Particles on the surface of an atom?

[tyogav]

There are many particles inside an atom.

What particles make up the surface of an atom? When we see graphical illustrations of spherical atoms, what are we actually seeing?

 

[The_Duck]

What particles make up the surface of an atom?

An atom has no surface. An atom is composed of a tiny nucleus and a cloud of electrons that surrounds the nucleus. The electron cloud has no definite edge or surface.

When we see graphical illustrations of spherical atoms, what are we actually seeing?

Whatever the illustrator decided to draw! If you are talking about atomic orbitals, these are showing of what regions of space have the highest probability to contain an electron.

 

[tyogav]

Thank you! This is the illustration I was keeping in mind. Am I missing something fundamental? https://m.flickr.com/#/photos/uclmaps/8536981794/

 

[mattt]

Usually, when drawing isolated atoms, they draw spheres, and you can guess the size of those spheres are meant to be the covalent-radius, the Van der Waals radius,...(one of the different definitions of radius of an atom).

As The_Duck pointed out above, atoms (and molecules) do not have an edge-surface or whatever you may call it. Also when they draw pictures of atomic (or molecular) orbitals, they are just some regions of space where some electrons have a given high probability ( normally 0.9 ) to be found in, but anyway there are infinitely many different regions of space where that same electron has a 0.9 probability to be found in, so they usually choose just one of those regions that are "nice" and "symmetric".

 

[mfb]

Thank you! This is the illustration I was keeping in mind. Am I missing something fundamental? https://m.flickr.com/#/photos/uclmaps/8536981794/

You see a layer of (probably) graphene in green and some atoms are drawn as reflective spheres.

 

[KL7AJ]

There are many particles inside an atom.

What particles make up the surface of an atom? When we see graphical illustrations of spherical atoms, what are we actually seeing?

You're actually seeing statistical data! The "surface" of an atom is actually a locus of points the outer electrons can occupy.

Imagine a spinning fan. On the whole, it looks like a solid disk, but there's space between the blades. In an atom, this is just a lot more extreme. If the nucleus was about the side of a grapefruit, the electron would be the point of a pin about 3000 miles away! (Actually the electron has NO size. :) )

Hope this helps...or not.

Eric

 

[Delta2]

All the atoms consist of elementary particles, which elementary particles are (according to current theory) point particles, that is at a given time they occupy a single point in space. However these elementary point particles are in continuous movement, that's how we get the impression that they occupy some finite volume (like a spherical volume) in space.

 

[KL7AJ]

All the atoms consist of elementary particles, which elementary particles are (according to current theory) point particles, that is at a given time they occupy a single point in space. However these elementary point particles are in continuous movement, that's how we get the impression that they occupy some finite volume (like a spherical volume) in space.

Another way it can be described is electrons are infinitely small, but they have a large personal space. "))

 

[tyogav]

Thanks everyone especially Delta2 and KL7AJ

 

[ChrisVer]

It has nothing to do with motion of electrons around the atom. What they mainly show is the probability of an electron existing in some region. And the solution is static...
At low energies however, at which you can't see the structure of an atom [electrons' energy mainly], the atom can be assumed as a solid sphere... That's what people thought in the past [before rutherford's experiment]...

 

[KL7AJ]

It has nothing to do with motion of electrons around the atom. What they mainly show is the probability of an electron existing in some region. And the solution is static...
At low energies however, at which you can't see the structure of an atom [electrons' energy mainly], the atom can be assumed as a solid sphere... That's what people thought in the past [before rutherford's experiment]...

This is why "obsolete" models can still be eminently useful. You can drill down this as deeply as you want...or not. :)

 

[Delta2]

It has nothing to do with motion of electrons around the atom. What they mainly show is the probability of an electron existing in some region. And the solution is static...

If the electrons didnt move at all and remain still then the probability to exist in some region would be zero except in a specific point where it would be 1.

 

[ChrisVer]

If the electrons didnt move at all and remain still then the probability to exist in some region would be zero except in a specific point where it would be 1.

except for that is impossible [it would have definite position and momentum], I didn't say it doesn't move. I'm saying that the probability to be found in some region has nothing to do with the electron moving (which would need pictures of it for several times to cover the probability- however the probability is there at any given time... that's why I called it static)

 

[Delta2]

except for that is impossible [it would have definite position and momentum], I didn't say it doesn't move. I'm saying that the probability to be found in some region has nothing to do with the electron moving (which would need pictures of it for several times to cover the probability- however the probability is there at any given time... that's why I called it static)

How can you say "it has nothing to do with electron moving" if the electrons don't move, there is no meaning talking about the probability to be found in a region of space, the only probability tha there is , is zero everywhere except a specific point.

