Then, in 1911, the truly turbulent times of the subatomic era began. Sir Ernest Rutherford, a physicist from New Zealand working at Cambridge University in England, peaked inside the atom—and found that it was mostly empty, except for a tiny, extremely dense core. In his experiment, he bombarded a thin gold foil (about 500 atoms thick, or 1/20 th the size of a human hair) with a stream of alpha particles. Emitted by a radioactive source, these alpha particles were known to be much smaller than an atom. Rutherford observed the angle at which these particles were deflected when they struck the gold film. To everyone’s surprise, he found that the vast majority of alpha particles passed through undeflected, and that only a few were redirected. He concluded that the prevailing picture of the atom—a jelly-like sphere of positive charge with negatively charged electrons, much like plum-pudding—was wrong. The experiment showed that the atom consisted instead of an extremely small positively charged core–the nucleus–that carried most of the atom’s mass, surrounded by a diffuse cloud of negatively charged electrons[20]. The charge of the nucleus exactly canceled that of the electrons, so that the atom as a whole was electrically neutral (see Fig. 1).

Figure 1: Simplified view of the lithium atom. The charges (expressed in terms of the fundamental charge e on an electron) are given in parentheses. Rutherford found that the nucleus is far smaller than the atom: if an atom were magnified to the size of a football field, the nucleus would be a cherry in its center.