# Valence electrons.

1. Jun 23, 2004

### Dual Op Amp

simple question, why do atoms seek to have eight valence electrons?

2. Jun 24, 2004

### Gokul43201

Staff Emeritus
Simple answer, because the solution of the Schrodinger Equation requires it.

3. Jun 24, 2004

### Dual Op Amp

What's the Schrodinger equasion?

4. Jun 30, 2004

### turin

It is, as I understand it, the quasi-wave equation for a little piece of matter (particle) which was the first major attempt to explain the strange quantum nuances that arose in the laboratories across the world at the beginning of the twentieth century. The equation is "good enough" to describe stationary configurations, i.e. your valence electrons, though, in a situation that complicated, there is no technique to exactly solve it that I know of.

5. Jul 1, 2004

the octet rule was known way befoure schroedinger found out his equation. The reason they seek to have 8 valence electrons is becuse this is the most stable arrangement for the atom. If the atom has less than 8, then the atom looses stability and becomes more reactive with other elements and compounds. (ie. Group 1 metals). And by the way, the valence shell can hace more than 8 electrons. It can have 2n^2 electrons.

6. Jul 1, 2004

### turin

A rule is not an explanation of itself. Just because a phsyical phenomenon was known before a particular theory does not mean that the theory is invalid or that the phenomenon itself is a better explanation of itself. In fact, most theories arise to explain phenomena that are already well known. The purpose of a theory is not to explain only one particular phenomenon, but to relate different phenomena that did not have an apparent relationship before the theory to provide a deeper explanation than an obscure rule.

7. Jul 1, 2004

### NateTG

Because they do.

Theories - such as physics - only predict behaviors, they do not justify them.

That said, it's probably possible to derive the behavior from other theories.

8. Jul 1, 2004

### ZapperZ

Staff Emeritus
Ok folks. I'm just surprised why people are not questioning this assertion in the first place. Where in physics and chemistry does it say that "atoms seek to have eight valence electrons"???

I have a He atom. Where are the "8 valenece electrons" here?

What about atoms with the p-orbital being the valence shell? What about the ones with the f-orbital? And what about the half-filled d-orbitals? Aren't all those stable atoms too? So what is this "rule" that atoms have to "seek" 8 valence electrons?

Zz.

9. Jul 1, 2004

### turin

If you want to get picky, then you can ask about particular atoms. Of course the 8 electron rule is not completely general. I doubt that the original question was about the number 8 so much as it was about why some specific number (2, 8, 18, etc.).

10. Jul 2, 2004

### ZapperZ

Staff Emeritus
Unfortunately, short of being a psychic, one doesn't know that. It is compounded by the fact that the original question is rather terse, and no references or sources were cited to where this "eight electrons" rule was found.

In fact, the ONLY atom that would want "eight electrons" in its valence shell is an atom with eight protons! This is only one particular atom. [Remember, the question asked about ATOMS, not molecules, not atoms in a solid state configuration, etc... or am I being "picky" again?] So there isn't even any notion of generalities here.

From where I stand, the whole original question is based on a false idea, or at best, extremely vague. So that's why I was puzzled that there was really an active followup with people attempting to answer it without anyone actually pointing out that the whole thing is based on a false premise in the first place.

Zz.

11. Jul 2, 2004

### turin

I don't get it. Are you saying that hydrogen and oxygen are not atoms when they form a water molecule? Yes, I think that is being picky (excessively technical). The word, "atom," comes roughly from the Greek "indivisible." I don't know about anyone else, but when I think of an atom of matter, I think of the nucleas. If that is not technically correct, then shame on me.

Regarding your question about "where in chemistry ..." Well, I didn't take chemistry after high school, but I do remember some kind of dot diagram that we drew in my highschool chemistry class. I specifically remember B, N, O, and C all reacting according to how the number of dots compared to 8. Then, I remember the explanation from the high school chemistry teacher that the atoms want to be like the noble gas atoms. This explanation actually did generalize the octet rule a bit to help explain hydrogen's desire for 2 valence electrons.

Perhaps, strictly classically, you could say that an atom fills its valence shell to the point that it becomes neutral, but if that's all there were to it, then it doesn't seem likely that there would even be chemistry. I am pretty convinced that there is chemistry, and furthermore, I am convinced that the octet rule (2n2) is not fundamental. I believe this is the issue, only because I remember how I felt in that chemistry class when I learned the strange rule. When I learned basic QM, it finally sort of made sense (as much sense as QM can make) in terms of energy eigenstates of the coulomb potential generated by the nucleas. (and yes, I realize it is emmensely more complicated than just a single weighted coulomb potential from the nucleas.)

The reason I like the Schroedinger Eq. explanation better than just stating the octet rule is that, while the octet rule is extremely limited to chemical reactions (and not even all of them), I believe the Schroedinger Eq. to offer an (at least qualitative) explanation for chemical reactions as well as many other things. In that sense, it is more satisfying to me, as a wanna-be physicist, to see how seemingly different natural phenomena are actually just different ways in which a more fundamental natural principle manifests.

