Electron Movement: Helium Model Explained

[staetualex]

Hello!

Anyone out there that can explain the helium model? Movement of electrons, forces, speed, etc? In an non excited atom.

 

[alxm]

Helium model?

Do you mean Bohr's model for Hydrogen?

 

[staetualex]

Yeah, i guess so. So, it's called Bor model, good to know.

 

[alxm]

Well, you could start by reading:
Hyperphysics: http://hyperphysics.phy-astr.gsu.edu/hbase/bohr.html
Wikipedia: http://en.wikipedia.org/wiki/Bohr_model

 

[staetualex]

Already tried that, but somehow, wikipedia thinks that only scientists are reading that. Of some reason, my brain can't read those symbols, many times confusing me more. I guess here is someone that can explain this things for monkeys too.

 

[jtbell]

Why helium? Why not start with something simpler, like hydrogen?

Anyway, the Bohr model, with electrons orbiting in planet-like orbits at fixed discrete distances from the nucleus, has been obsolete for over eighty years. Even for hydrogen, the Bohr model can't explain all the details, and I don't think it ever worked for helium. You have to use quantum mechanics.

 

[staetualex]

I chose the helium because inside hidrogen, the repulsion force between electrons don't exist. Ok, someone here can explain this trowing some quantum physics terms. Don't forget, act as i am a chimpanzee and i understand only simple things. Thanks!

 

[ZapperZ]

I chose the helium because inside hidrogen, the repulsion force between electrons don't exist. Ok, someone here can explain this trowing some quantum physics terms. Don't forget, act as i am a chimpanzee and i understand only simple things. Thanks!

But don't you think that if you are unable to walk, how are you expecting to be able to run? It appears that you have problems understanding the hydrogen atom via the QM description. Do you then think you have any reasonable hope of understanding something more complicated such as the Helium atom with the added complexity of the 2-electron state?

Zz.

 

[ytuab]

Helium model?

Do you mean Bohr's model for Hydrogen?

It's great! You can read other's mind.

Why helium? Why not start with something simpler, like hydrogen?

Anyway, the Bohr model, with electrons orbiting in planet-like orbits at fixed discrete distances from the nucleus, has been obsolete for over eighty years. Even for hydrogen, the Bohr model can't explain all the details, and I don't think it ever worked for helium. You have to use quantum mechanics.

But I think it is a very regrettable thing that in standard QM textbooks, they don't write about Bohr model and QM history in detail (Is it right?).

QM is a very strange world (uncertainty principle, spin...).
So I think it is very important to understand QM history very well.
(1920's~. especially about "spin")

I'm very glad if you read this book,
(The Story of Spin by Shinichiro Tomonaga) It's a very good textbook about spin.
(Or have you already read this?)

 

[alxm]

But I think it is a very regrettable thing that in standard QM textbooks, they don't write about Bohr model and QM history in detail (Is it right?).

Standard thermodynamics textbooks don't teach caloric theory. I don't view that as much of a loss.

So I think it is very important to understand QM history very well.

I disagree. I think it's lazy teaching (it being harder to invent a de novo approach) that to a large extent serves to perpetuate much of the initial confusion over QM, for no good reason. At least once a week someone seems to ask here "Why doesn't the electron fall into the nucleus?" - even though this question is now irrelevant, and was somewhat rhetorical to begin with.

I'm very glad if you read this book,
(The Story of Spin by Shinichiro Tomonaga) It's a very good textbook about spin.
(Or have you already read this?)

I've read it. I don't know why you bring it up in this thread though? It presupposes a fairly good knowledge of QM.

 

[staetualex]

Well, i like running. I would just want some explanations about those things(that bohr model). Can't a guy from here post some kind of "qm in 3 easy steps" or "Qm for noobs in 10 minutes" video clip on youtube? main thing, i am interested on why electrons spin around the nucleus. what is electric/magnetic force. The wave length, and so on.I rather stay away from books because they are boring and make me sleep.

not so later edit : Come one, people, let's focus, no flames and fights here!

 

[ZapperZ]

Well, i like running. I would just want some explanations about those things(that bohr model). Can't a guy from here post some kind of "qm in 3 easy steps" or "Qm for noobs in 10 minutes" video clip on youtube? main thing, i am interested on why electrons spin around the nucleus. what is electric/magnetic force. The wave length, and so on.I rather stay away from books because they are boring and make me sleep.

not so later edit : Come one, people, let's focus, no flames and fights here!

Electrons do not "spin around the nucleus". Please start by reading the FAQ in the General Physics forum.

Zz.

 

[frandango]

It's not really as complicated as people are making out, and it's a good question to wonder about what happens if we get more complicated than Hydrogen. I have only just joined this forum and am a bit surprised how snide a lot of these comments are! What we use for multi-electron atoms is to work with a 'fiddle factor' that uses perturbation theory. As you say, we must take into account the electron-electron repulsion. We work out what sort of contribution that would make, and then factor it into the energy calculations. For the ground state of Helium you end up with an approximate energy of -74.8eV which is close to the measured value (but the integrals to work this out are quite nasty). For lots of larger atoms, though, we can just use the Hydrogen model because the nucleus is shielded by lower-level electrons and we can pretend that there's only a single electron that's used for calculations. So even though Bohr's model isn't correct, it still "works" for many cases.

 

[staetualex]

Good to know there are supporting people around here. Thanks frandango!

 

[alxm]

It's not really as complicated as people are making out

No, it's actually a lot more complicated. There's a whole big field dedicated to trying to find better ways of solving the quantum many-body problem. I've got a half-dozen grad-level textbooks in my office that contain nothing other than various methods and approximations to calculate it. It is complicated.

I have only just joined this forum and am a bit surprised how snide a lot of these comments are!

Personally, I found the rudest comment to be the one an explanation because they think "books are boring" and can't be bothered to actually do any work. Nobody ever learned physics without work.

(But if you think that's okay, would you please pay my rent for me? I can't be bothered to go to work. It tires me. Could someone just do the work and give me a 10-minute rundown of what's going on at the office?)

What we use for multi-electron atoms is to work with a 'fiddle factor' that uses perturbation theory. As you say, we must take into account the electron-electron repulsion. We work out what sort of contribution that would make, and then factor it into the energy calculations.

Who uses first-order perturbation theory for atomic/molecular calculations other than as a textbook example?

For the ground state of Helium you end up with an approximate energy of -74.8eV which is close to the measured value (but the integrals to work this out are quite nasty).

That's, -2.74 Hartrees, the best value you can get with a wavefunction consisting of two hydrogenic wavefunctions. It's not at all near the measured value, which is -2.90372. It's not accurate enough for spectrocopy, it's not accurate enough for chemistry. And historically, it was not accurate enough to be able to say that the 'new' quantum theory worked for polyelectronic molecules. That was settled by Egil Hylleraas in 1929, by a direct variational calculation with the result -2.9032 Hartrees

For lots of larger atoms, though, we can just use the Hydrogen model because the nucleus is shielded by lower-level electrons and we can pretend that there's only a single electron that's used for calculations.

When is this used, and by whom?

So even though Bohr's model isn't correct, it still "works" for many cases.

No, it doesn't work in many cases at all. There aren't very many single-atom, single-electron systems. And you wouldn't use the Bohr model even then, because there's a well-known 'exact' solution to the Schrödinger equation for them.