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WEEKLIES ISSUE
2012-2013
Science—Atomic Structure
The world around us is composed of matter—stuff that takes up physical space and has mass.
For us humans, it is easiest to perceive the presence of that mass in the form of atoms. In this
weekly, we will talk briefly about the history of the atom, the current model as understood by
physicists and chemists, and the basic elementary particles.
History of the Atomic Structure
A
ROUND the year 400 BC, an ancient Greek philosopher asked
the question: “If you take any piece of matter and cut it in half
and in half over and over, at what point would you not be able to
subdivide further?” This philospher, Democritus, believed that you
could cut a piece of matter down to a fundamental unit that he called the
“atom,” which translates from the Greek as “not divisible.”
Democritus also held that every material had a different kind of atom that could have different
shapes, sizes, and weight—for example, he thought that atoms of iron were really heavy and
shaped like hooks so they would hold together and atoms of water were slippery, tiny, and
smooth so they would flow around each-other.
Though this idea of atoms is quite ridiculous by today’s standards, it was the first atomic model
to exist—it was also relatively unquestioned for over 2000 years.
T
HEN, about 2200 years after Democritus formulated his atomic
theory, the British chemist John Dalton conducted extensive research
on the formation of molecules and compounds (structures composed
from more than one atom). Dalton came to the conclusion that all atomic
matter must be composed of a specific number of elements in which all of
the atoms (of each element) are the same.
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This conclusion was a step forward from Democritus’ theory because it allowed multiple
compounds to be created from a specific set of atoms1 instead of declaring that each of the
resulting compounds consisted of different types of atoms. Instead of saying that rust had
completely different atoms than the iron that it was before it reacted with the air like Democritus
would, Dalton’s theory stated that the iron atoms were still there—they had not transformed into
rust atoms, just reacted with oxygen atoms from the air to make a compound. Published in 1803,
this conclusion was revolutionary!
However, Dalton thought, like Democritus, that atoms were indivisible.
O
VER the following century, various developments occurred in
laboratory techniques and technology that allowed the next
development in the atomic structure. In 1897, British physicist J. J.
Thomson conducted pivotal experiments with a
curiosity called a “Crookes Tube” that generated a
mysterious beam of (what appeared to be) light
that could be deflected by a magnetic field.
After a great deal of study, Thomson concluded
that the beam of “light” was not made of light at
all—it was a beam of tiny particles. These particles, subatomic and
negatively charged, came to be called electrons (Thomson called them “corpuscules”). Using
his discovery of the electron, Thomson was able to formulate the first revolution in the atomic
model since Democritus.
Thomson concluded that, because the electron has a negative charge and
atoms seem to be neutral, then the atom must consist of a large positive
charge that has the negatively-charged electrons “floating” around in it.
To Thomson, this model seemed to be like a popular British desert called
“plum-pudding;” therefore, the model came to be known as the “plumpudding model” of the atom.
M
ORE developments in the atomic structure occurred in the two decades that followed
the plum-pudding model. Ernest Rutherford conducted a landmark experiment in
1909 called the gold-foil experiment in which he bombarded a
gold foil with alpha radiation, discovering the atomic nucleus—a small,
dense, positively-charged center of the atom that contains almost all of the
atom’s mass. With that result, Rutherford developed the
“planetary model” of the atom in which electrons orbited
the central nucleus like the planets orbit the sun.
1
The Law of Multiple Proportions
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OME flaws could be found in this model, however. Under Rutherford’s criteria, the
electrons would eventually spiral out of control into the central nucleus. Also, landmark
results from Max Planck and Albert Einstein revealed that atoms absorb and emit energy
in discreet “packets” that could not be supported by Rutherford’s model.
S
The Danish physicist Niels Bohr, one of
Rutherford’s students, developed a slightly
different planetary model in which the orbits of
the electrons were “quantized,” or confined to a
set of orbitals or energy levels; jumping between
those orbits would require emission or absorption
of quanta just as theorized by Planck and Einstein.
This model came to be known as the Bohr model and was in use by the
scientific community from 1913 until about 1927.
A
FTER a few more years of research, Ernest Rutherford again contributed to the atomic
model. Researching the nature of the atomic nucleus, Rutherford discovered the
proton—the particle in the nucleus of the atom that was responsible for the positive
charge (and most of the mass of the atom). Rutherford found that each atom had the same
number of protons and electrons. The masses of the nuclei, however, did not
always agree with the expected masses of the protons. Rutherford postulated
that the extra mass was due to a neutral particle that he called the “neutron.”
The neutron was eventually discovered in 1932 by British physicist James
Chadwick, who also discovered definitive proof of the idea of the isotope—
atoms of the same element, though they must have the same number of
electrons and protons when they are neutral, can have different numbers of
neutrons (and therefore different masses).
R
ESEARCH by the Austrian physicist Erwin Schrödinger (right)—who is known largely
for his Schrödinger Equation—and German physicist Werner
Heisenberg (left)—known for his Uncertainty Principle—revealed a
world of the atom that is much more chaotic and messy than
the Bohr model would have us believe.
