## Physics Question #1243

Shaun Carney, a 17 year old male from the Internet asks on February 5, 2003,

What keeps electrons from eventually falling into the nucleus of an atom if they are attracted by the protons?

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### Barry Shell answered on February 5, 2003

To understand the reason, you first have to realize that at the scale of an atom things are not like what we see or experience in our full-sized world. The common view of an atom consisting of a nucleus with electrons flying around it in neat orbits like the planets orbit the sun is not quite right. Atoms positively do not look like this at all. Not one bit. There was a period in time early in the last century when scientists thought maybe they did, but since about the 1920's this idea has been replaced. Frankly, we humans in our world of ordinary objects simply cannot accurately picture what an atom looks like.

Fortunately, modern quantum mechanics has come up with very accurate mathematical explanations that describe atoms extremely well. The results are lots of wonderful things like lasers, cell phones and computers. So we "know" what an atom is and how it behaves, but we have a lot of trouble imagining how it appears.

The world of quantum physics is not "normal". Without going into the math (which proves everything perfectly) you just have to trust the following description. The electron exists as both a "thing" like a little ball, but also as a "cloud", a "wave", a "distribution" or a "probability". Think of this particle/wave duality as the way you probably love and hate your parents at the same time. It sounds weird and impossible, but for most teenagers it's a fact of life. Well, for electrons this wave particle duality is also a fact of life so it's better to picture electrons as clouds of energy that can also be particles, or to put it another way: as existing in a quantum state.

So to answer your question: within these quantum energy states, the energy manifests itself as momentum, or to think of it another way, "the energy that keeps you going". For instance, if you are on a skateboard and you are going into a half-pipe, you get some momentum going down and this momentum stays with you, giving you the energy to get up the other side of the half-pipe. Well, electrons have momentum like this, and it's this momentum that keeps them from falling into the nucleus, just like your momentum on the skateboard keeps you from stopping at the bottom of the half-pipe. This energy is enough to counteract the positive/negative attraction of protons for the electrons, too. Think of it like a puddle at the bottom of the half-pipe. It might catch your wheels for a bit, but your momentum is more than enough to keep you going, so like the electron, you don't stop at the bottom of the half-pipe. In the electron's world, there's no wheels or friction, and everything is balanced, so the electron (or at least its energy wave) just keeps zooming around the nucleus, as you would keep zooming up and down the half-pipe if you had frictionless wheels and perfect balance.

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