Question

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

Short answer

(a) \(\mathrm{IF}_{3}\) is a polar molecule due to its T-shaped geometry and polar I-F bonds. (b) \(\mathrm{CS}_{2}\) is a nonpolar molecule because of its linear geometry and the cancellation of polar C-S bond dipoles. (c) \(\mathrm{SO}_{3}\) is nonpolar because of its trigonal planar geometry and symmetrical arrangement of polar S-O bonds. (d) \(\mathrm{PCl}_{3}\) is polar due to its trigonal pyramidal geometry and polar P-Cl bonds. (e) \(\mathrm{SF}_{6}\) is nonpolar because its octahedral geometry cancels the polar S-F bond dipoles. (f) \(\mathrm{IF}_{5}\) is polar due to its square pyramidal geometry and polar I-F bonds.

Step-by-step solution

Determine Lewis structure

To draw the Lewis structure, identify the valence electrons for both iodine (7) and fluorine (7). In this case, we have 1 iodine and 3 fluorine atoms, giving us 28 valence electrons. The Lewis structure for \(\mathrm{IF}_{3}\) involves placing iodine in the center with three fluorine atoms bonded to it and two lone pairs on the central iodine atom.

Molecular geometry

Based on VSEPR theory, the molecular geometry of \(\mathrm{IF}_{3}\) with 5 electron groups (3 bonded fluorine atoms and 2 lone pairs) around the central iodine atom is T-shaped.

Determine polarity

Due to the presence of polar bonds between iodine and fluorine and the asymmetric T-shaped geometry, \(\mathrm{IF}_{3}\) is a polar molecule. (b) \(\mathrm{CS}_{2}\)

Determine Lewis structure

For \(\mathrm{CS}_{2}\), carbon has 4 valence electrons and sulfur has 6. Since there are 2 sulfur atoms, the total number of valence electrons is 16. The Lewis structure shows the carbon atom in the center, double-bonded to each sulfur atom.

Molecular geometry

The molecular geometry of \(\mathrm{CS}_{2}\) is linear, with the three atoms aligned in a straight line.

Determine polarity

Although the C-S bonds are somewhat polar due to the difference in electronegativity, the linear geometry results in the cancellation of dipole moments. Therefore, \(\mathrm{CS}_{2}\) is a nonpolar molecule. (c) \(\mathrm{SO}_{3}\)

Determine Lewis structure

The central sulfur atom in \(\mathrm{SO}_{3}\) has 6 valence electrons, and each oxygen atom has 6 valence electrons. With 3 oxygen atoms, the total number of valence electrons is 24. The Lewis structure features a sulfur atom double-bonded to each oxygen atom.

Molecular geometry

Since there are 3 electron groups around the central sulfur atom, the molecular geometry of \(\mathrm{SO}_{3}\) is trigonal planar.

Determine polarity

The polar S-O bonds are symmetrically arranged, and their dipole moments cancel in the trigonal planar geometry. Thus, \(\mathrm{SO}_{3}\) is a nonpolar molecule. (d) \(\mathrm{PCl}_{3}\)

Determine Lewis structure

Phosphorus has 5 valence electrons, and each chlorine atom has 7 valence electrons. There are 3 chlorine atoms in the molecule, so the total number of valence electrons is 26. The Lewis structure has a central phosphorus atom single-bonded to each chlorine atom.

Molecular geometry

With 3 bonded chlorine atoms and 1 lone pair around the central phosphorus, the molecular geometry of \(\mathrm{PCl}_{3}\) is trigonal pyramidal.

Determine polarity

Due to the polar P-Cl bonds and the asymmetric trigonal pyramidal geometry, \(\mathrm{PCl}_{3}\) is a polar molecule. (e) \(\mathrm{SF}_{6}\)

Determine Lewis structure

Sulfur has 6 valence electrons, and each fluorine atom has 7 valence electrons. With 6 fluorine atoms, the total number of valence electrons is 48. The Lewis structure has a central sulfur atom single-bonded to each fluorine atom.

