Bohr Model

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ehntoo
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Bohr Model

Postby ehntoo » Tue May 27, 2008 9:13 pm UTC

I couldn't find this on the web anywhere, but how many electrons are in each shell of the bohr model?

Thanks.
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Herman
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Re: Bohr Model

Postby Herman » Tue May 27, 2008 9:22 pm UTC

I think the Bohr model just uses the rules you learn in Chemistry. (?) So 2 in the first shell, 8 in the second, 18 in the third...

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Re: Bohr Model

Postby Robin S » Tue May 27, 2008 9:33 pm UTC

I'm not sure that the Bohr model predicted the numbers of electrons in each shell, but if you are curious as to the actual numbers of electrons in different shells or subshells,
Wikipedia wrote:A subshell can contain up to 4l + 2 electrons; a shell can contain up to 2n2 electrons; where n equals the shell number.
This is a placeholder until I think of something more creative to put here.

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Re: Bohr Model

Postby troyp » Tue May 27, 2008 9:40 pm UTC

Herman wrote:I think the Bohr model just uses the rules you learn in Chemistry. (?) So 2 in the first shell, 8 in the second, 18 in the third...


No, the Bohr model would use 2, 8, 8,..
ie. it doesn't account for transition elements, since it doesn't model interactions between electrons.

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Re: Bohr Model

Postby brook_88 » Tue May 27, 2008 9:46 pm UTC

troyp wrote:
Herman wrote:I think the Bohr model just uses the rules you learn in Chemistry. (?) So 2 in the first shell, 8 in the second, 18 in the third...


No, the Bohr model would use 2, 8, 8,..
ie. it doesn't account for transition elements, since it doesn't model interactions between electrons.



Ditto that... managed to dreg that bit out of my memory of GCSE chemistry :) I hate the fact that for the next few years people just kept telling you what they had told you the previous year was a lie....

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Re: Bohr Model

Postby psyck0 » Tue May 27, 2008 9:54 pm UTC

brook_88 wrote: I hate the fact that for the next few years people just kept telling you what they had told you the previous year was a lie....


What, they stop eventually? I must reach that point...

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Re: Bohr Model

Postby evilbeanfiend » Tue May 27, 2008 10:27 pm UTC

eventually you reach a point where you have to tell yourself that what you told yourself last year was a lie ;)
of course its still true given some assumptions that turn out not to be true ;)
in ur beanz makin u eveel

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ehntoo
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Re: Bohr Model

Postby ehntoo » Tue May 27, 2008 10:34 pm UTC

So its 2,8,8...?
Do the 8's just keep repeating?
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Re: Bohr Model

Postby Minchandre » Wed May 28, 2008 3:37 am UTC

psyck0 wrote:
brook_88 wrote: I hate the fact that for the next few years people just kept telling you what they had told you the previous year was a lie....


What, they stop eventually? I must reach that point...


After you get to quantum chemistry they outright say "We have no idea how this really works, these are all just convenient approximations" so that's where the lying ends!

As for the original question, the Bohr model follows the real world - roughly. You put 2 in the first, then 8, then 8, then 8, then you come back to the third and put an extra 10 in there...it's ugly, but it doesn't work. :D

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Re: Bohr Model

Postby troyp » Wed May 28, 2008 9:37 am UTC

Minchandre wrote:As for the original question, the Bohr model follows the real world - roughly. You put 2 in the first, then 8, then 8, then 8, then you come back to the third and put an extra 10 in there...it's ugly, but it doesn't work.


This is the more sophisticated shell model that succeeded Bohr's. I'm pretty sure Bohr's did not account for the transition elements. Can a physics person confirm this?

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Re: Bohr Model

Postby brook_88 » Wed May 28, 2008 10:01 am UTC

When using the Bohr model I remember being told to remember "you put 10" in something.. but equally that I wouldnt have to use it ever....

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Re: Bohr Model

Postby ave_matthew » Wed May 28, 2008 12:34 pm UTC

Just a question, why learn (this version of) the bohr model for anything but it's nostalgic value?
Also I say 2, 8, 8, 18, 18, 32
Hydrogen helium
Li, Be, B, C, N, O, Fl, Ne => 8
Na, Mg, Al, Si, P, S, Cl, Ar => 8
Ca, K...that's all I remember in order => 18 :(
more . .. => 18
more .. => 32
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Re: Bohr Model

Postby brook_88 » Wed May 28, 2008 4:02 pm UTC

Lower level Chemistry uses it.. I was taught it for for GCSE chem..

