Discovering the Nucleus


In 1909, Ernest Rutherford and his two colleagues, Hans Geiger (more commonly know for his work with radioactivity; hence the Geiger Counter) and Ernest Marsden developed a very important experiment. Their experiment consisted of shooting the newly discovered alpha particles (positive particles made of two protons and two neutrons ejected during radioactive decay) at a piece of gold foil. They set up fluorescent screens to detect the particles, dimmed the lights, and let the alpha particles fly. What they discovered was that the majority of the alpha particles went right through the foil, occasionally being deflected a bit. The real surprise came when some of the alpha particles ricocheted right back at the source. Rutherford compared this to shelling a piece of tissue paper and being hit by your own reflected shots.
To sum it all up, Rutherford theorized that the alpha particles must be occasionally hitting other positive particles. By the statistics gathered, he found that the positive charge must be concentrated in the center of an electron shell. The startling thing is that there is a vast amount of empty space between the electrons and the positive nucleus. If the nucleus was the size of a pin-head in the center of St. Paul’s Cathedral, the electron cloud would be almost 100 meters away in the dome. Rutherford created a new atomic model in 1911 through these observations. In his model, there is a cloud of electrons with a small, concentrated nucleus made of protons in the center.

Next
Previous