Molecular geometry

With 6 electron groups around the central sulfur atom, the molecular geometry of \(\mathrm{SF}_{6}\) is octahedral.

Determine polarity

Although the S-F bonds are polar, the symmetric octahedral geometry leads to the cancellation of the bond dipoles. Therefore, \(\mathrm{SF}_{6}\) is a nonpolar molecule. (f) \(\mathrm{IF}_{5}\)

Determine Lewis structure

Iodine has 7 valence electrons, and each fluorine atom has 7 valence electrons. With 5 fluorine atoms, there are 42 total valence electrons. The Lewis structure has a central iodine atom single-bonded to each fluorine atom, with one lone pair on the central iodine.

Molecular geometry

With 5 bonded fluorine atoms and 1 lone pair around the central iodine, the molecular geometry of \(\mathrm{IF}_{5}\) is square pyramidal.

Determine polarity

Due to the polar I-F bonds and the asymmetric square pyramidal geometry, \(\mathrm{IF}_{5}\) is a polar molecule.

Key concepts

Lewis Structure

The Lewis structure is a vital tool for understanding the layout of atoms within a molecule. It provides a visual representation that allows us to predict the arrangement of valence electrons, which includes both bonding electrons and lone pairs. To draw a Lewis structure, first determine the total number of valence electrons in the molecule. For example, in \( \mathrm{IF}_{3} \), iodine has 7 valence electrons, and each fluorine also has 7, totaling 28 because there are three fluorine atoms involved. The structure shows iodine at the center connected to the fluorine atoms, with the remaining electrons forming lone pairs on iodine. By correctly drawing the Lewis structure, you can also infer information about potential molecular shapes and reactivity. This is because the arrangement of electrons influences molecular behavior drastically, including polarity and how the molecule might interact with others.

VSEPR Theory

VSEPR (Valence Shell Electron Pair Repulsion) theory explains how the shape of a molecule is determined by the repulsions between electron groups (bonding and lone pairs) in its valence shell. According to VSEPR theory, these electron groups will position themselves as far apart as possible to minimize repulsion and achieve the molecule's most stable geometry.
For instance, in \( \mathrm{IF}_{3} \), the presence of 3 bonded atoms and 2 lone pairs leads to a T-shaped molecular geometry. This results from five electron groups repelling each other to find the most stable arrangement. VSEPR theory is crucial for predicting molecular shapes, which is directly related to understanding molecular polarity and other chemical properties. Different molecular geometries result in different molecular polarities, impacting how molecules interact in chemical reactions and physical processes.

Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of atoms within a molecule. It's determined by the number and arrangement of bonding pairs and lone pairs around the central atom, and it's crucial for predicting physical and chemical properties.
For example, the geometry of \( \mathrm{CS}_{2} \) is linear, with carbon at the center double-bonded to two sulfur atoms. This results in a nonpolar molecule, despite the polar nature of individual C-S bonds, because the linear geometry allows the dipoles to cancel out. In contrast, a molecule like \( \mathrm{PCl}_{3} \) has a trigonal pyramidal shape due to a lone pair on the central phosphorus, leading to a polar molecule.
Understanding molecular geometry aids in predicting not only polarity but also reactivity and potential interactions with other molecules, which is important for various fields such as chemistry and material science.

Valence Electrons

Valence electrons are the outermost electrons of an atom and play a key role in chemical bonding and molecule formation. These electrons are involved in forming bonds between atoms, creating the foundational structure of molecules. Each element has a specific number of valence electrons that can be determined from its group number in the periodic table.
In practice, the number of valence electrons directly influences the shape and polarity of the compound. For example, sulfur in \( \mathrm{SF}_{6} \) has six valence electrons, allowing it to form six bonds with fluorine atoms in an octahedral shape. This specific arrangement leads to a nonpolar molecule, despite the polar nature of the S-F bond, because the geometry allows the dipoles to cancel each other out due to symmetry. By understanding valence electrons, we can better predict molecular structures and interpret their behavior in different chemical environments.