Molecular Polarity (Simplified): Study with Video Lessons, Practice Problems & Examples

The polarity of chemical bonds is determined by the unequal sharing of electrons, influenced by electronegativity. Nonpolar molecules, such as hydrocarbons, have a perfect shape with no lone pairs on the central atom and identical surrounding atoms. In contrast, polar molecules lack this symmetry, often due to lone pairs or differing surrounding atoms. Understanding molecular geometry, including shapes like tetrahedral and trigonal planar, is crucial for identifying molecular polarity, which affects properties like solubility and reactivity in chemical reactions.

Molecular Polarity (Simplified) Concept 1

Recall that the polarity of chemical bonds arises from unequal sharing of electrons between atoms based on electronegativity. When we discuss molecular polarity, we are referring to polarity that arises from an entire covalent molecule. In this context, we differentiate between nonpolar and polar molecules. Nonpolar molecules are any hydrocarbon, meaning compounds composed of only carbons and hydrogens, and any non-hydrocarbon with a perfect shape. A compound has a perfect shape when the central element has 0 lone pairs and the same surrounding elements. If either criterion is not met, then the molecule is classified as a polar molecule. Hence, any Lewis dot structure that doesn't have a perfect shape falls into this category. For example, consider molecules with 2 to 4 electron groups. In the first column, all these shapes have 0 lone pairs on the central element, and it's assumed that all the surrounding elements are identical. In these cases, all these molecules would be nonpolar. Once we start considering a central element with 1 lone pair, 2 lone pairs, etc., these are classified as polar molecules due to their imperfection in shape. Therefore, to be a perfect shape, the central element must have 0 lone pairs, and the surrounding elements must be identical.

Molecular Polarity (Simplified) Example 1

Determine if carbon tetrachloride (CCl₄) is polar or nonpolar. Alright. So we have carbon which is in group 4A, and chlorine which is in group 7A, and there are 4 of them, giving us 32 valence electrons total. Carbon will go in the center. It will form single bonds with the chlorines. Remember the surrounding elements have to follow the octet rule, so we put enough electrons around them to do that. And that takes care of our 32 valence electrons. Now we're going to say here that we have a molecule basically that we've drawn that has 4 bonding groups. Remember, bonding groups are just your surrounding elements, and it has 0 lone pairs. Here, our central element has no lone pairs, and we have the same surrounding elements. So this is a perfect shape. Since it's a perfect shape, that means the molecule will be nonpolar.

Determine if the compound of BCl₂F is polar or nonpolar.

• A. polar
• B. nonpolar

Determine if phosphorus trihydride, PH₃, is polar or nonpolar.

• A. polar
• B. nonpolar

Determine if difluorine selenide, F₂Se, is polar or nonpolar.

• A. polar
• B. nonpolar

Determine if carbon dioxide, CO₂, is polar or nonpolar.

• A. polar
• B. nonpolar

Here’s what students ask on this topic:

What determines the polarity of a molecule?

The polarity of a molecule is determined by the distribution of electron density across the molecule, which is influenced by the electronegativity of the atoms involved. If the electrons are shared unequally between atoms, the bond is polar. For a molecule to be nonpolar, it must have a symmetrical shape with no lone pairs on the central atom and identical surrounding atoms. If the central atom has lone pairs or the surrounding atoms are different, the molecule is polar. Molecular geometry, such as tetrahedral or trigonal planar shapes, plays a crucial role in determining this polarity.

How do lone pairs on the central atom affect molecular polarity?

Lone pairs on the central atom significantly affect molecular polarity by disrupting the symmetry of the molecule. When the central atom has lone pairs, the electron distribution becomes uneven, leading to a polar molecule. For example, in a tetrahedral molecule like CH₄, the absence of lone pairs and identical surrounding atoms make it nonpolar. However, if the central atom has lone pairs, as in NH₃ (ammonia), the molecule becomes polar due to the asymmetrical distribution of electron density.

Why are hydrocarbons considered nonpolar molecules?

Hydrocarbons are considered nonpolar molecules because they consist solely of carbon and hydrogen atoms, which have similar electronegativities. This results in an even distribution of electron density across the molecule. Additionally, hydrocarbons typically have symmetrical shapes, such as linear or tetrahedral geometries, which further contribute to their nonpolarity. The lack of lone pairs on the central atoms and the identical nature of the surrounding atoms ensure that hydrocarbons remain nonpolar.

What is the significance of molecular geometry in determining polarity?

Molecular geometry is crucial in determining the polarity of a molecule because it dictates the spatial arrangement of atoms and electron pairs. Shapes like tetrahedral, trigonal planar, and linear can lead to nonpolar molecules if they are symmetrical and have no lone pairs on the central atom. However, if the geometry is distorted by lone pairs or different surrounding atoms, the molecule becomes polar. Understanding molecular geometry helps predict the distribution of electron density and, consequently, the molecule's polarity, which affects its chemical properties and interactions.

How does molecular polarity affect solubility?

Molecular polarity significantly affects solubility because polar molecules tend to dissolve well in polar solvents (like water), while nonpolar molecules dissolve better in nonpolar solvents (like hexane). This is due to the principle 'like dissolves like,' where similar types of intermolecular forces attract each other. Polar molecules have dipole-dipole interactions and hydrogen bonding, which are compatible with the polar nature of water. Nonpolar molecules, lacking these interactions, are more soluble in nonpolar solvents where dispersion forces dominate.