Why Do Atoms Seek an Octet in Their Valence Shell?

[mesa]

So what causes an atom to take on an additional electron to fill an octet? Is it a case where the proton from the donor atom is closer to the electron than the proton in the atom 'receiving' the electron so there is no real energy needed to 'hold' it in place as charges cancel as maybe somehow shown as an expression using coulombs law?

Is it understood why atoms like to have an octet? I won't even fathom a guess on this one lol!

 

[Borek]

8 is buried in the quantum mechanics - it is a number of electrons on fully filled sp orbitals.

 

[mesa]

8 is buried in the quantum mechanics - it is a number of electrons on fully filled sp orbitals.

Does quntum mechanics have a good way of explaining this phenomena or is it still somewhat a mystery?

 

[DrDu]

I'd guess that it is well understood for about 90 years now. Any decent chemistry book beyond high school level should contain a decent explanation, e.g. Linus Pauling, General Chemistry.

 

[mesa]

I'd guess that it is well understood for about 90 years now. Any decent chemistry book beyond high school level should contain a decent explanation, e.g. Linus Pauling, General Chemistry.

Although I like the chemistry books I have and they explain the observable effects of the octet very well, none of them go into any detail about why it likes to form.

 

[praeclarum]

Here is why. As Borek mentioned, 8 is the number of electrons in the s and p orbitals. Why do atoms "want" to fill these? All atoms in a particular orbital, such as the 3s or 2p orbital, have the same energy, and the lower the number (the "3" in "3s," for example), the lower the energy of those electrons. Also, s electrons are more strongly attracted to the nucleus because they can "penetrate"--there is a higher probability that an s electron will be close to the nucleus than a p electron will be--this all has to do with probability densities.

So, for example, Fluorine wants to fill an octet because the electrons in the outermost 2p orbital are strongly attracted to the nucleus and it has an unpaired electron, meaning there is room for one more. Potassium, on the other hand, wants to "give up" an electron (or, rather, other atoms want its outermost electron more) because it only has one electron in the 4th energy level, which is not strongly attracted to the nucleus because there are so many electrons closer to the nucleus that repel the outermost electron--the outermost electron is much higher energy than all the others, and it does not take much energy to strip the electron from the atom.

 

[Amok]

Eight electron rule is a good a rule of thumb for making qualitative predictions (and it works like a charm for quite a few elements). Have never seen any straightforward QM explanation for it and don't think there's one since you can't solve the SE for anything more than monoelectronic atoms and molecules. I think you could simply say that the ground state energy of 8 electron atoms is particularly low compared to other atomic systems. I'm interested to know if anyone else has a good explanation for it.

 

[mesa]

Here is why. As Borek mentioned, 8 is the number of electrons in the s and p orbitals. Why do atoms "want" to fill these? All atoms in a particular orbital, such as the 3s or 2p orbital, have the same energy, and the lower the number (the "3" in "3s," for example), the lower the energy of those electrons. Also, s electrons are more strongly attracted to the nucleus because they can "penetrate"--there is a higher probability that an s electron will be close to the nucleus than a p electron will be--this all has to do with probability densities.

So, for example, Fluorine wants to fill an octet because the electrons in the outermost 2p orbital are strongly attracted to the nucleus and it has an unpaired electron, meaning there is room for one more. Potassium, on the other hand, wants to "give up" an electron (or, rather, other atoms want its outermost electron more) because it only has one electron in the 4th energy level, which is not strongly attracted to the nucleus because there are so many electrons closer to the nucleus that repel the outermost electron--the outermost electron is much higher energy than all the others, and it does not take much energy to strip the electron from the atom.

I understand this, but what I am curious about is why does an atom want '8' electrons in it's outermost shell (when not dealing with the first two electron shells)? And do we have a good understanding of it in QM?

 

[Mike H]

My friends who are organometallic chemists would question the notion that an atom wants 8 electrons in its valence shell. They would point to the existence of the "18-electron rule", which is pretty much standard mention in most organometallic chemistry texts. As alluded to earlier in this thread, the "octet rule" is more a guideline for atoms in certain periods than an actual rule that applies across the entire periodic table. And we all know that helium seems to be OK with only two electrons.

Since the anthropomorphization of atoms has already begun, I will continue in the same vein. An atom wants to be in as deep an energy minimum as it can find, where it won't want to give up or accept an electron. If the atom has a closed shell, it is not as likely to do either. Because if it's more energetically favorable to bond with another atom, your (for sake of argument) open-shell atom of interest will do so eventually.

Having said that, there are some philosophers of science who claim that the periodic table isn't rigorously justified by QM (mostly thinking Eric Scerri here, at least the last time I checked). Most of us who are actually chemists don't lose that much sleep over it.