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Author Topic: What are electron shells, and why don't electrons fall into the nucleus  (Read 16362 times)

Offline simeonie

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There was another topic that I have just been on about electrons and what makes them stay around the atom, which I kinda already new :D

But I was wondering, I always learnt that there are electron shells around an atom. There are 2 on the first shell and 8 on the others.

First question: What are the shells exactly, are they real physical solid things or just like energy forces or something.

Second question: Why aren't all of the electrons just pulled straight into the nucleus with the neurtons and protons if it is the protons they are attracted to.

Third question: I know as I mentioned before that in the first shell there are only 2 electrons and other there are 8. When you get a certain amount of shells and electrons can you ever fit more than 8 in?

« Last Edit: 27/06/2009 15:12:05 by BenV »


 

Offline DoctorBeaver

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Electron shells are not physical entites. They are spherical orbits in which electrons may be found.
The maximum number of electrons in any given shell is given by the equation 2n^2 where n=shell number (1 being the closest to the nucleus, 2 the next nearest & so on). So, the number of electrons in shell 1 would be 2*(1^2)=2, shell 2 is 2*(2^2)= 8.
Knowing the atomic number & the mass number of an element, then using the rule above you can work out how electrons are in each shell.
Take Sodium as an example. It has an atomic number of 11 and an mass number of 23. An atom of Na has 11 protons & 11 electrons.
The mass number is 23, so protons+neutrons=23. As there are 11 protons, there must be 12 neutrons(23-11)
So, using the 2n^2 equation above you can work out that Na has 2 electrons in the 1st shell, 8 in the 2nd and 1 in the 3rd.


« Last Edit: 09/01/2006 23:23:30 by DoctorBeaver »
 

Offline ukmicky

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quote:
Therefore an atom of Na has 11 protons & 11 electrons.



Eth


so what does my Nana have and should my Grandad be worried

Michael                 HAPPY NEW YEAR                    
« Last Edit: 09/01/2006 23:22:30 by ukmicky »
 

Offline DoctorBeaver

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I've got a degree in idiocy. I'm a BA Nana! :D
 

Offline Solvay_1927

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Simon/Simeonie,
good questions. Your 3 questions are the sort of things that people were asking nearly 100 years ago, and in order to answer them, they had to develop the new and bizarre physics of "quantum theory".

A good starting point would be wikipedia:
http://en.wikipedia.org/wiki/Quantum_mechanics

To help answer your first two questions (I think Eth has sufficiently answered your third one), the following extract is taken from the link above:
quote:
It is necessary to use quantum mechanics to understand the behavior of systems at atomic length scales and smaller.
For example, Newtonian mechanics say unlike charges attract so that an electron in a hydrogen atom which has a negative charge will be attracted to the nucleus which has a positive charge at great speed and collapse making it highly unstable. However, in the natural world the electron normally remains in a stable orbit around a nucleus seeming to defy classical electromagnetism.

Quantum mechanics was initially developed to explain the atom and especially the orbit and position of the electron. Quantum mechanics uses probability distributions to explain such effects.
Probability in the context of quantum mechanics has to do with the likelihood of finding a particle, such as an electron, in a particular region around the nucleus at a particular time. Therefore, electrons cannot be pictured as localized particles in space but rather should be thought of as clouds of negative charge spread out over the entire orbit. These clouds represent the regions around the nucleus where the probability of finding an electron is the largest.
So in normal atoms with electrons in a stable orbit, the probability of the electron being at the nucleus is nearly zero according to the Heisenberg Uncertainty Principle.
Therefore, the laws of quantum mechanics, unlike Newton's deterministic laws, lead to a probabilistic description of nature.


Hope that helps. (If it doesn't - if it just makes you more confused - then don't worry, you're in good company. Nobody really understands quantum theory.:))
Paul.
 

Offline daveshorts

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Electrons and shells is all to do with quantum mechanics. This has a load of difficult equations and solving them is even more difficult especially if there are lots of interacting things to deal with. I think these days it is just about possible to model about 90 atoms with really big computers smoking a lot.

