Why don't electrons fall into the nucleus?
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Original post by zed963
Seen as the movement of the electron is random and its displacement changes every second, isn't there a centrifugal force involved?
Yes, because although the electron is orbiting at a constant speed, it's direction and therefore it's acceleration must be changing, so a force is exerted towards the centre of the orbit (in this case, I think it would be the electrostatic attraction between the electron and nucleus.)
Original post by saddle098
From what I've gathered, an electron can only travel in an orbit that has a total distance equal to a multiple of its wavelength... Something to do with constructive interference. Anyone fancy explaining this?
If I correctly understood the concepts, the electron has wave nature, and for a stable situation it should be in phase with itself around the nucleus. See also:
http://yougems.reflectionsinfos.com/queries/viewquery/4786
http://hyperphysics.phy-astr.gsu.edu/hbase/bohr.html#c5
(edited 10 years ago)
Original post by suzylemonade
What are you on about?
Shes talking about another type of chemistry *wink wink xD
Original post by Zenarthra
Shes talking about another type of chemistry *wink wink xD
Evidently. She didn't seem to know much about the actual subject itself
Original post by majmuh24
What is the concentration of Ag+ in a solution if it takes 2.30 mins using a current of 2.00A to plate all the silver from 0.25L of a solution containing Ag+?
Work out the answer to that and then get back to me on chemistry being simple.
Work out the answer to that and then get back to me on chemistry being simple.
Bit rich coming from the guy who asked why electrons weren't subject to air resistance in an atom
These are some great questions you are asking however are not explainable through the use of classical physics (as other people above mentioned). Obviously you have heard of quantum mechanics and you are familiar with some of the concepts. I guess the first place to start with answering these questions about the stability of atoms is ask "What does the structure of the atom look like?" As you know Rutherford gave us the insight into this and hence the orbital image of the atom formed. Now many problems occur with this picture, one being the stability of atoms. Classical electromagnetism tells us that accelerating charges emit EM radiation. Hence if an electron is truly orbiting the nucleus it would be emitting EM waves and consequently losing energy. This clearly makes no sense as atoms must be stable in general. So we need a new theory of the atom. Here is where QM comes in.
Old QM never really attempted to resolve these problems from a theoretical standpoint. They simply set up a series of axioms that basically were to the effect of "electrons don't emit EM waves". No one really explained why, they just neglected the obvious problems at the time. Then came along modern quantum theory (Copenhagen Interpretation) . This is where QM was formalised into the theory we know today. However it came at a cost of throwing out a lot of the notions that make sense to us at a classical level. Specifically the notion of measurement. This is where I think your confusion lies, in the idea of measurement. So the image of a particle like a marble in your mind implicitly means you have measured your system and it as collapsed into a single state. Thus by visualising particles in this marble like picture you remove the key QM properties that are needed to explain the problems above.
As this is a huge subject and this post is getting rather long I will await to see if you have more specific questions about what I have written about above. Cheers.
Old QM never really attempted to resolve these problems from a theoretical standpoint. They simply set up a series of axioms that basically were to the effect of "electrons don't emit EM waves". No one really explained why, they just neglected the obvious problems at the time. Then came along modern quantum theory (Copenhagen Interpretation) . This is where QM was formalised into the theory we know today. However it came at a cost of throwing out a lot of the notions that make sense to us at a classical level. Specifically the notion of measurement. This is where I think your confusion lies, in the idea of measurement. So the image of a particle like a marble in your mind implicitly means you have measured your system and it as collapsed into a single state. Thus by visualising particles in this marble like picture you remove the key QM properties that are needed to explain the problems above.
As this is a huge subject and this post is getting rather long I will await to see if you have more specific questions about what I have written about above. Cheers.
Original post by majmuh24
If I correctly understood the concepts, the electron has wave nature, and for a stable situation it should be in phase with itself around the nucleus. See also:
http://yougems.reflectionsinfos.com/queries/viewquery/4786
http://hyperphysics.phy-astr.gsu.edu/hbase/bohr.html#c5
http://yougems.reflectionsinfos.com/queries/viewquery/4786
http://hyperphysics.phy-astr.gsu.edu/hbase/bohr.html#c5
Original post by saetretue
Great article.
