J. J. Thomson, director of the famed Cavendish Laboratory in Cambridge, England, had discovered the electron in 1897, as noted above. At the time, there was no clear picture of the structure of an atom, but if it had electrons that had negative charge, then by the laws of electricity it had to have positive charges that would neutralize them, since there was no net charge in atoms. Thomson postulated a model of the atom that was called the “plum pudding” model, which consisted of electrons (the raisins) scattered throughout a sea of positive charge (the pudding) so that all of the charges would cancel out.

Ernest Rutherford grew up in the rough frontier of New Zealand and began his scientific career studying the new field of radio waves at the University of New Zealand. In 1895 he accepted a scholarship to work under J. J. Thomson in the Cavendish Laboratory, where he wanted to commercialize his work with radio waves. But the discoveries of X-rays by Rontgen and radioactivity by Becquerel and the Curies changed the focus of Thomson’s lab to these strange new radia­tions. Thomson subsequently discovered the electron, and Rutherford left the

Cavendish lab in 1899 to start his own lab at McGill University in Canada, where he began his work on seminal discoveries that led to an understanding of the nucleus of the atom (9, 10).

Rutherford analyzed the radiation emitted by the radioactive elements ura­nium, radium, and thorium and discovered that there were two types of radiation particles emitted, which he named alpha (a) and beta (p). Alpha radiation was readily absorbed by a sheet of paper while p radiation could penetrate through solid objects. He subsequently determined that a particles were identical to the helium atomic nucleus and were positively charged, while p particles were identi­cal to the electron discovered by Thomson and were negatively charged. A French physicist, Paul Villard, subsequently discovered a third type of radiation that was similar to the X-rays discovered by Rontgen, called gamma (y) radiation (10).

While studying the radioactivity given off by thorium, Rutherford discovered that a radioactive gas was formed. He collared a colleague at McGill, the chem­ist Frederick Soddy, to help him analyze the gas, and they determined that it was radon (11). The only possible conclusion was that the radioactive element, thorium, was slowly turning itself into radon by emitting an a particle from the nucleus, a process called transmutation. They studied numerous radioactive ele­ments and determined that different ones decayed into new elements at different rates by emitting a and p particles. Each radioactive element lost half of its radio­activity in a specific time that varied greatly among the elements. They called this the half-life of the radioactive element, and it became a way to distinguish differ­ent radioactive decays. There were different variations of a radioactive element that had different half-lives, and they called these different variations isotopes (10). Exactly what an isotope is will be clear later.

Rutherford moved back to England to take up a position at the University of Manchester in the spring of 1907. He was fascinated by a particle radiation and he worked with the German physicist Hans Geiger to develop detectors that could measure a single a particle. Geiger subsequently developed the modern Geiger counter, which is extremely useful in detecting radiation. Rutherford thought he could study the structure of atoms by bombarding them with a par­ticles emitted from a radioactive source such as thorium. He made thin foils of heavy elements such as gold and measured the scattering (deflection) of a particles as they moved through the foil. On a whim, he told an undergradu­ate student, Ernest Marsden, to measure the scatter of a particles in a backward direction. Rutherford was never sure why he assigned Marsden this “damn fool experiment,” but it was another one of those serendipitous moments in science (7). To everyone’s complete surprise, Marsden actually measured a particles scat­tering backward from the gold foil. Rutherford was astonished. “It was quite the most incredible event that had ever happened to me in my life. It was almost as incredible as if you fired a 15 inch shell at a piece of tissue paper and it came back and hit you”(10) He concluded that the positive charges of an atom must be clustered in a very small volume with electrons circulating around them and the scattered a particles occasionally hit a nucleus nearly head-on and bounced back as if you rolled a pool ball into a bowling ball. When he published this finding in 1911, it ended the idea of the “plum pudding” model for an atom and led to the modern concept of the nuclear atom with its mass clustered in a tiny nucleus with electrons circulating around it (9).

But there were a couple of big problems with this nuclear model of the atom. According to classical physics, with all of the positive charge concentrated in the nucleus, the electrons would emit electromagnetic radiation and lose energy as they orbited the nucleus and would eventually fall into the nucleus, much like a satellite circling the earth will eventually slow and fall to earth. Furthermore, as Rutherford later demonstrated, the positive charge in the nucleus is made up of particles called protons. How could a stable atom exist with all of its charge in the nucleus, as Rutherford demonstrated? The positive charges on the protons would push them apart, and the negative electrons should fall into the nucleus. The idea of the quantum once again intruded into classical physics.