 Now this isn't really very practical to do if you are going to do some chemistry this afternoon, and it doesn't give you a feeling for what is going on. So what chemists do is solve the equations imagining that there was only one electron involved and have developed all sorts of neat fudges to take into account the rest of the universe.

It turns out that these solutions can be grouped by energy into what you call the electron shells. This works quite well until you get to the transition metals when real life impedes a bit on the beautiful picture and the spread of energies within a shell is greater than the difference between them. This makes it all a bit more complex and is why you don't do a lot about transition metals at GCSE.
 

Offline DoctorBeaver

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Dave - I would have thought that chemists were only interested in how many electrons are in the outer shell as that determines valence. I don't see how different energy levels would interest them overly much. If I'm wrong, please enlighten me.
 

Offline Soul Surfer

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Understanding electron shells.  

When I first did my quantum mechanics it was realatively new and I was being taught by the people who were involved in its discovery so we came to it from a much more classical approach than is taught today.  

Here is a semi classical description of why we get fixed orbits and shells for electrons.

If an electron is forced by a proton to follow a circular orbit its natural (classical)tendancy is to radiate electromagnetic energy and eventually collapse into the proton.  

Now it had been shown by classical diffraction experiments that electrons (and any other matter)can behave like waves as well as particles and that the wavelength of the electron depends on how fast it is moving.  the faster it moves the shorer the wavelenth.

Now if you do a classical analysis of a hydrogen atom containing a single proton with an electron orbiting it it just happens that if an electron is orbiting the proton at in certain ways at certain particular distances the "wavelength" of the electron matches its orbit around the proton and there is a sort of resonance.  One way of visualising this is that as the electron radiates energy  it also absorbs it and so continues in a stable state.

If you calculate these classical eletron orbitals you will find that they accurately match the quantum energy levels in the hydrogen atom.  As you add more protons and more electrons these calculations become much more complex because the interactions between the electrons have to be taken into account these electron electron interactios prevent another electron fom being in the same resonant cavity. The end result is the electron shells.

One final question remains why doesn't the electron ever lose all its energy and collapse onto the proton.  Well we didnt know then but we now know that a proton is not a single static entity but three quarks batting around like mad things  (they have more energy in their motion than in their rest mass!) so there is nothing stable for an electron to collapse on to.




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Offline simeonie

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Wow sould sufer you are one smart girl/boy. And thanks for that you were all very useful. I didn't quite understand the answer to why the electrons don't just get pulled straight into the nucleus though, because there must be some sort of force pulling it out making it orbit.

« Last Edit: 27/06/2009 14:40:54 by BenV »
 

Offline DoctorBeaver

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quote:
Originally posted by simeonie

Wow sould sufer you are one smart girl/boy.



Are you inferring that Ian is a transexual? :D
« Last Edit: 10/01/2006 17:42:06 by DoctorBeaver »
 

Offline rosy

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quote:
I would have thought that chemists were only interested in how many electrons are in the outer shell as that determines valence. I don't see how different energy levels would interest them overly much. If I'm wrong, please enlighten me.

Ouch! Allow me to enlighten you instead.
Chemists are very much interested in the energies of electrons!!
The energy and number of electrons in the outer shell determines many things about reactivity, but the energies of inner shell electrons are also important, especially when you're talking about the transition metals where, after all, as you move across the period you're not really filling the outer shell, but rather (in some senses) the next one in. The energies (and shapes) of the electron orbitals involved determine the reactivity.
Actually, we don't talk about "shells". We're more interested in orbitals, which are occupied by one or two electrons (or not at all) these orbitals being grouped into "subshells" and the subshells into shells. Where there's more than one electron present the subshells have different energies, and in molecules the breaking of the spherical symmetry of the atom gives the orbitals different energies within the subshells.
Chemists need to understand (bits of) all that in order to predict/rationalise reactions.
I haven't explained very well, because all the understanding of this stuff in my head involves complicated diagrams. But I'm really just making the point...
 

