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Getting to Know: Atomic Theory
The phrase “atomic theory” sounds mysterious, doesn’t it? It evokes images of nuclear
laboratories and brilliant scientists; that could be you someday!
Atomic theory is simply the study of the atom. You probably
already know quite a few things about the atom, like how it is
made of different subatomic particles called protons, neutrons,
and electrons. You also probably know that atoms have mass
and are the basic building blocks of all matter. Understanding
more about atomic theory will help you learn how we have
come to know so much about the atom. In this lesson, you will
also learn about important scientists who furthered our
understanding of the atom, and you will learn about different
models that have been used to explain the atom.
Is this still the current model of
the atom? Read on to find out!
Who invented the atomic theory?
Atomic theory was not invented by a single scientist. Instead, many different scientists have
been interested in the building blocks of matter. Over many years, different scientists have
built on the work of people before them to add to our knowledge of the atom.
Misconception 1: I heard my teacher refer to cells as building blocks.
Are cells and atoms the same thing?
No, cells and atoms are not the same things. Atoms are the building
blocks of all matter; that includes cells as well as nonliving things.
Atoms make up cells, whereas cells make up all living things.
Ancient philosophers in Greece, India, and China all conceptualized the idea of the atom long
before our technology had developed enough to prove it. It was the Greeks, approximately
300 B.C., who gave us the name atomos, which means “not able to be divided.” They believed
that if you broke a piece of bread into smaller and smaller pieces, eventually you would have
a piece of bread so small it could not be divided further—a bread atom.
It was many years before their idea was picked up by John Dalton, a pioneer of atomic theory.
He lived from 1766 to 1844. Dalton proposed that atoms were essentially solid spheres that
combined to form matter. His thinking helped us understand how atoms of different
elements combined to form molecules. For example, he noticed that water could be broken
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down into two other substances, hydrogen and oxygen, but that hydrogen and oxygen could
not be divided further. Dalton was on the right path, but there was still much to learn about
the atom. John Dalton inspired the scientists who came after him.
J. J. Thompson was the next notable scientist to add to the atomic theory. He used x-rays to
take some of the first pictures of the insides of people’s bodies and in his research discovered
negatively charged particles, which he called electrons. However, he did not know how the
electrons were arranged in the atom. He imagined that they were probably spread out evenly
between the positive charges, much like plums in a pudding. Thus, his contribution is called
the “plum pudding” model. Although Thompson was the first to identify the electron, he did
not understand the other subatomic particles and did not
realize that atoms had uncharged subatomic particles, or
neutrons.
Another scientist, Ernest Rutherford, conducted what is now a
famous experiment, called the gold foil experiment. He shot
helium nuclei through a very thin sheet of gold foil—a sheet so
thin that it was only one-atom thick. He expected all of the
particles to pass straight through the gold foil, but he found
that a tiny portion of the particles deflected off of the gold foil
and bounced at odd angles. Some even shot straight backward!
He realized that the atoms of gold were not evenly distributed,
but instead were composed of a tiny, dense center surrounded
by a lot of empty space.
Rutherford bombarded gold
foil with tiny particles.
Wow, interesting! I guess the next scientists learned about how the electrons
move in circles, right?
That’s right. Niels Bohr developed the model that you are talking about in 1920. He observed
that when energy was added to an atom, electrons could use that energy to move farther
away from the nucleus. However, they did not just move anywhere. They were much more
likely to be in some places than others, just like a person climbing a ladder. When someone
climbs a ladder, he or she can be on the third step or the fourth step but not somewhere in
between. When the electrons dropped down to lower energy levels, they released the energy
back as light at specific frequencies. The color of the light told Bohr how much energy was
needed to travel from one level to another. Bohr imagined that the electrons moved in
circular orbits like planets, which is not the most accurate model of the electron paths,
although it is still a helpful way to visualize how electrons participate in bonding between
atoms.
So if Bohr’s model does not tell the whole story, what else is there?
Although Bohr’s model is still used sometimes, there is another atomic theory that is more
prevalent. It is called the electron cloud model. Rather than showing electrons moving in
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concentric rings, the electron cloud model shows the spaces where an electron is likely to be
or unlikely to be. When the locations of an electron are plotted over time, it looks like a cloud.
This probability model shows that there are several different shapes that electrons can fill,
although we can never know with certainty where a specific electron is at a specific time.
All of these changing ideas about the atom should tell you one thing: we are still learning
about the atom, its properties, and its subatomic particles.
Misconception 2: I remember learning long ago that the atom is the
smallest thing there is. However, now I know that there are even smaller
things, like protons, neutrons, and electrons. Are those the smallest things
ever known?
No, amazingly, protons, neutrons, and electrons are not the smallest
known things! Scientists have discovered that even subatomic particles are
made of smaller particles. These tiny particles are called quarks. Quarks
combine to form particles called hadrons. For example, two up quarks and
one down quark combine to make the familiar hadron, a proton.
Meanwhile, a neutron is made of two down quarks and one up quark.
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