Discovery of Protons
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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
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Alpha
Beta
Gamma
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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.
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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.
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Getting fuzzy
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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.
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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..
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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.
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Positive rays
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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.
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A new, fundamental atomic particle
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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?
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