Offline DoctorBeaver

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Thanks for that, Rosy. I've never known a lot about chemistry & I was just going by something I read the other day that said valence was determined by the outer shell. In my ignorance I didn't allow for the fact that there's more to it.
 

Offline Solvay_1927

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Ian, thanks for the "semi-classical" description, it's very useful.

But regarding the last para:
quote:
Originally posted by Soul Surfer
One final question remains why doesn't the electron ever lose all its energy and collapse onto the proton.  Well we didnt know then but we now know that a proton is not a single static entity but three quarks batting around like mad things  (they have more energy in their motion than in their rest mass!) so there is nothing stable for an electron to collapse on to.

If there is a direct force of attraction between the proton/quarks and the electron, then won't the electron still spiral into the nucleus?  Or at the very least, the electron would end up moving in a circle (ellipse?) the same size as the circle (ellipse) that the quarks are moving in? How come the electron maintains a "presence" in a space up to 100,000 times greater (in diameter) than the diameter of the nucleus?

Or is this where the "semi-classical" analogy breaks down and we just have to accept "quantum weirdness"?
« Last Edit: 14/01/2006 01:56:30 by Solvay_1927 »
 

Offline gsmollin

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Indeed, the classical approach cannot explain electron orbitals. However, I will try to give a very simple explanation.

The electron does not ever merge with the proton in the atom because it does not have the energy to do it. This process is called "inverse beta decay" and it takes a lot of energy, and a neutrino. The proton is composed of 2 up quarks and a down quark, and the neutron is 2 down quarks and a up. (Or vice-versa, I can never remember.) Anyway, inverse beta decay is the process of combining a proton, electron, neutrino, and some energy into a neutron. The quarks must change flavor to do this. The whole process will not occur just because electrons are attracted to protons, electrically. That fact satisfies charge conservation, but is only part of the process.

The orbital is a sort of resonance where each orbital level includes a quantum of energy. In addition, electrons are fermions, so they exclude each other in the atom, and always stay apart, in space and energy. In a broad-brush explanation this is why there are discrete orbitals.

"F = ma, E = mc^2, and you can't push a string."
 

Offline DoctorBeaver

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Offline ComplexQM

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Hi gsmollin,
Hi,
It is true that fixing the electron shell positions is supposed to be impossible due to the Uncertainty Principle but it is not.
I have a yahoo group complexquantummechanics but cannot link you to it
What you need to do is use two new waveforms I introduce and realise how they decay.
Have a good day,
Alex


Indeed, the classical approach cannot explain electron orbitals. However, I will try to give a very simple explanation.

The electron does not ever merge with the proton in the atom because it does not have the energy to do it. This process is called "inverse beta decay" and it takes a lot of energy, and a neutrino. The proton is composed of 2 up quarks and a down quark, and the neutron is 2 down quarks and a up. (Or vice-versa, I can never remember.) Anyway, inverse beta decay is the process of combining a proton, electron, neutrino, and some energy into a neutron. The quarks must change flavor to do this. The whole process will not occur just because electrons are attracted to protons, electrically. That fact satisfies charge conservation, but is only part of the process.

The orbital is a sort of resonance where each orbital level includes a quantum of energy. In addition, electrons are fermions, so they exclude each other in the atom, and always stay apart, in space and energy. In a broad-brush explanation this is why there are discrete orbitals.

"F = ma, E = mc^2, and you can't push a string."
 

Offline syhprum

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ComplexQM

Pop 'complexquantummechanics' into google and you will be guided to the site, it worked for me after I had registed as a Yahoo member.

http://tech.groups.yahoo.com/group/complexquantummechanics/
« Last Edit: 04/03/2008 20:58:39 by syhprum »
 

Offline Dr. Dan

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"F = ma, E = mc^2, and you can't push a string."

I presume the last part refers to string theory and the way they tried to push it on us, but couldn't...   <grin> ;D

 

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