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Q & A: Why don't electrons fall into the nucleus?

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Q:
Why doesn't the electron get sucked into the nucleus since the nucleus is positive and the electron is negative?
- matt (age 16)
mentor, oh, usa
A:
Matt -

That's a really great question!


The picture we often have of electrons as small objects circling a nucleus in well defined "orbits" is actually quite wrong. The positions of these electrons at any given time are not well-defined, but we CAN figure out the volume of space where we are likely to find a given electron if we do an experiment to look. For example, the electron in a hydrogen atom likes to occupy a spherical volume surrounding the proton. If you think of the proton as a grain of salt, then the electron is about equally likely to be found anywhere inside a ten foot radius sphere surrounding this grain, kind of like a cloud.

The weird thing about that cloud is that its spread in space is related to the spread of possible momenta (or velocities) of the electron. So here's the key point, which we won't pretend to explain here. The more squashed in the cloud gets, the more spread out the range of momenta has to get. That's called Heisenberg's uncertainty principle. Big momenta mean big kinetic energies. So the cloud can lower its potential energy by squishing in closer to the nucleus, but when it squishes in too far its kinetic energy goes up more than its potential energy goes down. So it settles at a happy medium, and that gives the cloud and thus the atom its size.

We've actually already answered a very similar question: try checking out Why do Electrons Move? for some more details -Tamara

(republished on 07/21/06)

Follow-Up #1: Does the electron have a position?

Q:
But if we are saying that there is a probability of finding something here or there, this means that it must be somewhere at a definite place, it is just that we do not have the means to know without errors where is it. So if the position is definied but cant be determined, then how actually is the motion of the electron. I dont want to determine where it is, what is its momentum etc.
- Amol Bhave (age 16)
Jabalpur, Madhya Pradesh, India
A:
Your interpretation is what common sense would initially lead us to believe. However, we know that it isn't true. The electron is truly spread out. If these quantum variables (such as the electron position) that seem to be spread out had actual hidden values, then a set of experimental predictions known as the Bell Inequalities would be obeyed. In actual experiments the Bell Inequalities are consistently violated. Therefore the spread-out variables really are spread out.

The process of interacting with a large apparatus that's sensitive to where the electron is causes the formation of a state in which the electron's cloud has much less spread. This occurs via a quantum process called decoherence. There's no consensus about whether a single such state arises or a collection of states covering the whole range of possibilities.

Mike W.

(published on 12/05/11)

Follow-Up #2: electron cloud in atom

Q:
So according to you, the electrons will be present mostly in the volume where the kinetic & potential energy are almost same.Does the electron move in a circular path in this region?if so why doesn't it radiate energy?
- ashwin gopal (age 16)
ernakulam,KERALA,INDIA
A:

The electron is present as a cloud. Averaged over the cloud, the positive kinetic energy is half as big as the negative potential energy.

More importantly, the cloud really is the state of the electron. It's not a picture of where some dot-like particle probably is. It isn't anywhere in particular. It also doesn't have any particular velocity.  In a hydrogen atom, it's certainly not going in a circle. The cloud doesn't go anywhere at all. There's no reason for it to radiate.

The world at a small scale cannot be put together out of anything like the pictures we're used to at a large scale.

Mike W.


(published on 08/02/13)

Follow-Up #3: what are electrons doing?

Q:
I want to ask about the electron. I love quantum mechanic and astrophysics. I've heard about this electron cloud and I google it for a while. I think I get the message, that is - At any given moment electron can be found at random place in the electron cloud (described with the wave function). What I don't understand is how you can see the electron and determine his position? And if you take snapshot how can you be sure that the electron doesn't move around this cloud? I mean, the electron must be moving in some pattern way but I think that he is so fast that we think that he is at many places at the same time or he simply disappear on one place and show up at another. Can this be true? Can you tell me the cutting edge, there must be something more than just saying that the electron can be everywhere and nowhere. Thank you!
- Alexander (age 21)
Skopje
A:

Alexander- Your question has a lot in common with many others, so I've put it in a thread.

There are a variety of ways that an electron, which is always a spread-out wave,  can be found to be in a smaller-than-usual region. Maybe it bumps into a piece of film, which localizes the wave to the region of one little bit of silver.  Maybe the electron just got emitted from the fine tip of a needle, so you know it's very close to that tip. Maybe a high-energy gamma ray bounces off of it, so that the position of the electron can be nearly determined from the gamma ray position and direction.

As we discuss at numerous points (search for "Bell Inequality") the ranges of positions and of velocities are not due to our ignorance of some fast rattling around, but are a description of what the electron wave actually is.  It's not that it is "in" the cloud, it is the cloud. Once the electron cloud has a small size than you can be sure that the electron is rapidly moving, in a broad range of directions, so that cloud will start to spread out.  If the cloud is already highly spread out, the electron may not be moving much. This relation is called the Heisenberg Uncertainty Principle.  Things people wrote about it before the Bell Inequality violations were established are generally outdated. They are still usually repeated in popular presentations and in some textbooks.

Mike W.


(published on 09/10/13)

Follow-Up #4: particle clouds

Q:
So from reading your thread where you answer several questions relating to the position of the electron in which you say that the electron has no defined measureable location but intstead exists as a wave probability function a set of positions in space where you might find the electron each position with a certain probability and that the electron doesn't exist, even from an absolute perspective, at any one of these points at any given moment but as all of these points at once in a sort of cloud. This much I understand but what I don't understand is why, why is it that an electron has this cloud like existance instead of a definite location I heard you mention the Heisenberg uncertainty principal that the more you know about a particles velocity the less you know about its position or visa versa but why is this that an electron exists as a cloud whereas on a macroscopic scale I have certainty of position and velocity, why is an electron a quantum wave probability function cloud thingy?
- Eric Fernandez (age 16)
Helena, MT, United States
A:

We don't really know why the world is made up of quantum waves rather than of little dot-like things, or some other possibility. That's just how it is. The Heisenberg uncertainty principle isn't really the reason for the quantum form of things, just one of the many mathematical consequences.

The thing that has to be explained, in a world made of quantum objects, is not why little things are fuzzy. That's just part of the basic description. What we have to explain is why we experience a world where big things have fairly definite positions. They do still have, so far as we can tell, a little quantum fuzz, but it just isn't very noticeable on a big scale. What we have trouble talking about is why we see no Schrödinger cats that are both alive and dead. That's the issue addressed, more or less, by various interpretations of quantum mechanics.

Mike W.


(published on 10/31/13)

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