Rutherford/Marsden/Geiger and Empty Space
In the early twentieth century a group of British scientists led by Ernest Rutherford were experimenting with radioactive chemicals and determined that there were three different types of radioactive particles. They found that the least energetic of the particles was an alpha (α, 42He) particle, the second most energetic, a beta (β, 0-1e) particle and the most energetic, a gamma (γ, 00γ) particle. All nuclear particles begin in the nucleus and burst forth, but it is possible to focus the particles into a beam. By focusing the beams through a magnetic field, Rutherford determined that the most massive particle, the alpha particle was attracted to the negatively charged plate and must therefore be positive, the beta particle was attracted to the positively charged plate and must be negative, and the gamma particles were unaffected by the magnetic field.
While working with alpha particles, Geiger and Marsden discovered a truly remarkable fact-atoms are mostly empty space. The team was firing the alpha particles, from a polonium source, into sheets of gold foil. The gold foil was surrounded by a zinc sulfide (ZnS) screen that would fluoresce when it was struck by the positively charged alpha particles. They hypothesized that, if the Thompson model was correct, that the massive, energetic alpha particles would rip through the thin pieces of gold with only minor deflection. Early results showed that Thompson was correct, however, after further testing it was found that, despite most of the particles passing through with minor deflection as expected, several fluorescing spots appeared at sharp angles and still others were reflected back toward the source.
only plausible explanation for these results would be that the atom must have a
compact, positively charged center and be composed primarily of empty space
where the electron can move around. The
positive center was given the title nucleus.