral molecule we looked at in (a), all of the fluorine atoms in the trigonal bipyramid are not in equivalent positions. There is a spacing of 120° between the fluorine atoms arranged in the horizontal triangle (the equatorial positions), whereas the two atoms above and below the plane of the triangle (axial positions) form an angle of 90° with the plane of the triangle.
Discussion
These examples can be difficult to see using only the text. Model kits or software models can help you visualize the three-dimensional nature of the shapes described here.