[Bionic Person]

I've been trying to understand this thing for awhile. Why is the octet rule, doublet rule for hydrogen, obeyed in nature? I read that the atoms are of a lower energy when they take on a noble gas configuration, but how is this so? Can someone explain to me why the octet rule is obeyed? I am quite sick of hearing that it is because atoms really really want to be like a noble gas! That doesn't make any sense. Atoms don't have feelings... or do they?

It seems to me that the net nuclear charge of both atoms may play a role in this. For it would seem to me that a hydrogen atom with two electrons would be very unstable unless it were to share an atomic nucleus with another atom, but this nucleus and nuclear charge would have to be specific in order to accommodate the hydrogen anion.

[Borek]

If you calculate energies involved, noble gas configuration comes out as the most stable one, requiring most energy to either remove or add an electron (as compared with other configurations). That's the way it is - and to be honest, I hate explanations that go into "that's because" as typically they are quite handwavy and just add new unfounded concepts to explain something.

Noble gas configuration has all valence orbitals fully filled, which - in a way - makes them "less messy" than all other configurations.

Think about a polygonal nut. As long as it is polygonal, you can easily remove it with a wrench, but the more round it becomes, the harder it is to rotate. Once the nut is perfectly round it is impossible to use. In a way that's a similar case - you can think of a noble gas atom as being perfectly round, all other atoms being polygonal.

In many ways that's a poor analogy, but I think it shows why the limiting case can be special.

[Bionic Person]

But what exactly attracts them to each other? I suppose it must be the electromagnetic force, but what exactly is causing it? Let's take hydrogen chloride for example. Hydrogen and chlorine are neutral atoms, but there must be some kind of force of attraction between them.

You make a very good analogy, as though they are puzzle pieces that fit together to form a whole. The octet rule was originally explained by the "cubicle atom." A cube has 8 points. When all 8 points are filled, the cube is stable, but this leaves a lot to the imagination.

Is it that there is some kind of opening, for lack of a better word, to the nuclei of each atom which is then closed by the covalent bond? They form a closed-shell configuration.

How do you explain this in terms of physics?

[Borek]

But what exactly attracts them to each other? I suppose it must be the electromagnetic force, but what exactly is causing it? Let's take hydrogen chloride for example. Hydrogen and chlorine are neutral atoms, but there must be some kind of force of attraction between them.

Attraction between isolated atoms would be that of London forces, but it is not this force that matters. When these atoms combine new orbitals are created, and these new orbitals have lower energy - this difference in energy is what keeps atoms in HCl molecule together. These new orbitals can be expressed combining old, atomic orbitals, and they have to be in a way similar - that is, they have to share some of the mathematical properties of the atomic orbitals. One of them is number of electrons on these orbitals - it happens the most stable configuration is again the "round one", using the same number of electrons as the noble gas.

I am not convinced I can explain it better, QM is something I am not even poking with a long stick.

[Corribus]

As the esteemed Borek has already mentioned, no matter how many layers of the onion you peel back, you're always eventually going to have to accept something as an axiomatic truth: it just is what it is. Where you draw that line will depend on a number of factors, including your education level, your specific interests, and the fundamental limits of knowledge.

At a molecular level, all important forces are electrostatic in nature. However, the application of these forces at the quantum level often gives rise to effects that aren't observed in classical physics. For example, a large part of bonding is due to so-called electron exchange, which has no classical analog - it's kind of like the ability of electrons to be in more than one place at the same time. Ultimately, though, the same basic rules apply: electrons create electric fields and magnetic fields that can interact with each other in either favorable or unfavorable ways. When atomic shells are filled, there is an electric field symmetry in place that, to a first order approximation, removes all "handles" for interaction. An unfilled shell has an asymmetry that allows the atom to interact with other nearby charged particles (provides a handle for sticking together). Think of bringing two magnets together. Once two magnets are sticking, the asymmetry is resolved and more magnets will not stick.

Of course this is just first order approximation. There are things like induced dipoles and such that we lump together as van der waals forces, but again: how far do you want to go?

Push too far and you start asking questions like: what is charge and why do they repel/attract? These are questions for theoretical physicists and maybe philosophers.

[Bionic Person]

Hell yea. I tried going that far. Why they repel and attract has something to do with the exchange of photons, I believe. The electromagnetic field is actually composed of light to my understanding. That is cool to think about, but I don't entirely understand it, of course.

I'll continue to search. If it means studying quantum mechanics then so be it. Another question, you do not study quantum mechanics in physical chemistry? I thought this was something chemists had to learn.

[Corribus]

Quantum mechanics is the foundation of modern chemistry, and you will learn quite a bit of it in a physical chemistry course. However the chemists' approach to quantum mechanics is quite a bit different from what you'd learn in a physics class dealing with the same basic subject matter.

[Bionic Person]

Hmm... very interesting. My desire is to be in a cross-field of some sort I think. Kind of like Gilbert Lewis or Marie Curie, you know? Physicists and chemists. Spectroscopy is something that really amazes me, for an example of a subject where physics and chemistry converge.

This might be off topic, but I was reading an article about quantum biochemistry. It sounds ridiculous, but it's actually really interesting and may even be a developing field. I think at these intersections of multiple disciplines are where really exciting phenomena can be observed.

As for the octet/doublet rule. I actually have somewhat of a better understanding of it. At my level, I don't think I will be capable of fully grasping it, so I'll come back to it later. So, thanks guys for the help.