Question

Which of the following are nonpolar molecules, even though they have polar bonds? (a) \(\mathrm{CH}_{2} \mathrm{Cl}_{2}\) (c) \(\mathrm{SCl}_{2}\) (e) \(\mathrm{PBr}_{3}\) (b) \(\mathrm{SiCl}_{4}\) (d) \(\mathrm{ClO}_{2}\)

Short answer

The following molecules are nonpolar: \(\mathrm{CH}_{2} \mathrm{Cl}_{2}\) and \(\mathrm{SiCl}_{4}\).

Step-by-step solution

Identify Molecular Shapes and Polarity

Identify the shapes of each molecule using Valence Shell Electron Pair Repulsion (VSEPR) theory and then determine if these molecules can be nonpolar. Even in molecules with polar bonds, symmetrically arranged polar bonds would lead to a nonpolar molecule.

Analyze Molecule (a) \(\mathrm{CH}_{2} \mathrm{Cl}_{2}\)

The \(\mathrm{CH}_{2} \mathrm{Cl}_{2}\) molecule has a tetrahedral shape which is symmetrical. Due to the symmetry, the polarities of the C-Cl bonds cancel each other making the \(\mathrm{CH}_{2} \mathrm{Cl}_{2}\) molecule nonpolar.

Analyze Molecule (b) \(\mathrm{SiCl}_{4}\)

The \(\mathrm{SiCl}_{4}\) molecule has a tetrahedral shape too, hence, the polarities of the Si-Cl bonds cancel out each other and it is a nonpolar molecule.

Analyze Molecule (c) \(\mathrm{SCl}_{2}\)

The \(\mathrm{SCl}_{2}\) molecule is V-shaped or bent. It is not symmetrical, hence, the molecule is polar.

Analyze Molecule (d) \(\mathrm{ClO}_{2}\)

The \(\mathrm{ClO}_{2}\) molecule is a bent or V-shaped molecule. It's not symmetrical and so the molecule is polar.

Analyze Molecule (e) \(\mathrm{PBr}_{3}\)

The \(\mathrm{PBr}_{3}\) molecule has a trigonal pyramidal shape. It is not symmetrical and this makes the molecule polar.

Key concepts

VSEPR Theory

The Valence Shell Electron Pair Repulsion (VSEPR) theory is a foundational tool in chemistry that predicts the shape of individual molecules based upon the repulsion between the electron pairs in the valence shell of the central atom.

According to VSEPR theory, electron pairs will arrange themselves as far apart as possible to minimize repulsion. This leads to predictable geometries like linear, bent, trigonal planar, tetrahedral, and more, depending on the number of electron pairs surrounding the central atom. It's thus essential for understanding how these shapes affect properties such as polarity.

To apply VSEPR theory, one should:

• Count the number of valence electrons around the central atom.
• Arrange them to minimize repulsion.
• Take lone pairs into account, as they occupy more space and can change the angle between bonding electron pairs, thus altering the molecular shape.

Nonpolar Molecules

Nonpolar molecules are molecules in which the charges are evenly distributed. This can happen when

• the molecule is composed of atoms with similar or identical electronegativities,
• or if the structure is symmetrical, causing any bond polarities to cancel out.

Nonpolarity ensures that no dipole (a positive and negative charge separation) is present in the molecule. Bonding between atoms with no significant difference in electronegativity or in symmetrical arrangements as seen in molecules like \textbf{CH}\(_2\)\textbf{Cl}\(_2\) and \textbf{SiCl}\(_4\) lead to this nonpolar characteristic.

Nonpolar molecules generally have a couple of distinctive behaviors: they tend to be insoluble in water, which is polar, and they will not conduct electricity. These features greatly influence how nonpolar substances are used in real-world applications.

Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of atoms within a molecule. Geometry not only dictates the chemical and physical properties of a molecule but also its reactivity and polarity.

The VSEPR theory serves as a guide for predicting geometries. For example, a linear geometry occurs with two electron groups around the central atom, while tetrahedral geometry occurs with four groups.

Importance of Symmetry in Molecular Geometry

Symmetry is particularly crucial when determining molecular polarity. If a molecule like \textbf{SiCl}\(_4\) exhibits a symmetrical tetrahedral shape, the dipole moments of each bond may cancel out, yielding a nonpolar molecule, despite these bonds being polar in nature.

Bond Polarity

Bond polarity refers to the distribution of electrical charge over the atoms joined by the bond. The difference in electronegativity between bonded atoms leads to unequal sharing of electrons, with the more electronegative atom pulling the shared electrons closer to itself and acquiring a slight negative charge.

The less electronegative atom, in turn, gets a slight positive charge. The resulting dipole moment, a vector quantity, has both magnitude and direction and is what defines bond polarity.

• When two atoms are of equal electronegativity, the bond is nonpolar.
• When of differing electronegativity, the bond is polar, which can be seen in molecules such as water (\textbf{H}\(_2\)\textbf{O}).

However, as mentioned before, a molecule can possess polar bonds and still be nonpolar overall if the molecular geometry allows for the dipole moments to cancel out.