Last edited: Jul 2, 2004
12. Jul 2, 2004

### ZapperZ

Staff Emeritus
Original queston: "simple question, why do atoms seek to have eight valence electrons?"

When I read something like that, I take it at FACE VALUE, meaning isolated atoms. If the person meant H2, or O2, then those are molecules. The eigenstates for molecules differ GREATLY than those of atoms. Just look at the hybridizations of the valence orbitals alone! Therefore, you cannot apply what you solved and know for "atoms" to "molecules" and get away with it. You literally have to resolve (if this is even possible without using any approximation) the Hamiltonian.

Moral of the story: when atoms are in close proximity to each other, especially of different species as in molecules and solid structures, atomic orbitals solutions can be easily thrown out of the window. The hybridized orbitals can look NOTHING like the orbitals of individual atoms. Thus, applying what molecules and atoms in solids would do to individual atoms is extremely suspect.

I still want to know where atoms in general SEEK to have 8 electrons in the valence shell. If this is true, we would be seeing atoms like H and He with extra electrons as the lowest energy state, and therefore, they would be common. Are they? I also tend to think this whole discussion is rather moot until the person who originally posted this question explains further.

Zz.

13. Jul 2, 2004

### turin

I will concede with you on this. I don't want to argue.

14. Jul 2, 2004

### Dual Op Amp

I'm talking about the covelent bonding rule. For example the molecule of oxygen shares a double bond with another oxygen atom (O2). The oxygen atom has six valence electrons, and seeks to have eight. The atoms share two electrons, and for a brief moment one of them own all eight valence electrons. This is because one oxygen atom donates an electron, and so does the other. The two electrons orbit around the two atoms. Okay, so the oxygen has six, donates one, and now has five.
When the two electrons orbit around it, it has seven valence electrons. That's not suitable. There's where the double bond come's in. Actually, each oxygen atom donates two valence electrons. So, it now has four valence electrons. The four valence electrons orbit the oxygen atoms. For a very brief moment one atom has it's eight valence electrons. This is proven in every almost every molecule. Water, methane, ozone, oxygen. If you calculate the valence electrons, it always adds to eight. My question was why does it seek to have eight. I didn't mean to incite anything, I'm sixteen years old and interested.

Last edited: Jul 2, 2004
15. Jul 4, 2004

### Dual Op Amp

I still don't understand, does it have anything to do with the particle's interactions?

16. Jul 10, 2004

### Dual Op Amp

Okay, I've learned a little more, but what I don't know is why the atom is more stable because it's valence shell is filled.
So, my question is, why is the atom stable when it has it's outer shell filled?

17. Jul 11, 2004

### Dr Transport

I believe that the energy of an atomic level is minimized by filling a shell ( s-, p-, d- etc...) shell completly.

18. Jul 11, 2004

### Dual Op Amp

Yes, but why?

19. Jul 11, 2004

### turin

Observe post #2.

20. Jul 12, 2004

### Dual Op Amp

Yes, I see post two. I also observed what came next, but none of you say why, when the valence shell is filled, it becomes stable.

21. Jul 12, 2004

### Dr Transport

two electrons of opposite spin have an enegy lower than two electrons of opposite spin. In the case of 8 electrons in an orbital, the electrons pair up in opposite spins. The reason for 2, then 8, then 18 electrons in filled shells is the following (I was waiting for someone else to get started on this point), is the following.

In an atom of the $$^{2S+1}L_{J}$$ variety, $$L$$ is the angular momentum,$$S$$ is the spin and $$J$$ is the total angular momentum $$J= L + S.$$ Now $$-J \leq L+S \leq +J$$, so for an atom $$S = \pm 1/2$$, and $$S$$ is less than $$L$$ in general, so for $$L = 0, 2S+1 = 2, J = 1/2$$,$$L = 1, 2S+1 = 2, J = 3/2$$, $$L = 2, 2S+1 = 2, J = 5/2$$. Now there are $$2J+1$$ angular momentum states, this is the basis behind the number if states in a closed shell.

(you guys are making me reach way, way back into my atomic physics coursework days......)

22. Jul 13, 2004

### Dual Op Amp

Okay, now I'm confused.

Last edited: Jul 14, 2004
23. Jul 13, 2004

### ZapperZ

Staff Emeritus
Yeah, but why does the 4s gets filled up first before the 3d? <runs and hides> :)

Zz.

24. Jul 13, 2004

### Dr Transport

don't remember, i said i had to reach way back into my atomic physics courses........ if memory serves me correctly, the 4s state has a lower energy than a 3d.

25. Jul 13, 2004

### ZapperZ

Staff Emeritus
Of course it does! That's why it gets filled up first. But do you remember why? It isn't obvious since one would think the shell with n=3 should be filled up first before the n=4 shell.

<Hums the Jeopardy tune>

Zz.