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Physics
The Structure of Matter
Eric Ratcliffe
This paper was written for Dr. Austin!s Modern Physics course.
The purpose of this paper is to discuss the history of the structure matter, from
the ancient Greek view to the modern accepted view. We will also discuss
Lord Rutherford!s experiment that lead to the accepted structure and describe
a similar experiment performed on the accelerator at Auburn University.
HISTORICAL PERSPECTIVE
The Greek View
The concept of matter was first introduced by Aristotle as “something out
of which that has the capacity or potentiality to be.”[1] Another way the Greeks
explained what matter is “the immense variety of different substances forming
the world results from a combination of comparatively few simple elements.”[2]
The Greeks also determined that matter was not the smallest substance, but
that it is “formed by a very large number of very small particles”[2] which they
called atoms (Greek for indivisible [2] or unsplittable [3]).
Democritus thought that there were four elementary substances made up
of atoms, these substances were: air, water, stone and fire. He described each
of these substances with certain properties: air atoms, dryness and lightness;
water atoms, heaviness and wetness; stone atoms, heaviness and dryness;
and fire atoms, hotness and slipperiness.[2] The Greek philosophers tried to
explain various transformations of matter as resulting from different combinations of atoms having these properties. One example is their explanation of
different metals. They believed that metals were a combination of stone atoms
along with different amounts of fire atoms. These philosophers believed that
the fire atoms were the cause of each metal!s shininess; therefore, iron had
very few fire atoms in its composition since it is very dull, on the other hand,
gold had the maximum allowed fire atoms. [2] We know that the Greeks were
wrong; each metal is an elementary substance itself and is not composed of a
combination of atoms. [2]
Aristotle had a different take on matter. He used the Greek word hyle,
meaning "that out of which!, to describe matter. Hyle in Greek literally means
“forrest, wood, woodland”; a derivation of this is “wood cut down, timber”. From
here Aristotle further derived the word as timber used in the construction of
something, then he generalized this meaning to "that out of which!. [1]
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The Latin thinkers at the Hellenistic schools did not like Aristotle!s use and
meaning of the word hyle, therefore they began to think of new meaning and
word. They came up with the word materia. Materia is derived from the Latin
root ma-, "to make!, and has the meaning of "that which is used by a maker,
that out of which he makes things!. [1]
The word matter as we use it today comes from the Latin thinkers! materia. We have learned enough about the origin of matter; let us move onto the
structure of matter and atoms.
Thomson!s Early Atomic Structure
J.J. Thomson was the first to discover the electron. In 1897, Thomson
experimented with a glass tube, which he filled with a rarefied gas. On one end
of the tube he placed a cathode; Thomson placed an anode in an extension
on the side of the tube, the other end broadened to a wider area with a flat end
which he covered with a fluorescent material. This fluorescent material would
be illuminated when it was bombarded by fast-moving particles. This tube that
Thomson created is very similar to those used in televisions today. [4]
Thomson placed a metal object in the broader area of the tube and
observed that there was a shadow cast by the object. From this observation
he concluded that the particles were moving in straight lines, similar to rays
of light. [4] Next, Thomson examined the deflection of the beam of particles
caused by electric and magnetic fields. When he applied an external electric
field to the tube, the beams bended towards the positive pole of the field; this
verified that the beams of particle were negatively charged. [4]
From his experiments, Thomson was not able to determine the charge of
his newly discovered "electron!, but he was able to determine that the chargeto-mass ratio of the electron was 1.76x108 coulombs/gram. [4]
Thomson wrote in a paper, published in 1899, that the atom contains a
large number of smaller bodies. In a normal atom, the bodies are assembled
in a system that is electrically neutral. You can get the negative bodies to
detach themselves from the whole by electrifying the gas. The part left over
from the detached negative body carries a positive charge. [5] Thomson also
discovered that the atom gave off many spectrum lines and by the Zeeman
Effect, concluded that it was not just the electrons that gave off the spectrum
lines. [5]
Jeans! Theory of the "Ideal! Atom
In 1901, J.H. Jeans wrote a paper titled "The Mechanism of Radiation!,
in which he discussed his ideas on the structure of the atom [5]. He stated
that each atom is a collection of positive and negative point charges. These
charges repel and attract by the laws of inverse square of distance; these
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charges should settle to an equilibrium and then vibrate around that position,
which causes the emission of the spectrum lines. [5]
Jeans then goes on to state that Earnshaw!s Theorem proved that there
is no position in which there will exist a stable equilibrium. If this is true, the
point charges should annihilate one another, and therefore, cause the atom
to not have even existed [5]. To solve this problem, Jeans proposed that the
electron and its positive counterpart are not point charges, but that they have
size. Then he proposed that the law of inverse square would not always hold
true for all distances; but that at very close distances the two charges would
repel each other, independently of their charge. [5]
Jeans "ideal! atom would have the properties as follows. The size of the
ions in each atom is to be small enough such that the number of ions in the
atom can be considered infinite [5]. Jeans also showed that if the atom was
arranged as he described, that one should be able to determine the size of the
atom [5].
