Question

The molecules \(\mathrm{BF}_{3}, \mathrm{CF}_{4}, \mathrm{CO}_{2}, \mathrm{PF}_{5},\) and \(\mathrm{SF}_{6}\) are all nonpolar, even though they contain polar bonds. Why?

Short answer

In summary, the molecules BF3, CF4, CO2, PF5, and SF6 are nonpolar because they have symmetric molecular geometries. Despite containing polar bonds, the symmetric distribution of these bonds in each molecule results in the cancellation of their polarity, leading to overall nonpolar molecules.

Step-by-step solution

Define Polar and Nonpolar molecules

A polar molecule is a molecule in which there is an uneven distribution of electron density, leading to regions of partial positive and negative charges within the molecule. A nonpolar molecule, on the other hand, has an even distribution of electron density, and there are no regions with partial charges.

Understand the role of molecular geometry in determining molecule polarity

The shape of the molecule plays a crucial role in determining its polarity. In some cases, polar bonds within a molecule can "cancel out" if the molecule has a symmetric geometry. This results in a nonpolar molecule, even though it contains polar bonds.

Analyze the geometry of each molecule

Now, let us analyze the geometries of the given molecules to determine why they are nonpolar despite having polar bonds: 1. BF3: Boron trifluoride has a trigonal planar geometry, meaning the three F atoms are around the central B atom in a flat, triangular arrangement. This symmetry causes the polarity of the individual B-F bonds to cancel out, leaving the molecule as nonpolar. 2. CF4: Carbon tetrafluoride has a tetrahedral geometry, meaning the four F atoms are symmetrically distributed around the central C atom in a three-dimensional arrangement. This symmetric distribution causes the polarity of the individual C-F bonds to cancel out, leaving the molecule nonpolar. 3. CO2: Carbon dioxide has a linear geometry, meaning the two O atoms are located on opposite sides of the central C atom in a straight line. This symmetry causes the polarity of the individual C-O bonds to cancel out, leaving the molecule nonpolar. 4. PF5: Phosphorus pentafluoride has a trigonal bipyramidal geometry, meaning the five F atoms are symmetrically distributed around the central P atom in a three-dimensional arrangement. This symmetric distribution causes the polarity of the individual P-F bonds to cancel out, leaving the molecule nonpolar. 5. SF6: Sulfur hexafluoride has an octahedral geometry, meaning the six F atoms are symmetrically distributed around the central S atom in a three-dimensional arrangement. This symmetric distribution causes the polarity of the individual S-F bonds to cancel out, leaving the molecule nonpolar.

Conclusion

In conclusion, the molecules BF3, CF4, CO2, PF5, and SF6 are nonpolar despite containing polar bonds because their molecular geometries are symmetric, and this symmetry causes the polarity of the individual bonds to "cancel out."

Key concepts

Molecular Geometry

The shape of a molecule is fundamental in determining whether the molecule is polar or nonpolar. Molecular geometry refers to the three-dimensional arrangement of atoms in a molecule.
For instance,

• BF₃: This molecule is trigonal planar, meaning the three fluorine atoms evenly surround the central boron atom in a flat plane.
• CF₄: It has a tetrahedral shape, with four fluorine atoms symmetrically around a central carbon atom in a three-dimensional form.
• CO₂: This molecule is linear, as the oxygen atoms stand directly opposite each other at the carbon center.
• PF₅: This molecule illustrates a trigonal bipyramidal geometry.
• SF₆: It possesses an octahedral geometry.

These symmetric shapes are crucial because they influence how electron density and dipoles are distributed in the molecule.

Symmetric Distribution

Symmetry plays a critical role in determining the overall polarity of a molecule. When a molecule has a symmetric distribution of its atoms, the polarities of individual bonds can cancel each other, leading to a nonpolar molecule.
The symmetry ensures that any pulls on the central atom by the outside atoms are balanced out by opposing pulls on opposite sides.

• Example: In CO₂ , the linear symmetrical alignment causes the dipoles from the C-O bonds to oppose and cancel one another.
• For molecules like SF₆, the octahedral geometry means all the dipoles from the S-F bonds are uniformly distributed, also leading to cancellation.

This symmetry is why many molecules with polar bonds can still be nonpolar overall.

Polar Bonds

Polar bonds are formed when two bonded atoms have different electronegativities, causing electron density to be-shared unequally. This results in one atom becoming partially negatively charged and the other partially positively charged.
For instance, in all these molecules:

• The bonds between the central atom (like C in CF₄) and the surrounding atoms (like F) are polar due to differences in electronegativity.
• Despite the polar nature of these bonds, the overall symmetry of the molecule can lead these polar effects to balance each other.

Understanding polar bonds is vital as they are the reason behind partial charges in molecules.

Nonpolar Molecules

Nonpolar molecules exhibit no overall charge because any bond polarity they possess is ultimately cancelled out by their geometric symmetry and bond arrangement.
Even if a molecule contains polar bonds, it can still be categorized as nonpolar if the symmetric nature neutralizes the individual dipole moments.

• Example: Despite PF₅ having polar P-F bonds, its trigonal bipyramidal structure ensures that these polarities cancel, making the molecule nonpolar overall.
• BF₃ and its trigonal planar shape also exhibit nonpolarity due to symmetry cancellation.

The overall nonpolarity of a molecule profoundly influences its behavior and interactions with other substances.