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Re: Bohr Model

Postby psyck0 » Wed May 28, 2008 6:34 pm UTC

It's useful for learning about valence shells and bonding, before you get to hybrid orbitals.

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Re: Bohr Model

Postby ave_matthew » Thu May 29, 2008 2:41 pm UTC

psyck0 wrote:It's useful for learning about valence shells and bonding, before you get to hybrid orbitals.

Ah true, but for the things i have forgotten...
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Re: Bohr Model

Postby watch_wait_plot » Thu May 29, 2008 3:31 pm UTC

I was under the impression (like several others here) that the Bohr Model didn't account for transition elements. He just based the model off of the H atom. Which is one reason out of several why the model failed when n>1. It's just 2, 8, 8, 8, ad infinitum.

Edit:
from Wikipedia
The first Bohr orbit is filled when it has two electrons, and this explains why helium is inert. The second orbit allows eight electrons, and when it is full the atom is neon, again inert. The third orbital contains eight again, except that in the more correct Sommerfeld treatment (reproduced in modern quantum mechanics) there are extra "d" electrons. The third orbit may hold an extra 10 d electrons, but these positions are not filled until a few more orbitals from the next level are filled (Filling the n=3 d orbitals produces the 10 transition elements). The irregular filling pattern is an effect of interactions between electrons, which are not taken into account in either the Bohr or Sommerfeld models, and which are difficult to calculate even in the modern treatment.

I'm not sure if it is saying that the Bohr model does account for this or if it does not...It's not worded all that well but I think it says it doesn't...

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Re: Bohr Model

Postby opsomath » Thu May 29, 2008 6:31 pm UTC

As I understand it, the Bohr model does not produce the numbers of atoms in shells at all. If you (inadvisably) wish to use it to calculate something for a non-hydrogenlike atom, you just have to assume the electrons don't interact, or vary the charge on the nucleus in an "effective charge" approximation based on the idea that the positively charged nucleus and negatively charged other electrons compensate each other partially.

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Re: Bohr Model

Postby troyp » Fri May 30, 2008 3:59 am UTC

watch_wait_plot wrote:I was under the impression (like several others here) that the Bohr Model didn't account for transition elements. He just based the model off of the H atom. Which is one reason out of several why the model failed when n>1. It's just 2, 8, 8, 8, ad infinitum.

Edit:
from Wikipedia
The first Bohr orbit is filled when it has two electrons, and this explains why helium is inert. The second orbit allows eight electrons, and when it is full the atom is neon, again inert. The third orbital contains eight again, except that in the more correct Sommerfeld treatment (reproduced in modern quantum mechanics) there are extra "d" electrons. The third orbit may hold an extra 10 d electrons, but these positions are not filled until a few more orbitals from the next level are filled (Filling the n=3 d orbitals produces the 10 transition elements). The irregular filling pattern is an effect of interactions between electrons, which are not taken into account in either the Bohr or Sommerfeld models, and which are difficult to calculate even in the modern treatment.

I'm not sure if it is saying that the Bohr model does account for this or if it does not...It's not worded all that well but I think it says it doesn't...

It's saying that:
a) the Bohr model doesn't account for the transition elements - it omits the extra 10 "d" electrons.
b) the Sommerfield model does includes the "d" electrons, but:
c) neither model accounts for the fact that the "d" electrons are filled "out of sequence"

Note:

troyp wrote:No, the Bohr model would use 2, 8, 8,..
ie. it doesn't account for transition elements, since it doesn't model interactions between electrons.

From what I read I seem to be wrong here in the reason I gave.
The fact that the Bohr model doesn't model electron interactions is not the reason it deosn't account for the transition elements: the Sommerfield model also omits electron interactions, but does include the transition elements. Tne interactions between electrons are needed to account for the filling order, but not the number in each orbit.
Correct me if I'm wrong. :D

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Re: Bohr Model

Postby 3.14159265... » Fri May 30, 2008 4:52 am UTC

number of electrons on the n-th shell: 2(n^2)

Prove it by induction and the fact that (n+1)^2 - n^2 = 2n + 1
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Re: Bohr Model

Postby 111222333 » Fri May 30, 2008 6:56 am UTC

The Bohr models says the no. of electrons in the shell n is 2n^2; except in the valence shell, which can have a maximum of 8 electrons. The issues with the Bohr model are in relation to the Spectra, and the "sub shell" filling, not its prediction of the no. of electrons.


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