OP, I believe your specific remaining queries are answered in the above link, and particularly the "Probability density vs. radial probability" section.
May I also suggest that if you want your mind blown by more QM, you google 'Elitzur-Vaidman bomb detector'.
(edited 10 years ago)
I don't think quantum mechanics is the key issue here. Think of the earth: it orbits the sun without falling into the sun, despite this being a Newtonian system. Electrons orbit the nucleus because they have angular momentum. Since angular momentum must be conserved, and an electron at rest in the nucleus would have zero orbital angular momentum, electrons cannot simply collapse into the nucleus without losing their angular momentum. Electrons do not lose their angular momentum to air resistance because air resistance is simply molecules - that is, their electrons - colliding near enough for their electrostatic forces to repel one another. Any angular momentum lost in this way by some electron will be gained by others.
Quantum mechanics is certainly needed to answer some questions about the behaviour of electrons in atoms but not, I think, the OP's question.
QM does in fact predict that the electron can exist inside the nucleus; that is, there is a finite probability of measuring its location there. Because the nucleus is very small, though, the uncertainty principle means that if the electron's position is measured as being inside the nucleus, it must therefore have a large momentum. So the electron cannot be at rest in the nucleus, in the way that the earth could end at rest in the sun if it were to lose its angular momentum.
Quantum mechanics is certainly needed to answer some questions about the behaviour of electrons in atoms but not, I think, the OP's question.
QM does in fact predict that the electron can exist inside the nucleus; that is, there is a finite probability of measuring its location there. Because the nucleus is very small, though, the uncertainty principle means that if the electron's position is measured as being inside the nucleus, it must therefore have a large momentum. So the electron cannot be at rest in the nucleus, in the way that the earth could end at rest in the sun if it were to lose its angular momentum.
(edited 10 years ago)
Original post by vincentasaurus
Bit rich coming from the guy who asked why electrons weren't subject to air resistance in an atom
Sorry, I was making the mistake of applying macroscopic ideas on a microscopic scale
I'm sure better people than me have made the same mistake 
.Original post by saetretue
Thanks for that, the whole 'radial probability vs. probability density' clears a lot up (I thought it was a bit strange that the probability density should be focused at the nucleus for the n=1 state of Hydrogen
)Original post by ClickItBack
Great article.
OP, I believe your specific remaining queries are answered in the above link, and particularly the "Probability density vs. radial probability" section.
May I also suggest that if you want your mind blown by more QM, you google 'Elitzur-Vaidman bomb detector'.
OP, I believe your specific remaining queries are answered in the above link, and particularly the "Probability density vs. radial probability" section.
May I also suggest that if you want your mind blown by more QM, you google 'Elitzur-Vaidman bomb detector'.
Will do, although whether I'll understand it is a different matter!
Thanks for the link. 
However problems occur here when you consider accelerating charged particles. According to classical EM, if a charged particle accelerates it emits EM radiation. Google Lamor Formula for more explanation. So unfortunately QM is most definitely needed to explain this phenomena.
Original post by Observatory
I don't think quantum mechanics is the key issue here. Think of the earth: it orbits the sun without falling into the sun, despite this being a Newtonian system. Electrons orbit the nucleus because they have angular momentum. Since angular momentum must be conserved, and an electron at rest in the nucleus would have zero orbital angular momentum, electrons cannot simply collapse into the nucleus without losing their angular momentum. Electrons do not lose their angular momentum to air resistance because air resistance is simply molecules - that is, their electrons - colliding near enough for their electrostatic forces to repel one another. Any angular momentum lost in this way by some electron will be gained by others.
Quantum mechanics is certainly needed to answer some questions about the behaviour of electrons in atoms but not, I think, the OP's question.
QM does in fact predict that the electron can exist inside the nucleus; that is, there is a finite probability of measuring its location there. Because the nucleus is very small, though, the uncertainty principle means that if the electron's position is measured as being inside the nucleus, it must therefore have a large momentum. So the electron cannot be at rest in the nucleus, in the way that the earth could end at rest in the sun if it were to lose its angular momentum.
Quantum mechanics is certainly needed to answer some questions about the behaviour of electrons in atoms but not, I think, the OP's question.
