(a) Consider the following two molecules: PCl3 and BCl3. Which molecule has a nonzero dipole moment?

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Identify the molecular geometry of each molecule. PCl_3 is a trigonal pyramidal molecule, while BCl_3 is a trigonal planar molecule.

Understand that a molecule's dipole moment depends on both the difference in electronegativity between the atoms and the molecular geometry.

For PCl_3, the phosphorus atom is more electronegative than the chlorine atoms, and the trigonal pyramidal shape leads to an asymmetrical distribution of charge, resulting in a nonzero dipole moment.

For BCl_3, the boron atom is less electronegative than the chlorine atoms, but the trigonal planar shape is symmetrical, causing the dipoles to cancel out, resulting in a zero dipole moment.

Conclude that PCl_3 has a nonzero dipole moment due to its asymmetrical shape, while BCl_3 has a zero dipole moment due to its symmetrical shape.

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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. The shape of a molecule is determined by the number of bonding pairs and lone pairs of electrons around the central atom. For PCl3, the geometry is trigonal pyramidal due to one lone pair on phosphorus, while BCl3 has a trigonal planar shape with no lone pairs on boron.

Dipole Moment

A dipole moment is a measure of the separation of positive and negative charges in a molecule, resulting from differences in electronegativity between bonded atoms. Molecules with an uneven distribution of charge, such as PCl3, exhibit a nonzero dipole moment, indicating polarity. In contrast, BCl3, being symmetrical, has a dipole moment of zero.

Polarity of Molecules

Polarity in molecules arises from the presence of polar bonds and the overall shape of the molecule. A molecule is polar if it has a net dipole moment due to an asymmetrical distribution of charge. PCl3 is polar because of its trigonal pyramidal shape, while BCl3 is nonpolar due to its symmetrical trigonal planar structure, leading to cancellation of dipole moments.

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