© Wikimedia Commons, Public Domain. Author: Bensaccount, 10 July 2006.
A hydrogen atom is around one hundred thousand times larger than the proton that forms its nucleus. If we think of the electron as smeared over a spherical volume, this volume is enormous compared to the proton. This volume is the result of balancing the electron's electric attraction to the proton with the high kinetic energy that results from confining the electron in a small region of space. In essence, it is the Heisenberg uncertainty principle that determines the size of this atom. (Unit: 5)