Quantized motion of a particle pushed around by waves

When a marble rolls down a sloped surface it eventually oscillates about a low point. With friction, it finally comes to a rest at the low point. Electrons are pulled towards an atomic nucleus like the marble is pulled downhill towards a low point by gravity. There are no frictional forces on the electron, but an electron circling around a nucleus creates radiation and these electromagnetic waves carry away energy and slow the electron down. Therefore, the electron is expected to lose energy and to spiral towards the nucleus, until it eventually strikes the nucleus. However, this does not happen in real atoms. When the electron is circling at a certain distance from the nucleus is stops radiating and continues to circulate at that distance. The theory of quantum mechanics explains this phenomenon. There are usually several such preferred distances, called quantum levels, where the electron can circulate without radiating. Therefore, when an electron comes close to a nucleus, it stays for a while at a preferred distance, then jumps to another preferred distance, and so on, till it eventually reaches the smallest preferred distance, called the ground state. At each jump it emits a burst of radiation. Because the electron does not approach the nucleus in a gradual fashion, but in jumps, the motion is called quantized motion.