Why is CCl4 a nonpolar molecule, but is PCl3 a polar molecule?

Ch.6 Ionic and Molecular Compounds

Timberlake - Chemistry: An Introduction to General, Organic, and Biological Chemistry 13th Edition

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Timberlake 13th Edition

Ch.6 Ionic and Molecular Compounds

Problem 80

Chapter 6, Problem 80

Why is CCl₄ a nonpolar molecule, but is PCl₃ a polar molecule?

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Step 1: Understand the concept of molecular polarity. A molecule is polar if it has an uneven distribution of electron density, resulting in a dipole moment. This typically occurs when there are polar bonds and the molecular geometry does not cancel out the dipole moments.

Step 2: Analyze the structure of CCl₄ (carbon tetrachloride). The central carbon atom is bonded to four chlorine atoms in a tetrahedral geometry. Each C-Cl bond is polar due to the difference in electronegativity between carbon and chlorine.

Step 3: Determine the net dipole moment for CCl₄. In a tetrahedral geometry, the dipole moments of the four C-Cl bonds cancel each other out because they are symmetrically arranged around the central carbon atom. This results in a nonpolar molecule.

Step 4: Analyze the structure of PCl₃ (phosphorus trichloride). The central phosphorus atom is bonded to three chlorine atoms and has one lone pair of electrons. This gives the molecule a trigonal pyramidal geometry.

Step 5: Determine the net dipole moment for PCl₃. The lone pair on phosphorus creates an asymmetry in the electron distribution, and the dipole moments of the P-Cl bonds do not cancel out. This results in a polar molecule.

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Key Concepts

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

Molecular Polarity

Molecular polarity refers to the distribution of electrical charge across a molecule. It is determined by the shape of the molecule and the electronegativity of its atoms. A molecule is polar if it has a net dipole moment due to an uneven distribution of charge, while nonpolar molecules have a symmetrical charge distribution.

Electronegativity

Electronegativity is a measure of an atom's ability to attract and hold onto electrons in a chemical bond. In polar molecules, there is a significant difference in electronegativity between bonded atoms, leading to unequal sharing of electrons. In contrast, nonpolar molecules have atoms with similar electronegativities, resulting in equal sharing of electrons.

Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of atoms within a molecule. The shape of a molecule affects its polarity; for example, CCl₄ has a tetrahedral shape that allows for symmetrical charge distribution, making it nonpolar. In contrast, PCl₃ has a trigonal pyramidal shape, leading to an uneven distribution of charge and making it polar.

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