Click here to
Atomic Structure
The Discovery of ...
... Protons
Discovery of Protons

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


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.

Getting fuzzy

But when a small amount of air was introduced into the apparatus, the outline caused by the alpha rays hitting the detector became fuzzy and blurred. Conclusion - the alpha rays were being scattered by "hitting" the molecules of gas in the air; they were solid particles!

If Rutherford covered half the slit through which the alpha rays were passing with a very, very thin piece of mica (about 0.003 cm thick), the rays were scattered as before. Conclusion - most of the alpha rays go through the solid mica, but when they hit something, once again they are deflected.

But what where they?

To answer this question Rutherford first had to isolate the particles that made up the alpha ray. To do this he used an ingenious idea. He placed some alpha emitting radioactive material in a very thin walled glass tube, and then placed this tube inside a second tube made of much thicker glass.

trapping alpha rays

.. the outer chamber has a thick glass wall that traps the alpha rays coming from the source inside the thin-walled inner chamber..

When the alpha particles left the radioactive substance, they had enough energy to pass easily through the thin glass of the first tube and into the space between the two tubes.

They did not have enough energy, however, to pass through the thicker glass of the outer tube and lost energy bouncing off the walls of the heavier container. At this lower energy state they could no longer go through the thin glass, either, so they were trapped! Rutherford had isolated the alpha particles and could now study their properties.

One thing he did was to excite the alpha particles with an electrical discharge until they positively glowed. By analyzing the lines in the glow given off by the alpha particles he deduced that they were the same as those given off by atoms of helium. Conclusion - the alpha particles were the same mass at helium.

When Rutherford passed alpha particles through a strong magnetic field the rays were bent, but in the opposite direction to that seen when electrons were used. Conclusion - alpha particles carried a positive charge that was at least twice that of a hydrogen ion, the particle that had the smallest known positive charge then known.

Since he already knew that the mass of the alpha particle had a mass of four times that of hydrogen (the mass of a helium atom), and now he knew that it had twice the charge of a hydrogen atom, he could conclude that the alpha particles he was isolating had a mass of 4 units and a positive charge of two units.

Positive rays

But this was not the end of the story. Rays that had many of the properties of alpha rays were also being seen in the cathode ray tubes used to discover electrons. These rays (variously called "channel rays", "Kanalstrahlen", or "positive rays") moved in the opposite direction to the electron, cathode, rays and would pass through metal and other objects with great ease. Conclusion - they were smaller than alpha particles.

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?

© 2003, Professor John Blamire