Henri Becquerel discovered that uranium gives off invisible rays that affected photographic film in 1896. The Curies built upon his work, named the phenomenon radioactivity and were also able to discover other radioactive elements. All this set the stage for Ernest Rutherford, who didn’t merely play his part, but took the scene by storm.
Born to a farmer in New Zealand, Rutherford began his graduate work by studying the effect of X-rays on different materials. With the discovery of radioactivity, however, he shifted his studies to particles emitted by uranium and its compounds.
By studying the absorption of radioactivity by thin sheets of metal foil, Rutherford was able to find at least two components of the radiation, which he named α (alpha) and β (beta). A third form of radiation, γ (gamma) rays, was soon discovered. Rutherford was also able to prove that radioactivity involved the transmutation of one element to another and also discovered the concept of radioactive half-life.
All these meant that in 1908 Rutherford won the Nobel Prize in Chemistry. In 1909, Hans Geiger and Ernest Marsden performed the Geiger-Marsden experiment, also known as Rutherford gold foil experiment, under the direction of Rutherford. This experiment sowed the seeds for what was to become nuclear physics and shattered existing notions about the atomic structure.
In this experiment, radioactive decay of radium generated a beam of alpha particles which were directed normally onto a very thin sheet of gold foil placed in an evacuated chamber. Using a zinc sulfide screen at the focus of a microscope, particles deflected at any given angle could be detected.
J. J. Thomson’s plum pudding model was the popular theory at that time. According to this, negatively charged electrons were floating or moving in a sea of positive charge, like plums in a pudding. This meant that the big alpha particles should have passed through the gold foil except for a few minor deflections. Only that, it wasn’t to be…
The actual results of the experiment showed that even though many alpha particles passed through as expected, many others were deflected and very few through angles larger than 90 degrees.
Reflecting on these results, Rutherford said, “It was quite the most incredible event that has ever happened to me in my life. It was almost incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you.”
In his paper in 1911 based on this experiment, Rutherford rejected Thomson’s model and instead proposed his own planetary model. This model comprised of a high central charge concentrated into a very small volume when compared to the atom, with this central volume also accounting for the bulk of the atomic mass.
The World War I made its way, but Rutherford’s model was here to stay. All future atomic models stemmed out of this, it helped us understand and appreciate the structure of the atom and also paved the way for dividing the indivisible. It’s safe to say that nuclear physics had made its way with a bang.
ganesh.a.s@thehindu.co.in
