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The Discovery of the Atom and the Scientists that
Contributed New Knowledge of the Atom
Nathaniel Higbie
New Albany High School
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Abstract
Atomic theory has progressed significantly since Democritus, back in 400 BC, first proposed that
all matter is made of atoms. This paper discusses the improvements made over atomic theory
over the last few hundreds of years. It will mention scientists from John Dalton, to JJ Thompson,
to Albert Einstein. These scientists made a variety of different improvements to atomic theory,
whether discovering, electrons, neutrons, or making the photoelectric model. All of them
conducted numerous different experiments leading up to each of their unique discoveries. They
restlessly pursued their work and finally came up with huge contributions to atomic theory as
they knew it at the time. This paper will analyze in depth the roles that fifteen different scientists
had in adding to the fast growing knowledge of atomic theory. Each one of these scientists used
the work and accomplishments of previous scientist pursuing the same knowledge to add on to
and make improvements upon.
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The Discovery of the Atom and the Scientists that
Contributed New Knowledge of the Atom
Over time, many different developments have been made in atomic theory, some of them
having changed the world. Numerous different scientists have made their own unique
contributions to the knowledge that the world possesses of atoms. If not for people like Albert
Einstein and John Dalton, the world would not be as technologically advanced as it is today.
The person that first introduced the idea of an atom was Democritus in 400 BC.
He theorized that all matter is actually made up of tiny units called atoms. Most people in that
time period, however refused to believe this theory. Even the famous Aristotle rejected the
possibility that this could be true. Many centuries, later Democritus’s theory is finally proven
correct. The very origin of atomic theory rests in Democritus’s hands.
The next important development does not occur until a couple thousand years later. In
1803, John Dalton states all matter is made from indestructible atoms and that compounds are
created by combining atoms. His theory had clear flaws, however. The problem that remained is
that the knowledge of ratios was inadequate to determine the actual number of elemental atoms
in each compound. Nevertheless, John Dalton’s achievements greatly outweighed his flaws and
he earned the sobriquet “father of chemistry.” John Dalton also created the “solid sphere model”,
the first atomic model. He made this model from hypothesizing that atoms are solid spheres and
cannot be broken down into smaller pieces.
The next noteworthy advancement came 82 years after John Dalton, in 1885. This was
when Euguen Goldstein discovered positive particles and is credited with the discovery of
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protons. He accomplished this by discovering canal rays, which are beams of positively charged
ions.
Radioactivity is the next important addition to the understanding of atomic theory. In
1896, Henri Becquerel discovered radioactivity. He reached this conclusion by studying and
experimenting with uranium. The newly founded concept of radioactivity demonstrated that the
atom was neither indivisible nor immutable. Radioactivity also became an important tool in
revealing the interior of the atom.
In 1898, JJ Thompson discovered electrons. This was a major contribution to science
because it changed the way people looked at the atom. With this newly found discovery,
Thompson created a new atomic model that at both positive and negative charges embedded
within it. His model was known as the “plum pudding model”, or the “chocolate chip cookie
model”. In it he presented equal amounts of positive and negative charges so that the atom would
be electrically neutral. He also agreed with the last atomic model, the “solid sphere model” , that
atoms were sphere shaped, but he believed that positive and negative charges were embedded
within it. Prior to Thompson’s discovery, scientists believed that atoms were the smallest
fundamental unit of matter. Although Thompson discovered electrons, at the time he called them
corpuscles.
The next notable discovery in regard to the atom was when Max Planck came up with the
quantum theory of energy, which explains the nature and behavior of matter and energy on the
atomic and subatomic level. He came up with the formula of E=hv, or energy=Planck’s constant
times frequency. Because of the new constant he came up with, which is 6.626 × 10
−34
he could
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now find the wavelength if given the energy of something, or the energy if given the wavelength.
His discoveries are agreed by most to have been the birth of quantum physics.
In 1905, Albert Einstein discovered the photoelectric effect. The importance of this
discovery is as large as it is because he suggested for the first time that light is both a wave and a
particle. It was of so much significance that he won the Nobel Prize specifically for this
achievement in physics, despite his theories of relativity and musings on black holes taking all
the attention. The phenomenon of light being both a wave and a particle is known as the
wave-particle duality of light. It is fundamental to all quantum mechanics and also has greatly
influenced the development of electron microscopes and solar cells.
Six years later, a new version of the atomic model is developed. Ernest Rutherford, in
1911 created a model known as the “nuclear model” and discovered the nucleus. In this model,
of the atom, there is a tiny, dense, positively charged core called the nucleus, in which all the
protons and neutrons are located. Nearly all of the atom’s mass is concentrated in this nucleus
core. Orbiting this small core are negatively charged electrons, similar to planets around the sun.
Rutherford’s theory was profoundly different from the last atomic model, the”plum pudding
model”, in that Rutherford’s model had the atom consisting of mostly empty space rather than
everything packed into a solid sphere.
Merely two years after Rutherford’s discovery, Neils Bohr came up with his own atomic
model in 1913. He called it the “Bohr atomic model”. His model was an enormous step in atomic
theory because he figured out that not all the electrons in an atom follow the same orbit. Also, he
found out that the number of electrons in the outer orbit depend on the properties of the element.
Bohr agreed with the previous model, Rutherford’s “nuclear model” that the nucleus was
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positively charged and the electrons orbiting it were negatively charged, but made two major
improvements on it with the electrons having different orbits and that the outer electrons, known
as valence electrons, depended on the properties of the element.
The next major discovery in the field of atomic theory was involved x-ray technology. In
1913, Henry Moseley used x-ray technology to measure protons in an atom. He was able to
firmly prove the correlation between atomic number and the charge of the atomic nucleus. He
also demonstrated that the major properties of an element are determined by the atomic number,
not atomic weight. Moseley correctly came to the conclusion that there were 92 different
elements up to and including uranium. Mosely’s findings were a milestone in atomic theory and
furthering the world’s knowledge of the atom.
4 years after Moseley’s breakthroughs, Robert Milikan successfully measured the charge
of a single electron.
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