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London forces are weak intermolecular forces that attract or repel atoms or molecules. They occur when instantaneous dipoles form due to the bulk movement of electrons. They affect both non-polar and polar molecules and can affect the physical state of a chemical compound. They are a type of van der Waals force and are the only intermolecular forces acting between non-polar molecules or atoms. The strength of the London forces between molecules is determined by the shape and number of electrons in each molecule. The physical characteristics of chemicals can be profoundly affected by the strength of dispersion forces.
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 form, which occurs when a separation of positive and negative charge across a molecule is created by the bulk movement of electrons. London forces occur in both non-polar and polar molecules and can affect the physical state of a chemical compound.
A dipole exists when one part of the molecule 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 irregularity in the distribution of electrons throughout their structures. Instantaneous or temporary dipoles can also form in non-polar molecules. This type of dipole is created when electrons aggregate, creating a net negative charge in the area of greatest electron density and leaving the empty area with a net positive charge.
The forces acting between molecules with dipoles are collectively known as the van der Waals forces. The London forces are a type of Van der Waals force. When molecules with instantaneous dipoles approach each other, areas of like charge repel and those of opposite charge attract. The temporary dipole of one molecule can also shape the distribution of electrons of another molecule into an induced dipole through electrostatic force.
The London forces are the only intermolecular forces acting between non-polar molecules or atoms. Chlorine, bromine, and carbon dioxide are all examples of molecules whose interactions are shaped by these forces. In polar molecules, London forces can act in addition to other van der Waals forces, but their overall effect is minimal.
The strength of the London forces between molecules is determined by the shape and number of electrons in each molecule. Those with elongated shapes can experience greater charge separation, creating stronger London forces. Larger molecules with more electrons also tend to have stronger London forces than smaller ones, since the greater number of electrons allows for a greater charge potential difference across the molecule.
The 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 exactly the same number and type of atoms, is a liquid. The difference is due to the molecular shape. Although both compounds are non-polar, n-pentane molecules have an elongated shape which gives them stronger London forces and a greater ability to make contact. Similarly, it is easier for bromine to form a liquid than for chlorine, because bromine, being the largest molecule, has stronger London forces than chlorine.
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