Why are electrons so far away from the nucleus of an atom?
Electrons are indeed far away from the nucleus! If we could magnify the simplest hydrogen atom to the size of the earth, then the nucleus (a proton) would only be about the size of a basketball. It would be at the very center of the earth and that lonely electron would be found somewhere out in earth's atmosphere. All of the space in between the electron and the basketball-size nucleus is empty! (See Our Home In The Universe for other interesting comparisons.)
But why the electron is so far from the nucleus is no more mysterious than some of your everyday experiences. Imagine that you tied a baseball to the end of a long rubber band. You hold the other end of the rubber band in your hand and start swinging the ball around in a circle. The faster the ball goes, the more the rubber band stretches and the farther away is the baseball. In an atom, an electron is attracted to the nucleus by the "electromagnetic force", similar to your rubber band. Like your baseball, the faster the electron goes, the farther away from the nucleus it is. The electrons in an atom are moving pretty fast, so they are far away from the nucleus. If you dig a little deeper into the problem you will discover some differences, though. Unlike the rubber band which pulls harder on the ball as it is stretched more and more, the electromagnetic force gets WEAKER as the electron and nucleus get farther apart. This does not change the basic idea. It's as if the rubber band never breaks, but just stretches more easily the longer it becomes. Another difference is that nature only allows atomic electrons to have certain amounts of energy - they can only go at certain speeds. The science that explains this is called "quantum mechanics". The SLOWEST allowed speed is the one that puts that electron 8,000 miles away from the basketball-sized nucleus! It could have MORE energy and be even farther away!
Howard Fenker, Staff Scientist (Other answers by Howard Fenker)