Question

What is the relationship between the dipole moment and bond moment? How is it possible for a molecule to have bond moments and yet be nonpolar?

Short answer

The dipole moment is the resultant of all the bond moments in a particular direction and depends on the bond length and the bond angle. A molecule can have bond moments but still be nonpolar when the bond dipoles are arranged in such a way that they counterbalance each other resulting in a net dipole moment of zero.

Step-by-step solution

Understand The Dipole Moment

A dipole moment is a measure of the polarity of a molecule. It is calculated as the product of the charge's magnitude times the distance between the centers of the positive and negative charges. Mathematically, \( \mu = q \times d \), where \( \mu \) is the dipole moment, \( q \) is the charge, and \( d \) is the distance between charges.

Understand The Bond Moment

Bond moment, also known as bond dipole moment, involves two atoms connected by a bond. This arises due to the difference in electronegativity between the two atoms. The atom with higher electronegativity attracts the shared pair of electron which induces polarity in the bond.

Explaining The Relationship between Dipole Moment and Bond Moment

In a molecule, the resultant of all bond moments in a particular direction is considered the dipole moment. It is the vector sum of all individual bond moments considering their direction and magnitude, and also depends on the molecular structure. To calculate the dipole moment, the bond length, the bond angle, and the individual bond dipole moments should be known. If all of these are given, we can find the net dipole moment by using vector addition.

Explain How a Molecule Can Have Bond Moments and Yet Be Nonpolar

Even though a molecule contains polar bonds, it can still be nonpolar if the dipole moments of the bonds cancel each other out due to molecular symmetry. In nonpolar molecules, the bond dipoles are arranged in such a way that they orient in opposite directions and result in a net dipole moment of zero. For instance, in carbon dioxide (CO2), though the C=O bonds are polar, the molecule is linear and the dipole moments from each side cancel each other, making the molecule nonpolar.

Key concepts

Bond Moment

The concept of bond moment is pivotal in understanding the polarity between two atoms bonded together. Each bond moment, sometimes referred to as a bond dipole moment, is created due to the difference in electronegativity between the two atoms forming a bond. One atom will generally have a stronger pull on the electrons shared in the bond, creating a partial negative charge, while the other atom becomes partially positive. This difference in charge creates a dipole, represented as an arrow pointing towards the more electronegative atom.

Some key points to consider about bond moments:

• The bond moment is a vector quantity, meaning it has both magnitude and direction.
• It is affected by both the difference in electronegativity and the distance between the charges.
• Bond moments contribute to the overall dipole moment of the molecule but can vary depending on the molecular structure.

Understanding how bond moments accumulate and interact within a molecule helps in predicting the molecule’s dipole moment.

Polarity

Polarity in molecules stems from the presence of bond moments and is an essential feature that influences the molecule's properties. A molecule is considered polar when it has an uneven distribution of charge, owing to bond dipoles that don't cancel each other out.

Important aspects of molecular polarity include:

• Polarity results in a molecule having a net dipole moment, which affects its interaction with other molecules.
• Polar molecules tend to have higher boiling and melting points due to strong intermolecular forces like hydrogen bonding or dipole-dipole interactions.
• Solubility in water is often higher for polar molecules compared to nonpolar molecules.

It's crucial to ascertain whether the bond moments add up or cancel out when determining a molecule’s overall polarity. For example, water is polar because its O-H bond dipoles are oriented in a way that they reinforce each other.

Molecular Symmetry

Molecular symmetry plays a significant role in determining the polarity of a molecule, despite the presence of polar bonds. When a molecule is symmetrical, the bond dipoles can cancel each other due to their orientation, leading to a nonpolar molecule.

Considerations of molecular symmetry include:

• A symmetrical molecule can have polar bonds but still end up being nonpolar by achieving a net dipole moment of zero.
• Linear, tetrahedral, and square planar orientations are common structures where symmetry might lead to nonpolarity.
• The spatial arrangement determines how the bond dipoles interact and whether they cancel out.

For instance, carbon dioxide (CO₂) is a classic example where the molecule is symmetrical. The linear configuration allows the opposing bond dipoles to cancel each other, rendering the molecule nonpolar, despite the C=O bonds themselves being polar due to differences in electronegativity.