Question

Which of these molecules is (are) polar? For each polar molecule, what is the direction of polarity; that is, which is the partial negative end and which is the partial positive end of the molecule? (a) \(\mathrm{CO}_{2}\) (b) \(\mathrm{HBF}_{2}\) (c) \(\mathrm{CH}_{3} \mathrm{Cl}\) (d) \(\mathrm{SO}_{3}\)

Short answer

HBF2 and CH3Cl are polar; HBF2 is negative at F, CH3Cl is negative at Cl.

Step-by-step solution

Determine the Molecular Geometry

To identify if a molecule is polar, begin by identifying its molecular geometry using VSEPR theory. This helps assess whether the dipoles cancel out or result in an overall dipole moment.

Analyze Dipole Moments of CO2

The geometry of CO2 is linear. The molecule has two polar C=O bonds, but they are oriented 180° apart, causing their dipoles to cancel out. Thus, CO2 is non-polar.

Analyze Dipole Moments of HBF2

HBF2 has a trigonal planar geometry. The B-F bonds are more polar than the B-H bond, leading to a net dipole moment. Thus, HBF2 is polar with the B center being partially positive and the F atoms being partially negative.

Analyze Dipole Moments of CH3Cl

CH3Cl has a tetrahedral geometry. The C-Cl bond is polar and does not align with the bond dipoles from C-H, causing an overall dipole. Thus, CH3Cl is polar with the Cl end as partially negative and the C end as partially positive.

Analyze Dipole Moments of SO3

SO3 has a trigonal planar geometry. The S=O bonds are polar, but the dipoles cancel out due to symmetry. Thus, SO3 is non-polar.

Key concepts

Molecular Geometry

Understanding the molecular geometry of a molecule is crucial in determining its polarity. Molecular geometry refers to the three-dimensional arrangement of atoms within a molecule. The shape of the molecule affects how the individual bond dipoles within the molecule interact and whether they cancel out or produce a net dipole moment.
For instance, in carbon dioxide (\(\mathrm{CO}_{2}\),) we see that the linear geometry causes the dipoles of the C=O bonds to be positioned directly opposite each other. This symmetric arrangement results in the dipoles canceling, making \(\mathrm{CO}_{2}\) non-polar.
Contrarily, in chloromethane (\(\mathrm{CH}_{3}\mathrm{Cl}\),) which has a tetrahedral geometry, the bond dipoles do not align perfectly to cancel, giving it an overall dipole moment. Molecular geometry is a defining factor that influences a molecule's polarity.

Dipole Moment

The dipole moment is a vector quantity that measures the separation of positive and negative charges in a molecule, providing insight into its polarity. It is influenced by two main factors: the difference in electronegativity between the bonded atoms and the spatial arrangement of the bonds.
In molecules like \(\mathrm{HBF}_{2}\), the electronegativity difference between boron and fluorine creates polar bonds, and the trigonal planar shape prevents the cancellation of these dipoles, resulting in a net dipole moment. This means \(\mathrm{HBF}_{2}\) is polar with the partial negative charge on the fluorine atoms.
Conversely, in sulfur trioxide (\(\mathrm{SO}_{3}\),) despite having polar bonds, the symmetric trigonal planar arrangement causes the individual dipole moments to cancel, rendering it non-polar. The dipole moment helps us ascertain the direction of molecular polarity.

VSEPR Theory

The Valence Shell Electron Pair Repulsion (VSEPR) theory is a model used to predict the geometry of individual molecules. According to VSEPR theory, electron pairs around a central atom will arrange themselves as far apart as possible to minimize repulsive forces. This theory is instrumental in determining the molecular shapes that influence polarity.
For example, using VSEPR theory, we determine that \(\mathrm{CO}_{2}\) adopts a linear configuration which leads to the cancellation of its dipoles, resulting in a non-polar molecule. In contrast, \(\mathrm{CH}_{3}\mathrm{Cl}\) forms a tetrahedral geometry, promoting an overall dipole moment because the arrangement of its bonds doesn't allow for dipole cancellation.

• Linear geometries often result in non-polar molecules if bonds are identical.
• Tetrahedral geometries can result in polar molecules if bond types vary.
• Trigonal planar shapes might be polar or non-polar depending on symmetry.

VSEPR theory thus assists us in visualizing the structures that dictate molecular polarity.