Question

Predict whether each of the following molecules is polar or nonpolar: (a) IF, (b) \(\mathrm{CS}_{2}\), (c) \(\mathrm{SO}_{3}\), (d) \(\mathrm{PCl}_{3}\), (e) \(\mathrm{SF}_{6}\), (f) \(\mathbb{F}_{5}\).

Short answer

(a) IF: polar (b) CS2: nonpolar (c) SO3: nonpolar (d) PCl3: polar (e) SF6: nonpolar (f) PF5: nonpolar

Step-by-step solution

(a) IF

In the case of IF (iodine monofluoride), the molecule consists of one iodine atom and one fluorine atom. Since these two atoms have different electronegativities (iodine: 2.66 and fluorine: 3.98), there is an electronegativity difference, thus creating a net dipole moment. The molecular geometry is also linear, which doesn't cancel out the dipole moment. Therefore, IF is a polar molecule.

(b) CS2

Carbon disulfide (CS2) consists of one central carbon atom and two sulfur atoms. The molecular geometry of CS2 is linear, with sulfur atoms at opposite ends of the central carbon atom. As both sulfur atoms have the same electronegativity (2.58), no net dipole moment is present in the molecule. Therefore, CS2 is a nonpolar molecule.

(c) SO3

Sulfur trioxide (SO3) consists of one central sulfur atom and three oxygen atoms. The molecular geometry of SO3 is trigonal planar. The oxygen atoms are equidistant from the central sulfur atom, forming a symmetric distribution of electron pairs. As a result, the dipole moments of individual bonds cancel each other out, leading to, no net dipole moment. Therefore, SO3 is a nonpolar molecule.

(d) PCl3

Phosphorus trichloride (PCl3) is formed by a central phosphorus atom and three chlorine atoms. The molecular geometry of PCl3 is trigonal pyramidal. Due to the difference in electronegativities between phosphorus (2.19) and chlorine (3.16), a net dipole moment exists. Since the molecular geometry doesn't cancel out the dipole moment, PCl3 is a polar molecule.

(e) SF6

Sulfur hexafluoride (SF6) has one central sulfur atom bonded with six fluorine atoms. The molecular geometry of SF6 is octahedral, exhibiting a symmetric distribution of electron pairs. Despite the electronegativity difference between sulfur (2.58) and fluorine (3.98), the symmetry in the molecule causes the dipole moments of individual bonds to cancel each other out, resulting in no net dipole moment. Therefore, SF6 is a nonpolar molecule.

(f) PF5

In the case of phosphorus pentafluoride (PF5), one central phosphorus atom is bonded to five fluorine atoms. The molecular geometry of PF5 is trigonal bipyramidal. Due to its symmetric distribution of electron pairs, the dipole moments of the individual bonds cancel each other out, resulting in no net dipole moment. Even though there is an electronegativity difference between phosphorus (2.19) and fluorine (3.98), PF5 is a nonpolar molecule.

Key concepts

Electronegativity

Electronegativity is a chemical property that describes how strongly atoms attract electrons in a chemical bond. It is a key factor in determining the polarity of a molecule. A larger difference in electronegativity values between atoms in a molecule suggests that the electrons will spend more time around the atom with the higher electronegativity, creating an imbalance in the electron distribution. This can lead to the formation of a dipole, with one end of the molecule carrying a partial negative charge and the other end carrying a partial positive charge.

For example, in the molecule IF (iodine monofluoride), iodine has an electronegativity of 2.66, while fluorine has a substantially higher value of 3.98. This significant difference means that the fluorine atom will pull the bonding electrons closer to itself, resulting in a molecule with a considerable dipole moment, thus classifying IF as polar.

Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of atoms in a molecule. Understanding geometry is essential for predicting molecular polarity, as the spatial distribution of bonds can dictate whether the dipole moments of individual polar bonds add up or cancel out. Symmetrical molecules (like CO2, CS2, and SF6) tend to be nonpolar because their symmetrical geometric distribution allows for the dipole moments to cancel each other. On the other hand, asymmetrical molecules (such as NH3 and PCl3) have a nonsymmetrical distribution, which can result in a net dipole moment, classifying them as polar.

Take CS2, which features a linear molecular geometry, for example. Both of the bond dipoles are directly opposite to each other and end up canceling out, leading to a nonpolar molecule. However, in PCl3, which has a trigonal pyramidal geometry, the polar bond dipoles do not cancel, leaving a net dipole moment and thus a polar molecule.

Dipole Moment

A dipole moment is a measure of the separation of positive and negative charges in a molecule. It's not merely about having different charges but also how far apart they are and the size of the charge. Molecules with significant electronegativity differences between bonded atoms and an asymmetric geometry will have a net dipole moment, which is the vector sum of individual bond dipoles.

In molecules such as PCl3, the dipole moments arising from P-Cl bonds do not cancel each other due to the trigonal pyramidal shape, resulting in a molecule with a net dipole moment, meaning it is polar. Contrast this with SF6, where the octahedral geometry ensures that any dipole moments from S-F bonds do cancel out, resulting in a nonpolar molecule despite the difference in electronegativity.