London dispersion forces are a type of intermolecular force that occur between atoms and molecules that are normally electrically symmetric. They are caused by the temporary fluctuations of the electron distribution around the nuclei, which create instantaneous dipoles that attract each other. London dispersion forces are the weakest of the intermolecular forces, but they can have a significant effect on the physical properties of substances, such as boiling point, melting point, and viscosity.
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The Effect of Electron Number on London Dispersion Forces
The strength of London dispersion forces depends on several factors, such as the size, shape, and polarizability of the atoms or molecules involved. However, one of the most important factors is the number of electrons in the atoms or molecules. The more electrons an atom or molecule has, the more likely it is to form temporary dipoles, and the stronger the London dispersion forces will be. This is because electrons are more easily displaced by external electric fields when there are more of them.
For example, consider two noble gases: helium and xenon. Helium has 2 electrons, while xenon has 54 electrons. Both gases are non-polar and have spherical shapes, so the only difference between them is the number of electrons. However, this difference has a huge impact on their London dispersion forces and their physical properties. Xenon has much stronger London dispersion forces than helium, which means it has a much higher boiling point (165 K vs 4 K) and a much higher melting point (161 K vs 0.95 K)
The Effect of Electron Number on Molecular Properties
The number of electrons in a molecule also affects its molecular properties, such as its shape, polarity, and bond strength. These properties can in turn affect the strength of London dispersion forces between molecules.
For example, consider two hydrocarbons: methane and hexane. Methane has 10 electrons, while hexane has 50 electrons. Both molecules are non-polar and have tetrahedral shapes, so the only difference between them is the number of electrons. However, this difference has a significant impact on their London dispersion forces and their physical properties. Hexane has much stronger London dispersion forces than methane, which means it has a much higher boiling point (342 K vs 111 K) and a much higher melting point (178 K vs 90 K)
However, not all molecules with the same number of electrons have the same strength of London dispersion forces. For example, consider two isomers of hexane: n-hexane and 2-methylpentane. Both molecules have 50 electrons and are non-polar, but they have different shapes. n-Hexane has a linear shape, while 2-methylpentane has a branched shape. This difference affects their polarizability and their surface area, which in turn affect their London dispersion forces. n-Hexane has a higher polarizability and a larger surface area than 2-methylpentane, which means it has stronger London dispersion forces and a higher boiling point (342 K vs 301 K) and a higher melting point (178 K vs 146 K)
Conclusion
London dispersion forces are influenced by many factors, but one of the most important ones is the number of electrons in the atoms or molecules involved. The more electrons an atom or molecule has, the stronger its London dispersion forces will be. This can have a significant effect on the physical properties of substances, such as boiling point, melting point, and viscosity.