Thomson!s Theory of Atomic Structure
In 1904, Thomson formulated a model of the atomic structure in which
the atom was composed of a positively charged fluid, evenly distributed across
the whole atom, with the negative electrons randomly distributed throughout
the atom, like the seeds in watermelon [2]. This is model is the same as the
well-known "Plum-Pudding! model of the atom. Thomson, using this model,
was able to describe the emission of spectrum-lines; if the distribution of the
electrons was disturbed by an external force, the collision of two atoms for
example, the electrons would begin vibrating and trying to return to an equilibrium position, thus emitting light [2].
Another, slightly different, description of Thomson!s atomic model was that
there were possible stable distributions of rings of electrons rotating around in
a within a sphere of positive electricity [5].
In 1906, Lord Rayleigh created a new atomic model that was a compilation
of Jeans! "ideal! atom and Thomson!s rings of electrons. At this time it was still
unknown that there are a limited amount of electrons in each atom, Rayleigh
and other scientist still believed that there were a large number of electrons in
each atom [5]. Rayleigh also thought of the concept of the mass of the atom
being associated with only the positively charged portion of the atom. [5]
Rutherford!s Interpretation of the Scattering Experiments
Lord Rutherford is the person credited with the first correct description of
the atomic model [4]. His discovery resulted from his backscattering experiment. In this experiment, Rutherford used an #-particle (#-particles are helium
nuclei, two protons and two neutrons) emitting radioactive material and used
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these #-particles as projectiles. The #-particles then collided with a very thin
gold foil. Rutherford placed a fluorescent screen behind the foil to detect the
scattered #-particles. He then used a microscope to observe the tiny flashes
of light that occurred when the #-particles hit the screen after passing through
the foil. [4]
To collect the data he needed, Rutherford first observed the pattern which
the #-particles hit the fluorescent screen without passing through the foil. This
pattern was a sharply defined area opposite the hole which creates the beam
[4]. After this initial observation, he placed the foil in between the beam of
#-particles and the screen. Rutherford noticed that the beam was deflected at
large angles and some were being scattered backwards towards the origin of
the beam. He plotted a curve from the data he collected of the angle at which
the particles were scattered and the number of hits at each angle. [4]
After his experiment, Rutherford compared his results and findings with
J.J. Thomson!s atomic model. Rutherford noticed that the results were very
different from Thomson!s model. If Thomson!s model was accurate, then the
positive charge and the mass associated with it in the atom should not have
deflected the #-particles as much as Rutherford had observed. Thus, Rutherford concluded that “all positive charges, along with most of the atomic mass,
had to be concentrated in a very small central region of the colliding particles
[4].” He then named his new discovery the atomic nucleus. [4]
Next, Rutherford theorized about a new atomic model. He theorized that
since all of the positive charge and most of the mass of an atom was concentrated into the center of the atom, then the bulk of the body of the atom was
nothing but free space with the negative charges, electrons, floating around
[4]. He then realized that the electrons had to be moving around the nucleus
very rapidly, similar to planets orbiting the sun [4]. Thus, Rutherford had disproved Thomson!s “Plum-pudding / watermelon model” and introduced his
“Planetary model.” [4]
The Rutherford scattering equations, which are based on scattering from
a massive scattering canter, are derived in the Appendix A.
