Question

Explain the difference between a temporary dipole moment and a permanent dipole moment.

Short answer

Permanent dipoles are constant and due to charge separation in polar molecules, while temporary dipoles are transient and occur due to fluctuating electron density.

Step-by-step solution

Understand Permanent Dipole Moments

A permanent dipole moment occurs when there is a consistent separation of charges within a molecule, resulting in a molecule with one end that is partially positive and the other end partially negative. This is due to differences in electronegativity between atoms, where more electronegative atoms attract electrons more strongly. For example, in a water molecule (H₂O), the oxygen atom is more electronegative than the hydrogen atoms, resulting in a permanent dipole moment.

Define Temporary Dipole Moments

Temporary dipole moments arise in non-polar molecules when there is an accidental distribution of electron density, due to electron movement, creating a momentary uneven distribution of charges. This induces a transient dipole, which can induce a similar dipole in neighboring molecules, leading to London dispersion forces. These dipoles are short-lived and fluctuate as electrons move.

Compare and Contrast Characteristics

Permanent dipole moments lead to permanent polar interactions, such as hydrogen bonding in polar molecules, affecting boiling and melting points significantly. Temporary dipole moments are weaker and result in transient interactions, typically influencing non-polar molecules through temporary attractions. Unlike permanent dipoles, temporary dipoles are not dependent on molecular characteristics but rather on electron movement.

Key concepts

Permanent Dipole

A permanent dipole occurs within molecules where charge separation is consistently present. This is due to the difference in electronegativity among atoms within the molecule. When one atom is more electronegative, it attracts electrons more strongly, resulting in a partially negative area, while the less electronegative atom assumes a partially positive charge.
A classic example of a permanent dipole is seen in water (H₂O). In this molecule, the oxygen atom is more electronegative compared to the hydrogen atoms. Consequently, electrons are drawn towards the oxygen, giving rise to a permanent dipole moment. This consistent difference in charge creates permanent polar interactions among such molecules.

• Permanent dipoles are central in defining the properties of polar molecules.
• They play a significant role in phenomena such as hydrogen bonding, affecting physical properties like boiling and melting points.

Temporary Dipole

Temporary dipoles are quite different from their permanent counterparts. They occur briefly in non-polar molecules due to the random movement of electrons. As electrons shift, they may create a momentary uneven distribution of charge, resulting in a transient dipole. This localized dipole moment can temporarily induce a similar dipole in neighboring molecules.
This phenomenon is common in non-polar substances where the electron cloud is in constant motion. A temporary dipole leads to weak, fleeting attractions between molecules, unlike the stronger permanent dipole interactions.

• Temporary dipoles are crucial for understanding how even non-polar molecules can exhibit some form of attraction.
• These dipoles fluctuate quickly because they depend solely on incessant electron movement.

Electronegativity

Electronegativity is a key concept to understand when discussing dipoles. It refers to the tendency of an atom to attract electrons towards itself within a chemical bond. This fundamental property is what leads to the development of permanent dipoles in polar molecules.
The diverse electronegativity values of atoms dictate how electrons are distributed within a molecule. Atoms with higher electronegativity, like oxygen and fluorine, pull electrons more strongly, giving rise to partial negative charges and indirectly creating areas of partial positive charge where electrons are less concentrated.

• Electronegativity differences are the driving force behind the creation of dipoles.
• The concept explains why some atoms can dominate electrons, shaping the molecule's polarity and interactions.

London Dispersion Forces

London dispersion forces are critical to understanding the behavior of temporary dipoles. These forces are the result of fluctuations in electron density that lead to temporary dipoles in atoms and molecules. Even in non-polar entities, these weak intermolecular forces can cause an attraction between molecules.
The concept of London dispersion forces helps elucidate why non-polar substances can occasionally have measurable attractions. These forces are named after Fritz London, who first described them as a type of van der Waals force that occurs due to the continual motion of electrons.

• London dispersion forces are a universal interaction present in all molecules, polar or non-polar.
• They are especially significant in large atoms or molecules, where electron clouds are more likely to polarize.