Draw three-dimensional structures of PCl3 and PCl5, and then explain why one of the molecules has a dipole moment and one does not.

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Step 1: Draw the Lewis structures for PCl3 and PCl5. For PCl3, phosphorus (P) is the central atom surrounded by three chlorine (Cl) atoms. For PCl5, phosphorus is the central atom surrounded by five chlorine atoms.

Step 2: Determine the molecular geometry of each molecule using the VSEPR theory. PCl3 has a trigonal pyramidal shape with one lone pair of electrons on the phosphorus atom, while PCl5 has a trigonal bipyramidal shape with no lone pairs on the phosphorus atom.

Step 3: Evaluate the polarity of each bond. The P-Cl bond is polar because phosphorus and chlorine have different electronegativities.

Step 4: Determine the dipole moment of each molecule. In PCl3, the dipole moments of the P-Cl bonds do not cancel out due to the molecule's trigonal pyramidal shape, resulting in a net dipole moment. In contrast, in PCl5, the dipole moments of the P-Cl bonds cancel out due to the molecule's symmetrical trigonal bipyramidal shape, resulting in no net dipole moment.

Step 5: Conclude that PCl3 has a dipole moment while PCl5 does not, despite both molecules containing polar bonds. This is because the overall dipole moment of a molecule depends not only on the polarity of its bonds but also on its molecular geometry.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of atoms within a molecule. It is determined by the number of bonding pairs and lone pairs of electrons around the central atom, which influences the shape of the molecule. For PCl3, the geometry is trigonal pyramidal due to one lone pair, while PCl5 has a trigonal bipyramidal shape with no lone pairs.

Polarity and Dipole Moment

Polarity in molecules arises from differences in electronegativity between bonded atoms, leading to uneven distribution of electron density. A dipole moment is a vector quantity that measures the separation of positive and negative charges in a molecule. PCl3 has a net dipole moment due to its asymmetrical shape, while PCl5 is symmetrical, resulting in no overall dipole moment.

Valence Shell Electron Pair Repulsion (VSEPR) Theory

VSEPR theory is a model used to predict the geometry of molecules based on the repulsion between electron pairs in the valence shell of the central atom. According to this theory, electron pairs will arrange themselves to minimize repulsion, leading to specific molecular shapes. This theory helps explain the different geometries of PCl3 and PCl5 and their resulting polarities.

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