Timeline of Atomic Structure
Marissa Kopelman
Democritus (460 B.C.-370 B.C.)
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Democritus, a Greek philosopher, believed that all matter was
made up of indivisible and indestructible particles called
atoms.
His ideas did not explain chemical behavior and were not
based on the scientific method, having no experimental
support.
His theory was a response to that of Parmenides and Zeno,
who believed that the universe was formed of material that
was a mass that had no empty space.
Other Greek philosophers such as Aristotle and Plato
challenged and dismissed Democritus’ ideas.
1803 John Dalton
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The modern age of atomic discoveries began with John
Dalton, an English chemist and schoolteacher.
Dalton formed a scientific theory through experimental
methods based on Democritus’ ideas.
He formed hypotheses based on his experiments and his
study of the ratios in chemical reactions in which elements
combine.
Dalton used Lavoisier’s Law of Definite Proportions,
established in 1799, in his experiments combining elements
and proposed the Law of Simple Multiple Proportions.
1803 John Dalton (continued)
Dalton’s atomic symbols
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Dalton’s Atomic Theory was the
result of his observations:
1.
All elements are made of indivisible
particles called atoms.
All atoms of the same element have
identical properties and mass.
Compounds are formed by the
combination of atoms of different
elements.
Chemical reactions are the
separation, combination, or
rearrangement of atoms.
2.
3.
4.
1896 Marie Curie
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In 1896, French chemist Antoine Henri Becquerel made an accidental
discovery when he was studying uranium salts exposed to sunlight and
their ability to fog a photographic film plate. He left one sample that was
not being exposed to the sun on the plate and saw that it had fogged. His
associates Marie Curie and her husband Pierre Curie showed that the
uranium atoms emitted rays that fogged the plates.
Marie Curie, a Polish scientist, named the process that the uranium atoms
gave off rays, radioactivity. The particles and rays given off by a
radioactive substance was called radiation. This discovery proved that
Dalton’s theory that atoms are indivisible was incorrect. Radioisotopes, or
radioactive atoms, have unstable nuclei and emit radiation as they undergo
changes.
Curie discovered that the radioactivity of a substance was in proportion to
the amount of the radioactive material present.
The discovery of radiation allowed Becquerel and the Curies to be awarded
the Nobel prize in 1903.
1897 J.J. Thomson
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Thomson, an English physicist, discovered the electron through
experiments involving electric currents passing through low pressure
gases.
Gases were sealed in glass tubes with electrodes, metal disks, at each end,
which were connected to an electricity source. The anode, one electrode,
became positively charged as the other, the cathode, became negatively
charged. A cathode ray, a glowing beam, then traveled to the anode from
the cathode.
Thomson used electrically charged plates and a magnet to observe how the
cathode ray repelled the negative charge. He hypothesized that the
cathode ray consisted of fast moving negatively charged particles which he
called corpuscles, later called electrons.
1897 J.J. Thomson (continued)
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Thomson also performed another
experiment to test his hypothesis
in which he measured the
electron charge’s ratio to its mass.
This ratio was constant and was
not affected by anything in the
experiment, such as the type of
gas. This led to his conclusion
that electrons are a part of the
atom of every element.
At the time of this discovery,
Thomson knew that the same
charges repelled each other and
the opposite charges attracted.
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Thomson’s results allowed him to
develop the first model of the
atom with Lord Kelvin in 1904. It
was called the “plum pudding”
model because the negative
electrons represented the plums
embedded in the uniform positive
charge, representing the pudding.
1910 Robert A. Millikan
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Millikan, a U.S. physicist, set up
an experiment to find the amount
of charge of an electron. He
already knew Thomson’s
hypothesis that the mass of an
electron is at least 1000 times less
than the smallest atom.
Millikan used an oil drop
apparatus to measure the
electron’s charge. Oil droplets
were sprayed into a chamber by a
perfume atomizer. Some droplets
fell through a pinhole into a
middle chamber, ionized by xrays, with a positive plate on the
top and a negative plate on the
bottom. Particles that captured
electrons floated upward or fell
slowly because they were
attracted to the positive plate.
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Millikan watched how fast the
rise and fall of the drops were and
calculated the charge of the drop.
He used Thomson’s charge-tomass ratio of an electron and his
own results to calculate the mass
of an electron.
1910 Robert A. Millikan (continued)
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Millikan won the 1923 Nobel Prize for his work with this
experiment and for determining the value of Plank’s constant.
Millikan’s diagram from his Physical Review paper
1911 Ernest Rutherford
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Rutherford, physics professor and former student of Thomson, established
the idea that all the positive charge and mass in an atom is contained in a
region he called the nucleus with his new nuclear atom model. It was first
suggested that an atom has a central nucleus by Japanese physicist
Hantaro Nagaoka in 1904.
