[infinitex2]

Question to all chemists.  I have looked and could not find the answer to this hard question.  My Chem teacher answered very vaguely.  WHY do elements or atoms strive for that completed valence shell?  Why is 8 valence electrons worth losing some electrons or gaining some?  I know that atoms do tend to do this but my question is why.  If your answer is, "Because it is more stable with 8 valence electrons", then I ask why is stable what they "want"?  Please anyone answer my question.  Try to be direct about it.  If this is a "just because" answer than tell me.  Thanks!

 :-\Infinite x 2

[Mitch]

Can you think of a compound that doesn't have 8 electrons in the valence shell?

[Yggdrasil]

It basically has to do with potential energy.  Consider a ball on the top of a hill.  Because the ball will roll down the hill but not up the hill, one could say that the ball "wants" to move downward.  Now consider two hills next to eachother.  The ball would move down the hill, but it would not move up the second hill.  In this case, the ball "wants" to occupy a position between the two hills.

Now we all know the reason that the ball wants to occupy these positions is because of gravity.  The gravitational potential energy of the ball is lower when the ball is at a lower position than when it is at a higher position.  The same applies to atoms.  A configuration with 8 valence electrons is more stable because it is a state of lower potential energy.  As for why it has a lower potential energy it has to do with reducing the number of unpaired electrons and lowering something known as degeneracy.

[Mitch]

Its not like these atoms exist in a state that doesn't have 8 electrons naturally. I want to know if anyone has any examples?

[Yggdrasil]

The chlorine radical has 7 valence electrons.  Borane and boron trifluoride have 6 electrons on the central boron atom.

[Mitch]

The chlorine radical doesn't exist in nature as a radical form. Borane does have 8 effective electrons around it if you look at the crystal structure.

[wereworm73]

Simply put, a nice balance is finally established when the electron orbitals are all evenly filled (or emptied.  The octet rule involves the s-orbital & three p-orbitals, which can together hold a total of 8 electrons.     

[tamim83]

Its not like these atoms exist in a state that doesn't have 8 electrons naturally. I want to know if anyone has any examples?

Well, let's go back to chlorine.  Chlorine exists as a diatomic molecule (Cl₂), that is two chlorine atoms are covalently bonded to one another so that each atom has its octet filled.  If you think about it, most elements do not exist in "elemental" form or single atoms.  They form compounds because compounds the electron configurations of the compounds are at a lower, more stable potential energy. 

[Donaldson Tan]

Let me break down the your BIG question into a few parts.

a) why is a full shell considered stable?
b) why is adding electrons to the valence shell preferred over removing electrons to achieve stable configuration in some cases?
c) why is removing electrons from the valence shell preferred over adding electrons to achieve the stable configuration in some cases?
d) why are there some chemical compounds which don't have a full valence shell? eg. radicals

[Will]

Can you think of a compound that doesn't have 8 electrons in the valence shell?

Does persistant carbenes count? Like bis(diisopropylamino)phosphino[trimethylsilylcarbene], a red liquid.

[Mitch]

I didn't know carbenes could be kept at room temperature and in our oxidizing atmosphere. If so, then you found one, but you found a very esoteric one.

[Dan]

Can you think of a compound that doesn't have 8 electrons in the valence shell?

iron oxide