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Geissler/Crooks/Thompson: The Discovery of Electrons
Read Pages 228 to 233
A glass tube filled with air or another gas will
not conduct electricity. The tube has metal
electrodes at each end which are attached to
a high voltage source of electricity. An
ammeter measures the electric current.
If the air pressure inside the glass tube is
reduced, the gas will conduct electricity and
the gas inside the tube glows. The colour of
the light given off depends on the type of gas.
It does not matter what metal the electrodes
are made out of, the effect is the same.
If the air pressure is reduced almost to zero,
the gas still conducts electricity, but only the
glass itself at the positive end of the tube
glows.
If a small paddle wheel is placed in the glass
tube, it rotates when the battery is
connected. In a separate experiment, an
object in the tube cast a shadow in the glow
at the positive end of the tube.
Conclusion
A particle is given off by the negative metal electrode. It must be a particle because it makes the paddle
wheel rotate. It must be negative because it moves from the negative electrode to the positive electrode
(opposite charges attract and like charges repel). It is easy to remove from the atoms of the electrode
because only electric voltage is needed to remove it. Since the particles are given off by the metal of the
electrodes, they must be a component of the atoms of the metal.
Rutherford: Discovery of the Nucleus
Read Pages 238 to 240
By the early 1900s, people knew that atoms were made of electrons (negative charge) and protons
(positive charge), but it was thought that the electrons and protons were mixed together into a single
mass.
Radioactivity also had been discovered by this time. Ernst Rutherford conducted exhaustive studies of
radioactivity. He found that there were three types of radiation given off by radioactive material:
alpha particles: heavy particles with a positive charge.
beta particles: light particles with a negative charge.
gamma rays: not a particle, but a type of light with a much higher frequency.
In 1909, Ernest Rutherford devised an experiment to probe the structure of the atom using a beam of
alpha particles given off by a radioactive source. The alpha particles were shot through a thin foil made
of Gold. An alpha particle detector was placed around the gold foil to determine what happened to the
alpha particles after they passed through the Gold foil.
Much to his
amazement, most
of the alpha
particles passed
through the Gold
(which is a very
dense material)
as if nothing was
there. However, a few were deflected off to the side, and a smaller number even bounced back towards
the source. This was pretty impressive since alpha particles are very fast moving and the gold foil was
very thin. It was rather like firing a rifle at a sheet of paper and having the bullet bounce back off the
paper.
Conclusions
The conclusions were that atoms must be mostly empty space with a small, very massive core. Since the
positive alpha particles were deflected or repelled by this small core, the core also must have a positive
charge.
The Rutherford model differs from the Thompson Model in that:
-
the electrons are separate from the positive core of the atom
electrons orbit around the nucleus
almost all of the mass of the atom is concentrated in a very small nucleus
all of the positive charges are in the nucleus
atoms are mostly empty space with small electrons moving through this space
Many scientists began investigating atoms using methods similar to Rutherford’s as well
as other methods. One of the most important was the work of Henry Moseley.
Henry Moseley: The Significance of the Atomic Number
Henry Moseley, a student of Rutherford, bombarded samples of different elements with X-rays
and was able, through some pretty complicated math, that the atomic number of an element
must equal the number of protons in the nucleus. Remember, that the atomic number was only a
cataloguing number in the periodic table.
This conclusion was backed up by several other different experiments, such as the charges that
result when electrons are removed from atoms (if you can only remove so many electrons from a
neutral atom, must that number also equal the number of protons in the nucleus?)
The Neutron
If the number of protons in the nucleus increases by one as you from one element to the next in
the periodic table, why does the relative atomic mass not increase by one as you go from one
element to the next in the periodic table?
This was a very important question.
Since the relative atomic mass does not increase by one from one element to the next, there
must be another particle in the nucleus that has about the same mass as the proton, but does
not have a charge - in other words, a neutral particle. This particle was called the Neutron.
We now have independent evidence of the existence of neutrons. They seem to help hold all of
the positive protons in the nucleus together. Stable nuclei (stable atoms) have about the same
number of neutrons and protons, although larger nuclei seem to need proportionally more
neutrons. Nuclei with either too many neutrons or not enough neutrons emit radiation as they
either fall apart into several smaller nuclei (nuclear fission) or eject a
small number of particles to become more stable.