I was recently learning about intramolecular and intermolecular forces. It seems there are a variety of interactions ( Dipole-Dipole. Ion-Dipole etc ). Why must this be the case? Couldn't all chemical species just interact in the same manner. I feel like this would be more intuitive.
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3$\begingroup$ "Why must this be the case?" — I mean, that's just how it is. A rainbow has 7 colours, we don't go around asking why can't it just have 1. $\endgroup$– orthocresolApr 21 at 10:50
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11$\begingroup$ They all do interact in the same manner. Ion-dipole and other smart names are just labels that we humans put on things. $\endgroup$– Ivan NeretinApr 21 at 10:55
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2$\begingroup$ all the forces involved are the same electromagnetic forces. It is the details that are complex. Nature and quantum mechanics don't have to be simple and the normal classifications used in chemistry are a way to simplify the exceedingly complex nature of quantum scale electromagnetic interactions. $\endgroup$– matt_blackApr 21 at 11:10
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$\begingroup$ Bound displaced charges are more conveniently evaluated as a complex object with internal charge structure than as a set of independent charges. $\endgroup$– PoutnikApr 21 at 13:18
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5$\begingroup$ There aren't different intermolecular forces. But there are many, many levels of approximations and in chemistry it often happens that the approximations are taken at face value due to the way its taught and presented in lectures. All these interactions that you list are part of the multipole expansion of the electromagnetic interaction, which is truncated. The terms retained in your approximation depend on the situation that you want to model. For intermolecular interaction these are typically dipole and point charge terms, since these are the dominant terms. $\endgroup$– Hans WurstApr 21 at 13:30
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2 Answers
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Intermolecular forces are of a diverse variety:
- ion-ion interaction
- ion-dipole interaction
- dipole-dipole interaction
- ion-induced dipole interaction
- dipole-induced dipole interaction
- induced dipole-induced dipole interaction (dispersion/London forces)
Encyclopedia Britannica explains some of them.
But all these forces essentially stem from the same quantum mechanical effect. What makes them distinct from each other is the level of complexity involved in the species that the forces act on. The effect we are seeking is the displacement/distortion of electron cloud of a species.
Take, for example, Argon which is an inert gas but it liquifies at $-185.8$°C which states there is some level of inter-atomic forces acting among atoms to liquify the gas. These forces are the London forces that are caused by the temporary and random distortion of electron clouds within the atoms.
In single atom, the electron cloud is swirling around but this distribution of electron cloud is not perfectly symmetric due to wave-particle duality of electrons and the uncertainty principle. When this symmetry breaks, the electron cloud redistributed in terms of the electron density in the atom. Where the density is higher, there develops a partial negative charge (δ-) in that region whereas the region where the cloud density decreased gets an equally partial positive charge (δ+). Then a nearby atom that was perfectly symmetric would break it's symmetry in response to the temporary polarity generated in the first atom. Where there is (δ-) in the first atom, it will develop a (δ+) in it's vicinity and vice-versa. In this way more and more subsequent atoms are weakly held together over larger distances.
Now, talking about the aforementioned forces these are just variants of the same effect with some complexity of the species involved. But the same principle is acting. Either temporary or permanent redistribution of electron clouds (or electrons in case of ions) within the same species or from one species to the other.
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3$\begingroup$ I think this is a good answer, but quadrupole-quadrupole (e.g., benzene dimer) and other kinds of electrostatics also show up, not to mention hydrogen bonds, chalcogen interactions, sigma holes, … also not all intermolecular interactions require displacement / distortion of the electrostatic distribution - some are just undistorted electrostatics. $\endgroup$ Apr 22 at 1:56
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$\begingroup$ I agree that my answer covers only a few basic interactions because I wanted to give only a basic idea of how it works. There are many left out such as metallic bonding that are bit complex and my knowledge does not well extend to them. I request if anyone could elaborate on those forces, they may add their answer here. $\endgroup$ Apr 22 at 3:08
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Why must this be the case? Couldn't all chemical species just interact in the same manner.
It would be fine to say that intermolecular interactions are all based on how electrons and nuclei (specifically, protons) interact. However, this would be of little use when you are interested in the specific interactions between two species.
Why so many intermolecular forces?
By classifying intermolecular forces into different types, you can get a sense of the strength of interactions (e.g. ionic interactions are stronger than dispersion interactions when comparing a single pairwise interaction). You can also get a sense of how the interactions get weaker with distance (e.g. some have a $\frac{1}{r}$ law, some have a $\frac{1}{r^2}$ law).
You can also classify molecules or moieties by the kind of intermolecular forces they support, and arrive at rules of thumb such as "like dissolves in like".
