A momentary dipole, also known as an instantaneous dipole, is a temporary imbalance in the electron distribution around an atom or molecule. This phenomenon occurs even in non-polar species where electrons are, on average, evenly distributed. The constant, rapid motion of electrons causes them to temporarily cluster on one side of the nucleus. This random, asymmetrical arrangement creates a brief separation of charge, which is fundamental to understanding how matter interacts at the molecular level.
The Mechanism of Momentary Dipole Formation
The formation of a momentary dipole is rooted in the probabilistic nature of electron location, often described as an electron cloud surrounding the atomic nucleus. While quantum mechanics dictates that electrons are not fixed in orbit, they occupy a region of space where they are most likely to be found. This electron cloud is in constant, unpredictable motion, even for an individual, isolated atom.
For a tiny fraction of a second, the random movement of electrons results in a momentary, unequal distribution of charge. If more electrons are located on one side of the atom, that region acquires a slight, temporary negative charge (denoted as delta-minus). This simultaneously leaves the opposite side electron-deficient, resulting in a corresponding partial positive charge (delta-plus).
This fleeting charge separation constitutes the momentary dipole. It exists for an incredibly short duration before the electrons shift again and the charge imbalance disappears, only to reappear randomly in another orientation. This process is a continuous, natural fluctuation of the electron cloud, creating a constantly shifting polarity even in atoms like Helium or molecules like Methane.
The Resulting Force London Dispersion
A momentary dipole influences neighboring particles, leading to the formation of a weak attractive force. When an atom develops a temporary charge separation, its partial negative side can repel the electrons in an adjacent, otherwise neutral molecule. This repulsion distorts the second molecule’s electron cloud, inducing a corresponding, temporary dipole in it.
This process is called “induction,” where the instantaneous dipole in the first molecule creates an induced dipole in the second. The resulting attraction between the partial negative pole of the first molecule and the partial positive pole of the second molecule is the London Dispersion Force (LDF). LDFs are a type of Van der Waals force, representing a weak, short-range attraction that is present between all atoms and molecules.
These rapidly forming and disappearing attractions explain why non-polar substances can exist as liquids or solids. For instance, the weak LDFs between non-polar oxygen molecules are sufficient to allow oxygen to be condensed into a liquid at very low temperatures. Without the constant, fleeting attraction provided by the momentary dipoles, these non-polar substances would not be able to stick together.
Factors Influencing Dipole Strength
The strength of the momentary dipole and the resulting London Dispersion Force is influenced by three molecular characteristics.
Polarizability
The first factor is polarizability, which describes how easily an electron cloud can be distorted by an electric field, such as that created by a neighboring momentary dipole. Larger atoms and molecules possess valence electrons that are farther from the nucleus and thus are more loosely held. This makes them significantly more polarizable.
Molecular Size and Mass
Molecular size and mass correlate directly with the number of electrons present in the particle. Molecules with a greater number of electrons have a larger and more diffuse electron cloud. This increases the probability of a significant temporary charge imbalance occurring. For example, heavier halogen atoms like iodine exhibit much stronger LDFs than lighter halogens like fluorine, leading to iodine being a solid at room temperature while fluorine is a gas.
Molecular Shape
Molecular shape determines the overall strength of these temporary attractions. Molecules with a larger, more elongated surface area can achieve greater contact with neighboring molecules. This increased contact allows for more points of simultaneous, temporary attraction, strengthening the overall LDF. In contrast, molecules that are more compact or spherical, such as neopentane, have a smaller surface area for interaction, which results in weaker LDFs and a lower boiling point compared to its straight-chain isomer, n-pentane.