There is no meaning talking about uncertainty principle, wave function or probability clouds or whatever unless the electron is moving.

 

[DrClaude]

How can you say "it has nothing to do with electron moving" if the electrons don't move, there is no meaning talking about the probability to be found in a region of space, the only probability tha there is , is zero everywhere except a specific point.

There is no meaning talking about uncertainty principle, wave function or probability clouds or whatever unless the electron is moving.

This is an incredibly classical, and therefore incorrect, point of view. The electron's state is governed by quantum mechanics, which tells us that the electron is not occupying one position, then another. It is therefore meaningless to talk about the electron "moving." It is in a stationary state.

Another way to see it is that the electron is in a superposition of being in different places at the same time.

 

[Delta2]

So you actually saying that particles in quantum mechanics never move? They teleport from one position to another? And what is the meaning of the momentum/velocity of the particle if they don't move??

 

[DrClaude]

So you actually saying that particles in quantum mechanics never move?

No, far from it. I'm talking specifically about electrons in a bound state in an atom.

They teleport from one position to another?

Again, you are stuck on the idea of "moving". Teleporting would be the same as moving, but in discrete steps. What I am saying is that the electron is in many places at the same time, not jumping from one place to the next.

And what is the meaning of the momentum/velocity of the particle if they don't move??

The thing is that an electron in an atom also doesn't have a definite momentum, it is also in a superposition of various "fast" and "slow".

I don't want to derail this thread, so I won't go into more details, but it's one of the important conclusion of quantum mechanics. Electrons in an atom do not orbit around the nucleus, they are in diffuse orbitals, diffuse both from the point of view of regelar space and momentum space. It seems strange from the classical/macroscopic point of view, but this is what the theory, which is very well backed by experiments, tells us.

 

[ChrisVer]

An electron confined in some space \Delta x = \mathcal{O}(1-100 nm) will not have definite momentum due to the uncertainty principle... it's momentum will be undefined by \Delta p = \frac{1}{1-100 nm} in natural units.
my score doesn't stand for "minus", but for "to".

You can try to calculate the "classicaly" behaved expectation value of the electron's momentum in Hydrogen atom's ground state. What you'll get will be zero... as someone expects from bound states (look at Dr Claude's post above, and the posts in the link below):
https://www.physicsforums.com/showthread.php?t=508528

 

[DrClaude]

You can try to calculate the "classicaly" behaved expectation value of the electron's momentum in Hydrogen atom's ground state. What you'll get will be zero... as someone expects from bound states (look at Dr Claude's post above, and the posts in the link below):
https://www.physicsforums.com/showthread.php?t=508528

You should however note that $\langle p^2 \rangle > 0$.

 

[Delta2]

No, far from it. I'm talking specifically about electrons in a bound state in an atom.


Again, you are stuck on the idea of "moving". Teleporting would be the same as moving, but in discrete steps. What I am saying is that the electron is in many places at the same time, not jumping from one place to the next.


The thing is that an electron in an atom also doesn't have a definite momentum, it is also in a superposition of various "fast" and "slow".

I don't want to derail this thread, so I won't go into more details, but it's one of the important conclusion of quantum mechanics. Electrons in an atom do not orbit around the nucleus, they are in diffuse orbitals, diffuse both from the point of view of regelar space and momentum space. It seems strange from the classical/macroscopic point of view, but this is what the theory, which is very well backed by experiments, tells us.

No i strongly disagree, electron is a point particle, and at a given time it occupies a single point in space and it has a definite momentum. It is just that we don't have the technology to measure with infinite degree of accuracy its position or momentum , the current theory says that its impossible to measure with infinite degree both of them, and we also don't have the theory to predict exactly its position or momentum. The best current theory give us is a probability cloud in which the electron lies, but you are going way too far ahead to assume that just because the theory give us a probability cloud, this means that actually (in the physical reality) the electron occupies simultaneously all those points in the theoretical probability cloud.

 

[Nugatory]

No i strongly disagree, electron is a point particle, and at a given time it occupies a single point in space and it has a definite momentum. It is just that we don't have the technology to measure with infinite degree of accuracy its position or momentum,

Google for "quantum tunneling" for one of the many reasons why this model is not tenable.

way too far ahead to assume that just because the theory give us a probability cloud, this means that actually (in the physical reality) the electron occupies simultaneously all those points in the theoretical probability cloud.