QM does in fact predict that the electron can exist inside the nucleus; that is, there is a finite probability of measuring its location there. Because the nucleus is very small, though, the uncertainty principle means that if the electron's position is measured as being inside the nucleus, it must therefore have a large momentum. So the electron cannot be at rest in the nucleus, in the way that the earth could end at rest in the sun if it were to lose its angular momentum.
You don't need quantum mechanics to come up with a naive reason that electrons don't fall into the nucleus. Why don't planets fall into the sun? Because the force is centripetal, i.e causes electrons to orbit the nucleus rather than falling in. This model doesn't work, and there are many reasons why it doesn't, but the presence of an attractive force isn't one of them. I give this cop-out answer in light of other people's quantum mechanical interpretations because it is important to understand classical physics before getting bogged down in the more abstract world of quantum mechanics. And as people have said, the real reason electrons don't fall into the nucleus is both subtle and complicated. And in actuality electrons DO spend some time in the nucleus! In fact it is the ability of particular electrons to get close to the nucleus that allows chemistry as we know it to exist.
To answer the OP, the reason why electrons don't crash into the nucleus is a) it's not actually a particle, it's both that and a wave, a waveticle so to speak. b) the orbits it follows aren't circular.
Original post by WishingChaff
These are some great questions you are asking however are not explainable through the use of classical physics (as other people above mentioned). Obviously you have heard of quantum mechanics and you are familiar with some of the concepts. I guess the first place to start with answering these questions about the stability of atoms is ask "What does the structure of the atom look like?" As you know Rutherford gave us the insight into this and hence the orbital image of the atom formed. Now many problems occur with this picture, one being the stability of atoms. Classical electromagnetism tells us that accelerating charges emit EM radiation. Hence if an electron is truly orbiting the nucleus it would be emitting EM waves and consequently losing energy. This clearly makes no sense as atoms must be stable in general. So we need a new theory of the atom. Here is where QM comes in.
Old QM never really attempted to resolve these problems from a theoretical standpoint. They simply set up a series of axioms that basically were to the effect of "electrons don't emit EM waves". No one really explained why, they just neglected the obvious problems at the time. Then came along modern quantum theory (Copenhagen Interpretation) . This is where QM was formalised into the theory we know today. However it came at a cost of throwing out a lot of the notions that make sense to us at a classical level. Specifically the notion of measurement. This is where I think your confusion lies, in the idea of measurement. So the image of a particle like a marble in your mind implicitly means you have measured your system and it as collapsed into a single state. Thus by visualising particles in this marble like picture you remove the key QM properties that are needed to explain the problems above.
As this is a huge subject and this post is getting rather long I will await to see if you have more specific questions about what I have written about above. Cheers.
Old QM never really attempted to resolve these problems from a theoretical standpoint. They simply set up a series of axioms that basically were to the effect of "electrons don't emit EM waves". No one really explained why, they just neglected the obvious problems at the time. Then came along modern quantum theory (Copenhagen Interpretation) . This is where QM was formalised into the theory we know today. However it came at a cost of throwing out a lot of the notions that make sense to us at a classical level. Specifically the notion of measurement. This is where I think your confusion lies, in the idea of measurement. So the image of a particle like a marble in your mind implicitly means you have measured your system and it as collapsed into a single state. Thus by visualising particles in this marble like picture you remove the key QM properties that are needed to explain the problems above.
As this is a huge subject and this post is getting rather long I will await to see if you have more specific questions about what I have written about above. Cheers.
Thanks a lot for your post, it was really insightful! I've looked at this a bit more and it seems to be something to do with decoherence and wave function collapse, although I can't say I fully understand it.
Can't the question be 'explained' by thinking of the electron as a solid little particle with an*accompanying wave which pushes it around (i.e. wave-particle duality => two things rather than one, which is the viewpoint of the de Broglie-Bohm theory, whereas QM is just a dynamical theory and more to do with the statistical mechanics of particles moving along non-classical paths whereas de-Broglie is more of a probability calculus as referred to by a different poster.
Thanks for helping!
