# Electron/Proton Attraction

1. Oct 13, 2009

### Fuz

Hello again PF peeps! I'm sorry if this might sound like the unoriginal question of the day, but its been bothering me for quite some time now. Here it is:

Lets choose hydrogen to keep things simple... Why doesn't the electron of hydrogen spiral into the proton? If the EM force is supposed to attract opposite poles, then what is keeping the electron apart from the nucleus of hydrogen? Or any element for that matter.

I hope my question is worthy!

Fuz

2. Oct 14, 2009

### Staff: Mentor

Why doesn't Earth fall into the Sun?

In fact these two cases are completely unrelated, but it should be obvious for you that electron not falling on the nucleus is not that strange.

3. Oct 14, 2009

### Fuz

Well I really don't see why the Earth wouldn't fall into the sun, sadly.... Could someone please explain.

4. Oct 14, 2009

### alxm

We're supposed to explain your own question to you?

There's a valid (classical physical) argument to be made for why an electron should spiral into the nucleus, but the fact that they attract each other isn't it. If you don't understand the question, there's no point in giving the answer.

5. Oct 14, 2009

### pzona

The electron is moving isn't it? Without getting into the technical details, the fact that the orbiting body is, well, orbiting should give you a pretty good idea of why it doesn't fall into the central body. Have you ever taken a course on physics or astronomy? If not, these would definitely help out your understanding of this idea.

6. Oct 14, 2009

### Bohrok

Last edited by a moderator: Apr 24, 2017
7. Oct 14, 2009

### Fuz

What? I understand my own question... I'm asking if someone is kind enough to explain why the electron doesn't fall into the nucleus of its own atom.

Ok. I'm going to rephrase the question. Why DOES the electron orbit the nucleus?

8. Oct 14, 2009

### alxm

No, I don't think you do understand the question.

- Why wouldn't the electron be able to orbit the nucleus without falling in?
- What if the electron did enter the nucleus? (which they in fact do all the time) Why would that be a problem?
- Are you aware that this question, in its historical context, was rhetorical?

It was an objection to a particular model of the atom, where the electron would have spiraled into the nucleus. Which was obviously wrong (and would have been a problem in that model). The question was really: "What's wrong with this model?"

Without the context, the question doesn't make sense.

9. Oct 14, 2009

### Fuz

Slow down dude. I'm asking a simple question here. Why does the electron orbit the atom? This was never meant to be a cat fight.

10. Oct 14, 2009

### pzona

What everyone is trying to do is get you to answer your own question. It's better if you can do this by reasoning and thinking about it; you'll usually understand it better if you can answer a question yourself. Let's start off by asking the simple questions. Why would the electron fall into the nucleus? I realize you don't know, but try to answer the question anyway. What do you know about electron orbits? What types of forces are involved in the orbit itself, ignoring the positive nuclear charge?

No one here wants to start fights, but simply answering a question like this will probably just give rise to more questions, and we want to make sure you understand it thoroughly. Hint: think about the uncertainty principle. Would this be violated if the electron fell into the nucleus?

11. Oct 14, 2009

### Fuz

Hmmm... The electron would fall into the nucleus because the proton is positively charged and the electron is negativity charged. They are attracted to each other by the electromagnetic force. I think centripetal force (or centrifugal. I'm not sure which one) is keeping the electron in orbit instead of falling into the proton. I'm not sure why the electron is orbiting in the first place though.

12. Oct 14, 2009

### Ygggdrasil

1) How does the Earth revolve around the sun?
Here, you have to remember that the Earth is moving at great speeds. It is this movement that keeps the Earth from falling into the sun. Now recall Newton's first law of motion: inertia, a body in motion tends to stay in motion. Because of inertia, the Earth would like to move in a straight line. However, due to the gravitational force between the Earth and the sun, the Earth is constantly "falling" toward the sun. This gravitational force curves the Earth's preferred straight line of motion into a circle. You can think of this as the sideways motion of the Earth keeping it from ever reaching the sun (see http://en.wikipedia.org/wiki/Orbit#Understanding_orbits for a better explanation).

Early models of the atom were based on this "planetary orbit" model. Electrons travel very fast and their fast speeds allow them to orbit the nucleus like planets orbit the sun.

2) Why is this model is incorrect?
Now, if you have studied electricity and magnetism, you should see a problem with this planetary model. The electron is traveling in a circle which means that it is constantly accelerating (note: although it is not changing speed, it is changing velocity because velocity accounts for the direction of the electron as well as its speed), and the electron has a charge. From the laws of electricity and magnetism, we know that accelerating charged particles should radiate energy (in the form of electromagnetic radiation). Because energy is conserved, this radiated energy would decrease the speed of the electron (lowering its kinetic energy), causing it to slowly spiral into the nucleus. So, we're back where we started, classical physics predicts that an electron will fall into the nucleus!

3) WTF?! What's really going on?
The real explanation for why an electron does not fall into the nucleus comes from a fundamental concept in quantum mechanics: the Heisenberg uncertainty principle. Put simply, it states that you cannot know the position and momentum of a particle simultaneously. More rigorously stated, the product of the uncertainty of the position of a particle (Δx) and the uncertainty of its momentum (Δp) must be greater than a specified value:

$$\Delta x \Delta p \geq \frac{\hbar}{2}$$

Now, as the electron approaches the nucleus, it's uncertainty in position decreases (if the electron is 10nm away from the nucleus, it could be anywhere within a spherical shell of radius 10nm, but if the electron is only 0.1nm away from the nucleus, that area is greatly reduced). According to the Heisenberg uncertainty principle, if you decrease the uncertainty of the electrons position, the uncertainty in its momentum must increase. This increased momentum uncertainty means that the electron will be moving away from the nucleus faster, on average.

Put another way, if we do know that at one instant, that the electron is right on top of the nucleus, we lose all information about where the electron will be at the next instant. It could stay at the nucleus, it could be slightly to the left or to the right, or it could very likely be very far away from the nucleus. Therefore, because of the the uncertainty principle it is impossible for the electron to fall into the nucleus and stay in the nucleus.

Last edited: Oct 14, 2009
13. Oct 14, 2009

### Fuz

Wow. That was an awesome reply! Thanks Ygggdrasil!

14. Oct 15, 2009

### leroyjenkens

That seems kinda weird. Like the electron realizes its uncertainty is decreasing, so it makes a conscious effort to increase it, so that humans don't catch it.

15. Oct 15, 2009

### pzona

Yeah, it is really weird. It's one of the stranger things you'll find happening inside an atom.