Schrödinger’s Equation reveals that the idyllic orbits of the
Bohr model are in fact highly-complex shapes in three
dimensions. When that idea is partnered with Heisenberg’s
Uncertainty Principle—which states that the more that is
known about the motion of a particle, the less is known about its position—we
get a model of the atom where electrons move extremely rapidly around the nucleus in quantum
motion. The electrons, if we could see them, would appear to us as an electron cloud. This
reveals to us the quantum mechanical model of the atom.
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Modern Model of the Atom
E
VER a topic of debate in the scientific community, our understanding of the atom is
always changing. Though there are several attributes about that atom that are currently
accepted by physicists and chemists—whose research seems to support the model—each
of the models mentioned in the History section were accepted similarly in their time.
The current model of the atom consists of a positively-charged nucleus composed of protons and
neutrons. These protons and neutrons are, in turn, composed of much smaller particles that were
discovered in the 1960s called quarks. Quarks are held together (“bound”) by force-carrying
particles known as gluons.
C
OMPONENTS of the atom—namely protons, neutrons, and electrons—have been found
to have uses outside of the formation of atoms. Particularly through the discovery of
radiation, each of these particles can be beneficial in technology in health,
communications, and energy fields.
The proton is composed of three quarks bound together by gluons. In short hand, it can be
represented by the symbol p+ (p-plus) or just p. Besides their presence in the nuclei of atoms,
they are the primary fuel for the fusion processes of most stars, and can be used to perform
radiation therapy. Protons are considered to be stable, but can combine with electrons to produce
neutrons.
The neutron is also composed of three quarks bound together by gluons. In short hand, the
neutron can be represented by the symbol n0 (n-zero) or just simply n. Neutrons are primarily
found bound into atomic nuclei because they are unstable in an unbound state; after about 15
minutes, the neutron decays into a proton and an electron. Neutrons can be used to maintain
nuclear chain reactions that can generate electricity.
The electron is a lepton—a type of fundamental particle—and currently-accepted physics states
that it is not composed of smaller particles. Electrons are represented by the symbol e – (eminus). The flow of electrons creates what we perceive as electricity.
The Standard Model
O
THER types of particles are required under the current understanding of particle physics.
These particles—which include quarks, leptons, neutrinos, and gauge bosons—have
been identified as composing the Standard Model of quantum mechanics.
The Standard Model is capable of predicting large amounts of modern physics and can explain a
variety of phenomena on a particle scale. It is often contrasted with Albert Einstein’s Theory of
General Relativity because the Standard Model governs the physics of the very small and
General Relativity governs the physics of the very large.
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The Standard Model is not considered to be a perfect model—it cannot satisfactorily explain
gravitational interactions and—as of September 2012—defies unification with General
Relativity.
More General Information
O
VER the centuries in which the atomic model has evolved, many important terms have
been generated to describe the phenomena that result from the interactions of atoms and
their components. Below, we describe three terms that you should be familiar with.
Element
As we mentioned earlier, John Dalton concluded that all atoms of an
element are identical. Though this is only mostly true (due to the presence
of isotopes made possible by Chadwick’s neutron), it is one of the most
important parts of atomic theory.
Atoms that have the same number of protons and electrons in a neutral
state are considered to be atoms of the same element. This means that
those atoms all exhibit similar properties (melting point, durability, color,
etc.) and are interchangeable on a molecular scale.
There are, as of September 2012, 118 confirmed elements in existence.
These elements are organized by atomic properties on the Periodic Table.
The first periodic table was organized by the Russian chemist Dmitri
Mendeleev.
Ion
Because the electrons around an atom’s nucleus are only held in attraction
to the nucleus by electronic forces (positive and negative forces attract),
electrons can be removed or added to atoms when they have sufficient
energy. These resulting atoms with greater or fewer electrons than normal
are called ions. If an atom has more electrons than normal, it is a negative
ion or an anion. Similarly, if an atom has fewer electrons than normal it is
a positive ion or cation.
Molecule
Atoms can bond together by sharing electrons to reach more ideal
configurations. When two or more atoms share electrons in a bond, those
atoms are said to be part of a molecule. Molecules are often contrasted
with ionic solids or ionic compounds. Molecules can be found all around
us—from oxygen gas to plastics.
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Ionic Compounds
When one atom transfers an electron to another atom, those two atoms
become ions with opposing charges (one an anion, the other a cation).
Those two ions are then electronically attracted to each-other, and form an
ionic compound. Ionic compounds are often contrasted with molecules.
All salts are ionic compounds—including table salt, which is composed
from a sodium ion and a chlorine ion.
Future of the Atomic Model
L
ASTING for the better part of a century thus far, the Quantum Mechanical Model of the
atom has been one of the most successful models to date; most—if not all—of the
experiments conducted since the 1930s have been consistent with the model, which is
continually supported by work in the chemistry industry and research.
There are, however, some theories that go further than the Standard Model states that the
Quantum Mechanical Model should exist. These theories—of which there are many—theorize
peculiar phenomena such as vibrating bands of energy or multidimensional energy fields.
It is uncertain where scientific research will take us in terms of understanding the atom.
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