Original post by QuantumOverlord
You don't need quantum mechanics to come up with a naive reason that electrons don't fall into the nucleus. Why don't planets fall into the sun? Because the force is centripetal, i.e causes electrons to orbit the nucleus rather than falling in. This model doesn't work, and there are many reasons why it doesn't, but the presence of an attractive force isn't one of them. I give this cop-out answer in light of other people's quantum mechanical interpretations because it is important to understand classical physics before getting bogged down in the more abstract world of quantum mechanics. And as people have said, the real reason electrons don't fall into the nucleus is both subtle and complicated. And in actuality electrons DO spend some time in the nucleus! In fact it is the ability of particular electrons to get close to the nucleus that allows chemistry as we know it to exist.
See the post directly above yours for more info, he explains it much better than me.
Also, I know that the probability density function predicts that there is a very small probability of the electrons being found in the nucleus, but what happens in this event? Would it cause the emission of a neutrino to occur?
(edited 10 years ago)
Original post by majmuh24
See the post directly above yours for more info, he explains it much better than me.
Also, I know that the probability density function predicts that there is a very small probability of the electrons being found in the nucleus, but what happens in this event? Would it cause the emission of a neutrino to occur?
Also, I know that the probability density function predicts that there is a very small probability of the electrons being found in the nucleus, but what happens in this event? Would it cause the emission of a neutrino to occur?
No I don't think so. Stuff like electron capture, as far as I'm aware is a nuclear property and occurs to drop the nucleus into a lower energy state. I don't think the presence of a non zero wavefunction at the nucleus would cause radioactive decay.
So there are a lot of concepts being thrown around here and getting very mixed up in different fields.
Firstly each of these models you mention are viable interpretations of quantum theory, i.e. de-Broglie-Bohm interpretation. We still do not know today what is the correct interpretation of QM (although the Copenhagen interpretation is the most common). This image of an electron as a classical object being pushed by a wave doesn't seem right to me. I don't know as much on Bohm interpretation but I have linked a wiki source below.
http://en.wikipedia.org/wiki/De_Broglie%E2%80%93Bohm_theory#Overview
You have to be careful here as to what interpretation you are using. There appear to be many people throwing around a mix of interpretations and I would recommend choosing one that appeals to you and sticking with it.
http://en.wikipedia.org/wiki/Copenhagen_interpretation
Firstly each of these models you mention are viable interpretations of quantum theory, i.e. de-Broglie-Bohm interpretation. We still do not know today what is the correct interpretation of QM (although the Copenhagen interpretation is the most common). This image of an electron as a classical object being pushed by a wave doesn't seem right to me. I don't know as much on Bohm interpretation but I have linked a wiki source below.
http://en.wikipedia.org/wiki/De_Broglie%E2%80%93Bohm_theory#Overview
You have to be careful here as to what interpretation you are using. There appear to be many people throwing around a mix of interpretations and I would recommend choosing one that appeals to you and sticking with it.
http://en.wikipedia.org/wiki/Copenhagen_interpretation
Original post by majmuh24
Thanks a lot for your post, it was really insightful! I've looked at this a bit more and it seems to be something to do with decoherence and wave function collapse, although I can't say I fully understand it.
Can't the question be 'explained' by thinking of the electron as a solid little particle with an*accompanying wave which pushes it around (i.e. wave-particle duality => two things rather than one, which is the viewpoint of the de Broglie-Bohm theory, whereas QM is just a dynamical theory and more to do with the statistical mechanics of particles moving along non-classical paths whereas de-Broglie is more of a probability calculus as referred to by a different poster.
http://en.wikipedia.org/wiki/Copenhagen_interpretation
Thanks for helping!
Can't the question be 'explained' by thinking of the electron as a solid little particle with an*accompanying wave which pushes it around (i.e. wave-particle duality => two things rather than one, which is the viewpoint of the de Broglie-Bohm theory, whereas QM is just a dynamical theory and more to do with the statistical mechanics of particles moving along non-classical paths whereas de-Broglie is more of a probability calculus as referred to by a different poster.
http://en.wikipedia.org/wiki/Copenhagen_interpretation
Thanks for helping!
Original post by WishingChaff
However problems occur here when you consider accelerating charged particles. According to classical EM, if a charged particle accelerates it emits EM radiation. Google Lamor Formula for more explanation. So unfortunately QM is most definitely needed to explain this phenomena.
Yes, Bremsstrahlung kills the classical electron orbit idea. But, OP did not mention that
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