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

Problema 80

Capitolo 6, Problema 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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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.

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01:34

Molecular Polarity (Simplified) Concept 1

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.

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01:51

Dipole Moment (Simplified) Concept 1

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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