In his experiment, a beam of alpha particles was aimed at a thin sheet of
gold foil that was surrounded by a fluorescent screen. Most of the
particles passed through the gold atoms with no deflections, but some
bounced off the foil at large angles and some bounced straight back.
At this time it was believed that the particles should have easily passed
through with small deflection because of the positive charge that was
believed to be spread throughout the atom.
Because of the lack of deflection from most of the particles, Rutherford
stated the idea that the atom is mostly empty space. He said that the great
deflection of some particles was due to them hitting the positive charge in
the central region of the atom he called the nucleus.
1911 Ernest Rutherford (continued)
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Rutherford’s model of the
atom is called the nuclear
atom in which protons and
neutrons are in the nucleus
and electrons take up most
of the atom’s volume around
the nucleus.
1913 Niels Bohr
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Bohr, a Danish physicist and student of Rutherford, proposed the idea that
electrons were only found in specific orbits around the nucleus of an atom.
Bohr changed Rutherford’s model to include new information about how
an atom’s energy changes when light is emitted. Bohr knew that elements
gave off color when heated and when passed through a prism, only specific
lines or line spectra can be seen. This showed him that atoms could only
emit energy in precise quantities. The light that was emitted was due to
the electron movement in the atom and Bohr, as well as Rutherford,
suggested that electrons can only move in precise steps in the atom.
Bohr proposed the idea that electrons occupy energy levels in the atom and
when the atom is excited, such as during heating, the electrons can move
to higher energy levels. This amount of energy required
to move an electron to another level is called a quantum
of energy.
1913 Niels Bohr (continued)
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Bohr also hypothesized that each energy level had a certain limit to the
number of electrons it could occupy. He said that the maximum number
of electrons the first electron shell could hold is two electrons. Elements
with more than two electrons would have the extra electrons on the
additional electron shells.
Bohr won the Nobel prize of 1922 for his work on
the structure of the atom.
1924 Louis de Broglie
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Broglie, a French physicist, developed the theory of electron waves.
Broglie knew that Einstein had previously proposed that atomic matter
may have the same properties as waves. Experiments had also been made
which showed that there were restrictions on electron movement because
they needed to move around the nucleus. Waves are confined within
boundaries by a nuclear charge and are restricted in their motion and
shape. If there was an interference, they would be canceled out. Broglie
compared this definition of a wave to an electron, which also had
restricted motion.
Broglie’s theory had no experimental evidence, but offered an explanation
to the questions raised by the calculation of electron motion in the atom.
Ernest Schrodinger constructed a mathematical system based on de
Broglie’s theory. In 1927, Clinton Davisson, Lester Germer, and George
Thomson found the first experimental evidence of the similarity between
the properties of the wave and electron.
1926 Erwin Schrödinger
 Schrödinger, an Austrian physicist, developed and solved a mathematical
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equation on electron behavior in a hydrogen atom, using new experimental
results and theoretical calculations that went against previous
descriptions of the movement of electrons.
His mathematical model was based on the combination of the behavior of
waves and the de Broglie equations to result in an equation for the electron
distribution in an atom. In this model, called the quantum mechanical
model, the allowed energy an electron could have and the chances of
finding it in different locations in the atom around the nucleus are
determined. It shows the probability that an electron can be found in a
given location in space at a given time.
1926 Erwin Schrödinger (continued)
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Unlike Bohr’s one-dimensional model, Schrödinger’s three-dimensional
model required three quantum numbers or coordinates to show the atomic
orbitals in which the electrons were found. His model also does not involve
the idea of an electron taking a specific path around the nucleus.
These three coordinates in his wave equation were the principal (p), angular
(l), and magnetic (m) quantum numbers which show the size, shape, and
orientation in space of the atomic orbital.
This led to Schrödinger’s electron cloud
model which shows that there is a higher
probability of finding an electron in the
cloud’s denser regions.
1932 James Chadwick
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After Rutherford’s discovery of the positively charged nucleus,
it was not determined where the extra mass was in the atom.
Chadwick identified the neutron in an experiment in which
alpha particles were smashed into beryllium, which released
radiation that hit hydrogen atoms in paraffin wax. Protons
were released from the paraffin. Chadwick stated that only a
particle as heavy as the proton would allow it to be released.
He said it had no charge because it easily penetrated the
nucleus. These neutral particles were named neutrons by
Chadwick.
After Chadwick’s discovery, scientists
began the bombardment of neutrons
with many different materials.
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