EXPERIMENTAL FINDINGS
In this section, the Rutherford!s backscattering experiment using modern
technology will be described and analyzed. This experiment was performed in
collaboration with the Auburn University Physics Department. The #-particles
were produced in a Pelletron linear accelerator (a Van de Graaff type generation system) [6]. Depending on the charge of the ions, the accelerator is capable of yielding particles in the range of many millions of electron volts. It is
this stream of very collinear particles that impact the target beam.
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The targets were fabricated from a commercially available gold foil
mounted to a metal frame that had been designed for the experiment. To
mount the very thin gold foil and the frame, the gold was floated on water. The
target holder was then moved under the gold foil and lifted the holder so that
the foil completely cover the frame. Using the same process, a spare target
was made. Once the gold foil scattering targets were placed in the scattering chamber, the system was evacuated until the pressure was approximately
1x10-7 Torr.
After the 3 million electron-volt (3 MeV) #-particle beam from the accelerator was stabilized, the beam geometry (the cross section profile) was
adjusted. The beam profile was adjusted by carefully varying the positions of
beam blocking slits. Initially, the left slit was adjusted until approximately half
of the beam was blocked. Then the right slit was adjusted until the entire beam
was blocked from the detector. The left and right slits were then opened !
mm each, creating a 1mm wide opening. The process was then repeated for
top and bottom slits. This resulted in a beam that was 1mm by 1mm in cross
section.
For the data collection, the angle of the detector was varied and the number of scattered #-particles per time were recorded at the various angles. The
set angle was determined by lining up the detector mechanism with scribed
angle lines at the bottom of the vacuum chamber.
The data recorded at each angle setting was the collection time interval, the
area of the curve, and the angle at which the target was set. After recording the
data, the above process was repeated for the next angle setting. This process
of adjusting the angle of the target and recording data was repeated for each
angle of the analysis.
TABLE 1 Scattering Data for 3 MeV !-particles scattered from gold foil.
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As derived in Appendix 4, for a given scattering system the scattering is
inversely proportional to the fourth power of the sin($/2), that is, proportional to
sin-4($/2), where $ is the scattering angle. The scattering results from this experiment are compared with the Rutherford!s theory in Graph 1, in which, scattered particles are plotted versus scattering angle. Our results are in excellent
agreement with Rutherford!s results. But more importantly, the existence of a
relatively massive scattering center, that is, the gold nucleus is shown.
SUMMARY
In summary, the Greeks were the first to theorize about matter and atoms.
J.J. Thompson was the first to discover that the atom was not indivisible when
he discovered the electron. Thompson
also formed the “Plum-pudding/Watermelon model”. Later, Lord Rutherford, after examining the results from his
scattering experiment, discovered the atomic nucleus. With the discovery of
the nucleus, Rutherford formed the “Planetary model” of the atom. This atomic
structure model of a massive nucleus surrounded by much lighter electrons
solved a question that scientists had been asking for centuries.
Following Rutherford!s atomic model of a positively charged nucleus surrounded by orbiting electrons, the neutron was discovered. The neutron is a
neutral particle that shares the nucleus with the positively charged protons.
Subsequently it was discovered that the neutrons and protons are composed
of quarks. Where does this end? Are the quarks truly fundamental? This question is at the frontier of physics today.
References
[1] I. Leclerc, The Nature of Physical Existence (George Allen \& Unwin,
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The Structure of Matter
London, 1972)
[2] G. Gamow, The Atom and its Nucleus (Prentice Hall, Englewood Cliffs,
NJ, 1961)
[3]A.J. Rocke, Atom and Molecule, The Oxford Companion to the History of
Modern Science, Editor J.L. Heilbron (Oxford University Press, New
York, NY, 2003)
[4]G. Gamow, Mater Earth and Sky (Prentice-Hall, Englewood Cliffs, NJ,
1965)
[5]G.K.T. Conn, H.D. Turner, The Evolution of the Nuclear Atom (American
Elsevier Publishing Company, New York, NY, 1965)
[6]http://www.pelletron.com/charging.html
[7] S.T.Thornton, A.Rex, Modern Physics for Scientists and Engineers
(Thomson Brooks/Cole, Belmont, CA, 2006)
Graph 1
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