Question 13.12: Prediction of Molecular Structure II When phosphorus reacts ...
Prediction of Molecular Structure II
When phosphorus reacts with excess chlorine gas, the compound phosphorus pentachloride (PC l_{5} ) is formed. In the gaseous and liquid states, this substance consists of PC l_{5} molecules, but in the solid state, it consists of a 1 : 1 mixture of PC l_{4} ^{+} and PC l_{6} ^{-} ions. Predict the geometric structures of PC l_{5} , PC l_{4}^{+} , and PC l_{6}^{-} .
In the traditional Lewis structure for PC l_{5} five pairs of electrons surround the phosphorus atom. This requires a trigonal bipyramidal arrangement (see Table 13.8). When the chlorine atoms are included, a trigonal bipyramidal molecule results.
In the Lewis structure for the PC l_{4} ^{+} ion [5 + 4(7) - 1 = 32 valence electrons] four pairs of electrons surround the phosphorus atom in the PC l_{4} ^{+} ion. This requires a tetrahedral arrangement of the pairs. Since each pair is shared with a chlorine atom, a tetrahedral PC l_{4} ^{+} cation results.
The traditional Lewis structure for PC l_{6} ^{-} [5 + 6(7) + 1 = 48 valence electrons] is
Since phosphorus is surrounded by six pairs of electrons, an octahedral arrangement is required to minimize repulsions, as shown below on the left. Since each electron pair is shared with a chlorine atom, an octahedral PC l_{6} ^{-} anion is predicted.
Lewis structure for PC l_{5}
Lewis structure for PC l_{4} ^{+}
Table 13.8
Arrangements of Electron Pairs Around an Atom Yielding Minimum Repulsion
|
Number of Electron Pairs |
Arrangement of
Electron Pairs |
Example |
|
|
| 2 | Linear |  |
 |
|
| 3 | Trigonal planar |  |
 |
|
| 4 | Tetrahedral |  |
 |
|
| 5 | Trigonal
bipyramidal |
 |
 |
|
| 6 | Octahedral |  |
 |