You are right about that, but it's also going too far to assume that the electron has a definite position within the cloud, but we just don't know what it is. It's a natural assumption, it's an almost irresistible assumption if you're thinking of particles as if they were grains of sand except much smaller, but... It's just not the way world is.

If you want to pursue this question further, it might be best to start a new thread in the quantum mechanics section.

 

[ChrisVer]

that's exactly the essence of superposition. There is no way to say that the electron is here or there, it's here and there... superposition is not just some probability due to our non-knowledge of the system in consideration (where you could say that it's of course somewhere but I don't know where)... it is in the particles' nature.

 

[mfb]

No i strongly disagree, electron is a point particle, and at a given time it occupies a single point in space and it has a definite momentum. It is just that we don't have the technology to measure with infinite degree of accuracy its position or momentum , the current theory says that its impossible to measure with infinite degree both of them, and we also don't have the theory to predict exactly its position or momentum. The best current theory give us is a probability cloud in which the electron lies, but you are going way too far ahead to assume that just because the theory give us a probability cloud, this means that actually (in the physical reality) the electron occupies simultaneously all those points in the theoretical probability cloud.

It is not a measurement issue, the uncertainty is an intrinsic property of the objects. There is no way to explain the quantum-mechanical effects with particles at specific positions.*


*technical detail: The de-Broglie-Bohm theory has specific positions, but those don't do anything, the physical interactions are done by the pilot waves.

 

[Delta2]

It is not a measurement issue, the uncertainty is an intrinsic property of the objects. There is no way to explain the quantum-mechanical effects with particles at specific positions.*


*technical detail: The de-Broglie-Bohm theory has specific positions, but those don't do anything, the physical interactions are done by the pilot waves.

Cant be explained if we assume that the motion of the particles is not deterministic, or speaking more generally that the time evolution of a system is not deterministic? that is, given two time intervals [t1,t1+t] and [t2,t2+t] such that all the fields of interest take exactly the same values (that is for any field E, E(t1+x)=E(t2+x) 0<x<t) and the initial momentum of particle p(t1)=p(t2) yet the path the particle takes is not the same in the two time intervals. This wouldn't necessarily mean that the laws of the universe are different in each time interval.

 

[mfb]

Cant be explained if we assume that the motion of the particles is not deterministic, or speaking more generally that the time evolution of a system is not deterministic?

The evolution of a quantum-mechanical system is always deterministic and there are even deterministic interpretations of the application of quantum mechanics to our everyday world, but that's not the point.

Things like the double-slit experiment with single electrons just would not work if the electron would be at a specific point and nowhere else for every moment in time.

that is, given two time intervals [t1,t1+t] and [t2,t2+t] such that all the fields of interest take exactly the same values (that is for any field E, E(t1+x)=E(t2+x) 0<x<t) and the initial momentum of particle p(t1)=p(t2) yet the path the particle takes is not the same in the two time intervals.

There is no well-defined "path" a particle takes.

 

[Delta2]

.

There is no well-defined "path" a particle takes.

Whenever we do a measurement, the particle seems to be at a specific position, i suppose if we could do measurements at a sequence of times t_n=t_0+n\epsilon with ε arbitrary small it would seem that the particle has a well defined path.

Is there something particular in the measurement process or the measurement device that causes the particle to take a well defined path? Dont tell me welcome to the big mystery of QM, the measurement process and the wave function collapse :D.

 

[mfb]

Whenever we do a measurement, the particle seems to be at a specific position

No, that depends on the type of measurement you do. For other measurements, the particle might have a specific momentum (but not position), or specific polarization (but not position, or momentum), or whatever.

, i suppose if we could do measurements at a sequence of times t_n=t_0+n\epsilon with ε arbitrary small it would seem that the particle has a well defined path.

Making ε small will lead to an observed pattern that has nothing to do with the initial propagation of an unobserved particle - for example, you won't observe the electron bound to an atom any more if you measure its position frequent enough (with suitable methods), or you don't get double-slit patterns and so on.

Is there something particular in the measurement process or the measurement device that causes the particle to take a well defined path? Dont tell me welcome to the big mystery of QM, the measurement process and the wave function collapse :D.

But this is exactly the case. Note that there are interpretations without wave function collapses.