In 1903, Ernest Rutherford (a New Zealander who worked in Cambridge, England most of his life) performed a simple and elegant experiment showing that the a-particle is the nucleus of the helium atom. Rutherford positioned one glass tube inside a second glass tube. The inner tube contained a radioactive source that emitted a-particles. The outer tube contained a vacuum and at each end there was an electrode. The a-particles passed through a thin window, picking up two electrons on the way, and entered the outer tube as a gas. When Rutherford turned on the high voltage between the electrodes, the tube emitted light at very specific wavelengths (specific colors). He compared wavelengths of this light with the wavelengths of light produced by a similar tube that he had filled with helium gas. The colors of the light were identifical. Rutherford concluded that an a-particle is simply the nucleus of a helium atom and that when the a-particles reach the outermost tube they have picked up two electrons to become helium atoms.
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