Dispersion force, also called London force, refers to the weak attraction between the instantaneous dipole moments of molecules when they are close to each other. The dispersion force exists between all molecules.
Every molecule exists instantaneous dipole This kind of instantaneous dipole will also induce adjacent molecules to generate instantaneous dipole, so two molecules can attract each other by instantaneous dipole. The force generated by the instantaneous dipole is called dispersion force. The dispersion force is Philips London (Fritz London) quantum mechanics The method proves that, because the theoretical formula derived from quantum mechanics is similar to the light dispersion formula, this effect is called dispersion force, also called London force. [1] Any molecule, due to the constant movement of electrons and the constant vibration of the atomic nucleus, often has an instantaneous relative displacement between the electron cloud and the atomic nucleus, resulting in an instantaneous dipole. Molecules attract each other by instantaneous dipoles, which is called dispersion force. The dispersion force is mainly related to the deformability of molecules. The greater the deformability of molecules, the stronger the dispersion force. It exists between all molecules. [2]
Due to the constant movement of electrons and atomic nuclei in the molecule, the distribution of the molecular electron cloud presents a state of rising and falling, which makes it appear an instantaneous relative displacement with the atomic nucleus, producing an instantaneous dipole, and thus the molecule is deformed. In molecule Number of electrons The more Atomic number The more Atomic radius The larger the molecule, the easier it will deform. Instantaneous dipoles can make another molecule adjacent to them produce an instantaneous induced dipole, and the two instantaneous dipoles are always in a state of heteropolar proximity. The force between the instantaneous dipoles of molecules generated at any time is a dispersion force (because the expression of its action energy is Dispersion of light The formula is similar and named). Although the instantaneous dipole exists for a short time, the neighboring states of the different poles keep on repeating, so there is always a chromatic dispersion force between molecules. Undoubtedly, the dispersion force exists not only between nonpolar molecules, but also between polar molecules and between polar and nonpolar molecules. The dispersion force exists between all molecules. Dispersion force and molecular Deformability The stronger the deformation is, the easier it is to be polarized and the stronger the dispersion force is. Noble gas There is no chemical bond between molecules, but when they are close to each other, they can liquefy and release energy, which is proof of the existence of dispersion force. Quantum mechanical calculations show that the dispersion force is related to the molecular deformability. The greater the deformability, the stronger the dispersion force. Since all kinds of molecules have instantaneous dipoles, the dispersion force exists between polar molecules and polar molecules, polar molecules and non-polar molecules, and non-polar molecules and non-polar molecules. In general, the dispersion force is the main intermolecular force. The orientation force is important only for molecules with relatively strong polarity.
Dispersion force 、 Induced force and Orientation force Collectively referred to as Van der Waals （ Intermolecular force ）。 It is when people study the actual gas ideal gas When the deviation is raised. As it decreases rapidly with the increase of the distance between molecules, it is a short-range force, which shows the attraction of the close distance between molecules, with only a few action ranges Pimi 。 Its action energy ranges from a few to dozens of joules per mole Chemical bond Of Bond energy 1~2 orders of magnitude smaller. Unlike covalent bonds, intermolecular forces have no directionality and saturation. The intermolecular forces include three kinds of forces. Due to different interacting molecules, the proportions of these three forces are different, but the dispersion force is usually the most important. [1] The intermolecular force has the following characteristics: ① The intermolecular force is inversely proportional to the sixth power of the intermolecular distance. Therefore, when the molecules are slightly away, the intermolecular force suddenly weakens. Their operating distance is about 300~500pm. When the molecules maintain a certain contact distance and there is no overlap of electron clouds, the two molecules in contact with each other atom Half of the nuclear spacing is called atomic Vander Waals radius 。 The van der Waals radius of chlorine atom is 180pm, which is more than Covalent half path 99pm is much larger. ② The intermolecular force has no directivity and saturation. ③ The intermolecular force interaction energy is generally 2~20KJ · mol -1 , specific chemical bond energy (100~600kJ · mol -1 ）It is about 1-2 orders of magnitude smaller. Halogen molecule Physical properties can easily be qualitatively explained by intermolecular forces: F two 、Cl two 、Br two 、I two Are non-polar molecules. The order molecular weight increases, the atomic radius increases, and the electrons increase, so the dispersion force increases Deformability Increase, intermolecular force increases. Therefore, the melting and boiling points of halogen molecules increase rapidly in sequence, and F two 、Cl two It's gas, Br two Is liquid, and I two Is a solid. However, HF, H two O、NH two Three hydride The molecular weight of is obviously smaller than that of the corresponding homologous hydride, but their melting and boiling points are abnormally high, which is due to the existence of these molecules hydrogen bond 。 [3] 
