The Discovery of the Proton

Photographic plates (negatives) contain chemicals that change their composition when exposed to light - hence photography is possible. As a result of the chemical change it is easy to detect and then permanently capture a fleeting image that only lasts a fraction of a second. However, the chemical reactions taking place among the compounds on a photographic plate can also be triggered by sources of radiation other than light, and it was this ability that led directly to important discoveries about atomic structure.

For, in the late 1800s, Henri Becquerel made a mistake and stored some photographic plates he was using to detect fluorescence in the presence of potassium uranyl sulfate (which contains uranium). To his surprise, when he developed these plates, they had strong black spots where the uranium containing chemical had touched them.

As the plates had been stored in the dark, and away from fluorescing compounds, the exposure of the photographic plate must have been caused by some new type of radiation coming from the uranium atoms in the sulfate. It was an exciting discovery that was immediately used by a newly married female scientist - Marie Curie. She was the one who suggested a name for this new phenomenon - radioactivity

Alpha, Beta and Gamma

But this new "radioactivity" was not a homogeneous form of radiation. Becquerel and Marie Curie quickly showed that some of the radiation had the properties of electrons, but there was also a second form of radiation that had much less penetrating power. The physicist Earnest Rutherford named these two kinds of radiation "beta particles" for the penetrating electron emissions, and "alpha rays" for the new, heavier form of emission. ("alpha" and "beta" are the first two letters of the Greek alphabet).

French chemist P. Villard discovered a third form of radiation which was promptly named "gamma rays" (the third letter) which had many of the properties of X-rays, but of shorter wavelength.

There were three basic ways of detecting and therefore measuring the properties of these new "alpha rays"; they could expose a photographic plate, they could cause a tiny burst of light when they hit a screen of zinc sulfide, (this is called "scintillation"), or they could leave a trail in a special chamber filled with a "cloud" of gas (actually a rapidly cooled volume of air, supersaturated with water vapor). This latter was called a "Wilson cloud chamber" and could be used to track the path that a particle took by means of the "trail" left behind (a bit like the vapor trail left behind a high flying airplane).

Earnest Rutherford coated wires with radioactive materials that were good emitters of alpha rays and tested them under a variety of conditions. If the alpha rays were first passed through a narrow slit, and then allowed to travel through a vacuum, they formed a clear, crisp rectangular outline on a detecting device at the other end. Conclusion - alpha rays traveled in straight lines.

A new, fundamental atomic particle

When these lighter rays were deflected in a magnetic field the smallest particles found had the mass of a hydrogen ion and one positive charge. This was as small as they got. Conclusion - these "positive rays" consisted of particles of matter as small as that found in a hydrogen ion, and they carried one positive charge. They must be a fundamental particle and the exact opposite of the negatively charged electron.

Rutherford named them protons from the Greek word meaning "first". People who studied atomic structure now had two fundamental particles, the proton and the electron, and they seemed made for each other. While different in masses (the proton is 1,836 times more massive than an electron), the proton and electron each carried on electrical charge (+ or -). It was beginning to look as if atoms were made of equal numbers of protons and electrons, but how were they arranged and how did their properties contribute to the very different properties of the many different types of elements and atoms?