A dipole represents a separation of electric charge, where one end of a molecule or atom temporarily or permanently carries a slight positive charge and the other a slight negative charge. While some molecules inherently possess this charge separation, others do not. For those without a fixed charge imbalance, the concept of an induced dipole becomes relevant, describing a temporary form of charge separation.
How Induced Dipoles Form
An induced dipole arises when the electron cloud of an atom or molecule, normally evenly distributed, becomes temporarily distorted by a nearby charged particle or polar molecule. The external electric field from this entity attracts or repels electrons, causing them to shift. This shift results in a momentary uneven distribution of electron density, creating a temporary positive and negative region. This phenomenon is short-lived and exists only as long as the external influence persists.
Induced Versus Permanent Dipoles
The distinction between induced and permanent dipoles lies in their origin and duration. A permanent dipole occurs in molecules with an inherent, unequal sharing of electrons due to differences in electronegativity. This creates a fixed positive and negative end, such as in a water molecule where oxygen holds electrons more tightly than hydrogen. In contrast, an induced dipole is a temporary phenomenon, created when an external electric field distorts the electron cloud of a neutral atom or molecule. Induced dipoles do not result from the molecule’s intrinsic structure but from an outside influence.
Factors Affecting Induced Dipoles
The ease with which an electron cloud can be distorted to form an induced dipole is known as polarizability. This property is important for understanding the strength of induced dipoles. Larger atoms and molecules are more polarizable because their valence electrons are, on average, farther from the nucleus and less tightly held. This makes their electron clouds more susceptible to distortion by an external electric field. Consequently, molecules with more electrons or a larger overall size exhibit higher polarizability, leading to stronger induced dipoles.
Induced Dipoles and Molecular Interactions
Induced dipoles are key to understanding London Dispersion Forces, the weakest type of intermolecular force present between all molecules, including nonpolar ones. These forces arise from the constant motion of electrons, which can momentarily create an uneven distribution, forming an instantaneous dipole. This temporary dipole then induces a corresponding dipole in a neighboring atom or molecule, leading to a weak, temporary attraction. While individually weak, these interactions become significant in larger molecules and allow nonpolar substances, like noble gases or hydrocarbons, to condense into liquids and freeze into solids at lower temperatures.