Ernest Rutherford’s 1911 experiments with gold foil discovered that the atom’s positive charge is concentrated in one place – a nucleus, as we now know it. Although Rutherford did not personally conduct the experiment (his assistants, Ernest Marsden and Hans Geiger, of “Geiger Counter” fame, actually carried it out), he did design and direct it. The groundbreaking results ensured that the “Rutherford model”, as it was then known, gained quick, widespread acceptance in the scientific community.
Rutherford’s goal had not been to revolutionize chemistry. He was one of the greatest experimentalists of his time, but this experiment was merely intended to study the distribution of charge in the atom. The logic was that if an alpha particle with a charge of 2+ is fired at a super-thin gold foil (on the order of micrometers), the degree to which it is thrown off course allows you to reverse-engineer the structure of the atom that it passed through.
Rutherford was in for a big surprise when he found that the vast majority of his alpha particles went undisturbed, but that a select few came zooming straight back at the alpha emitter. He would later describe this as being like firing a cannon at tissue paper, and having the cannon ball fly back in your face. His reverse engineering told him that there was only one possible explanation: the atom’s positive charge was all at one location. And so the nucleus was discovered.
Rutherford’s results flew in the face of contemporary science. The “plum pudding” model of the atom, which dictated that protons and electrons are spread out evenly throughout the atom (like fruit n a plum pudding), was in complete contradiction to Rutherford’s findings. It was not long before the new way prevailed, and Rutherford had the brand-new “nuclear model” of the atom named after himself. Of course, the great Danish theorist Niels Bohr would unseat him barely a decade later – but that’s another story.
Rutherford’s model treats the atom as a solar system, of sorts. It argued (incorrectly, as Bohr and Louis de Broglie would later show) that the nucleus acted as a sun, located at the center. Instead of using gravity to attract the electrons, it relied on the electric force. The electrons orbited the nucleus, zipping around at speeds where the inward pull of the electric force equaled the outward pull of the centripetal force.
Even to this day, some of the fundamental ideas of Rutherford’s model are still used. Modern knowledge of the nucleus is not all that much better than it was at Rutherford’s time, though neutrons have been added to the modern model. Also, the idea that the electrons move in “orbitals” is, to some limited extent, still relevant, and is an especially useful mental tool for those who cannot visualize quantum wavefunctions.
