© Wikimedia Commons, Public Domain. Author: Bensaccount, 10 July 2006.
In the Rutherford model of the atom, the positive charge and most of the mass of an ordinary hydrogen atom, consisting of one electron and one proton, is that of the proton, which has a linear diameter about 10-5 smaller than the size of the atom itself. Thus, the volume of the nucleus is 1015 times smaller than the volume of the entire atom. As an electron by itself has essentially no intrinsic volume, it is common to say that an atom is mostly empty space. What, then, creates and determines the actual size of the atom? We will see that the answer is quantum mechanics, which, as it turns out fortuitously, also lays out the entire structure of the periodic table. (Unit: 6)