Ernest Rutherford

Ernest Rutherford (born August 30, 1871, Nelson, New Zealand and died October 19, 1937, Cambridge, England) was the first person to split an atom and as the world’s first successful “alchemist,” changed nitrogen into oxygen. He discerned that radioactivity is the product of subatomic change and thereby ended the chapter on Aristotelian physics which held that all matter is immutable and unchanging. He postulated the existence of the proton and the neutron, naming them both, discovered and named alpha and beta particles emitted by radioactive materials, discovered radon (a radioactive gas), tritium (H³) and He³, and he coined the phrase half-life. He provided definitive evidence for the extreme age of the Earth, invented the Geiger Counter with Hans Geiger (actually it is the Geiger-Rutherford detector of single ionizing particles), and provided one of the first theoretical models of the atom.

His list of practical applications is equally as impressive. He built one of the first radio receivers and at one time held the world record in distance over which `wireless' waves were detected, designed a time-apparatus capable of measuring time intervals of a hundred-thousandth of a second, designed and built the first modern smoke detector, as a pioneer in acoustic submarine detection, with W H Bragg, patented the first apparatus for determining the direction of submarine sound.

His list of students includes many of those who made some of the most remarkable breakthroughs in science, including Niels Bohr, James Chadwick, the discoverer of the neutron which Rutherford predicted, and John Douglas Cockcroft and Ernest Thomas Sinton Walton who pioneered work on the transmutation of atomic nuclei by artificially accelerated atomic particles.

He was awarded the Nobel Prize in Chemistry in 1908

Ernest Rutherford is buried at Westminster Abbey just west of Sir Isaac Newton's tomb and next to Lord Kelvin’s.

The Gold Foil Experiment

Between 1906 and 1909, Rutherford, Hans Geiger and Ernst Marsden conducted diffraction experiments using a sheet of gold foil through which were fired alpha particles (helium nuclei containing two positive charges). On the opposite side of the gold foil was a zinc-sulfide screen which would emit a flash of light when struck by the alpha particles. They expected that some of the alpha particles would be slightly deflected and thereby strike the zinc-sulfide screen at slightly varied angles from the point of emission due to repulsion by weak charges in the gold atoms. What they discovered were very large angles of diffraction and in some cases the alpha particles were bounced back in the direction they came from.

Alpha particles are approximately 8000 times as massive as an electron and would strike the foil at very high velocities. The severe angles and the backscatter made it clear that there were very strong forces present to create such drastic deflection. Rutherford's theory to explain this was that most of the mass of an atom is in a compact nucleus with a positive charge and electrons filled the bulk of the atom's space in orbit around the nucleus.

If the atom is composed largely of empty space, Rutherford postulated that most of the alpha particles would pass through without deviation from their straight trajectory. Only those that directly collided with the massive nucleus were deflected or scattered backwards. With this model he significantly changed the model of the atom.

Rutherford Model of the Atom

Prior to Rutherford’s work on the model of the atom, the prevailing model was Thomson’s "plum pudding" model. Thomson’s model held that positive and negative charges were evenly distributed in a sphere, not unlike raisins in plum pudding. Rutherford’s Gold Foil experiments, which showed what is commonly called Rutherford Scattering, indicated that rather than an even distribution, the atom was largely empty space with a massive nucleus that held a strong charge. His answer was to postulate a solar system type of model for the atom with negatively charged and very light electrons orbiting a massive, positively charged nucleus. However, as is true to some extent in our solar system, the electron orbits are not limited to an orbital plane.

One problem with this model is that orbiting electrons must by definition be accelerating as they orbit if they are to remain in orbit. In the process of accelerating they must also necessarily radiate energy. This in turn means that charged particles like electrons must lose energy and their orbits would deteriorate and they would spiral into the nucleus. In 1915, Ernest’s student, Niels Bohr improved the model by postulating quantization: the electrons could only lose energy in jumps or quanta so that the electrons cannot lose energy continuously. There are only certain orbits and nothing in between. While the model did not resolve other issues, it had a better correspondence with data known at the time.