Flexi Says: Quantum mechanics is needed to explain why electrons don't usually fall into the nucleus. Unlike large objects, which obey classical mechanics, electrons do not lose energy as they orbit. In addition, electrons don't gain or lose energy in a gradual, continuous manner like objects do as they slow down. Rather, the amount of energy that electrons have can only change in discrete steps. The lowest energy step keeps electrons in the lowest orbital outside the nucleus. Note that in some heavy radioactive elements, an electron can occasionally combine with a proton to form a neutron in a type of radioactive decay called electron capture.
