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London forces, also known as London dispersion forces, are weak intermolecular forces that attract or repel atoms or molecules. They are named after Fritz London, a German physicist. These interactions come into play when instantaneous dipoles are formed, which happens when a separation of positive and negative charge across a molecule is created by the mass movement of electrons. London forces occur in both nonpolar and polar molecules and can affect a chemical compound’s physical state.
A dipole exists when part of the molecule is has a net positive charge and another part has a net negative charge. Polar molecules, such as water, have permanent dipoles due to an inherent unevenness in electron distribution across their structures. Instantaneous or temporary dipoles may also form in nonpolar molecules. This type of dipole is created when electrons congregate, creating a net negative charge in the area of greater electron density and leaving the vacated area with a net positive charge.
The forces acting between molecules with dipoles are collectively known as van der Waals forces. London forces are a type of Van der Waals force. When molecules with instantaneous dipoles come close to each other, areas of like charge repel one another and those of opposite charge attract one another. The temporary dipole of one molecule may also shape the electron distribution of another molecule into an induced dipole through electrostatic force.
London forces are the only intermolecular forces acting between molecules or atoms that are nonpolar. Chlorine, bromine, and carbon dioxide are all examples of molecules whose interactions are shaped by these forces. In polar molecules, London forces may act in addition to the other van der Waals forces, but their overall effect is minimal.
The strength of London forces between molecules is determined by the shape and the number of electrons in each molecule. Those with elongated shapes can experience a greater separation of charge, creating stronger London forces. Larger molecules with more electrons also tend to have stronger London forces than smaller ones, since the larger number of electrons allows for a greater potential difference in charge across the molecule.
Physical characteristics of chemicals can be profoundly affected by the strength of dispersion forces. For example, neopentane exists as a gas at room temperature, while n-pentane, another chemical that contains the exactly the same number and types of atoms, is a liquid. The difference is due to molecular shape. Although both compounds are nonpolar, n-pentane molecules have an elongated shape that gives them stronger London forces and a greater ability to make contact. Similarly, it is easier for bromine to form a liquid than it is for chlorine to do so, because bromine, as the larger molecule, has stronger London forces than chlorine does.