Download John Dalton, who lived in the late 18th century and the early 19th

John Dalton, who lived in the late 18 century and the early 19 century, was raised in England in a world of
mathematics, science and meteorology. Apart from naming the atom, he also was responsible for the birth of the
atomic theory . He came up with the atomic theory from several sources, including his own studies with
meteorology and gases, and from previous experiments done by previous chemists. Democritus had already
discovered atomos, (uncuttable, indivisible, very small particles) which are now known as atoms. From his
experiments with gas particles, he came up with the second law of the atomic theory: all atoms of a given
element are identical. The atoms of one element are different from the atoms of all the other elements. The Law
of Definite Proportions led Dalton the discovery of the third law: Compounds are composed of atoms of more
than one element; the ratio of the number of atoms of any two of the elements present is an integer or simple
fraction. Finally, the fourth atomic theory - chemical reaction involves only the separation, combination, or
rearrangement of atoms, not the creation or destruction – was derived form the Law of Conservation of Mass.
John Dalton’s model of an atom was very different from our model of the atom. In his time, protons, neutrons,
and electrons (sub-particles) had not been discovered, so atoms were considered the smallest unit of matter. The
models and the atomic theory were useful in further experiments done by Marie Curie and Henri Becquerel, as
well as those done by J.J. Thompson.
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Henri Becquerel was born into family of scientists in Paris, France on December 15, 1852. He received
his formal education in France at Ecole Polytechnique. After Wilhelm Conrad Rontgen developed the first Xray in 1896, Becquerel became fascinated the study of naturally-occurring phosphorescence, which is when
certain compounds glow without being exposed to light. He decided to study the plane-polarization of light. In
his later research, Becquerel discovered that uranium emitted an unknown form of energy, and he conducted a
series of experiments on it. He wrapped uranium salts in photographic paper and placed the sample in his desk
drawer, away from any other source of light, and yet the uranium salts still showed signs of phosphorescence.
Becquerel called this unknown energy “penetrating rays.” He contributed to the understanding of the atom by
discovering that beta particles, which is one type of radiation, comprise the nucleus of an atom.
Marie Curie was born in Warsaw, Poland on November 7, 1867. She moved to Paris in September, 1891
in order to study physics at Sorbonne University. In 1895, she married Pierre Curie. Her and her husband’s
early research was based off of Becquerel’s later work, and together they made significant progress in the
developing field of radioactivity. Curie discovered that the strength of the energy rays emitted from uranium
solely depended on the quantity of the sample and not on its physical condition. She tested other elements for
similar properties and named this phenomenon “radioactivity.” Curie published thirty-two scientific papers and
three books based on her findings. She contributed to the understanding of the atomic structure by concluding
that radioactivity is an atomic property, and because atoms emit particles, there must be something smaller than
the atom. Curie and Becquerel’s work set the stage for Millikan and J.J. Thompson to discover elemental
isotopes and the electron.
J.J. Thomson and Robert Millikan
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J.J. Thomson was born near Manchester, UK in 1856. In 1897 he announced the findings of his cathode ray
experiment, a three-part experiment where he deflected the rays using an electric field. Thomson concluded the
rays were made up negatively charged particles, which he called corpuscles, found the mass to charge ratio of
the particles (1.759 x 108 coulombs/gram), and hypothesized that the particles were a universal component of all
matter. These conclusions led him to his Raison Pudding Model of the atom. Robert Millikan was born in
Illinois in 1868 and won the Nobel Prize in 1923. Using his oil-drop experiment in which he ionized atoms with
x-rays and Thomson’s work, Millikan found the exact charge of an electron to be 1.59 x 10-19 Coulombs (the
actual charge is 1.602 x 10-19 Coulombs. Together these two scientists discovered the atom was divisible,
disproving Dalton’s theory, and discovered multiple properties of the electron.
Rutherford and Chadwick
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Ernest Rutherford discovered the nucleus. He performed the Gold Foil Experiment in 1909, which led to his
discovery of the nucleus. The Gold Foil Experiment consisted of shooting positively charged alpha particles
generated from radium at a very thin piece of gold foil. What Rutherford found was that the vast majority of
alpha particles passed through the gold foil, but 1 in every 10000 particles were deflected at a high angle. This
showed two things. First, the majority of the atom consisted of empty space, which is why the vast majority of
atoms passed through the gold foil. Second, at the center of the atom lay a positively charged nucleus, which
explained why very rarely some alpha particles would be deflected, as when a positively charged alpha particle
hit a positively charged nucleus the like charges would repel. He concluded that at the center lay a positive
nucleus and everything else, the majority of the atom, was empty space.
James Chadwick discovered the neutron. He performed the Neutron Experiment in 1932, which lead to the
discovery of the neutron. The experiment consisted of shooting alpha particles generated from polonium at a
thin sheet of beryllium. When the alpha particles hit the beryllium a mysterious radiation would be produced.
He then put a thin piece of paraffin wax, a proton-rich substance, in the path of the radiation. When the
radiation hit the paraffin wax protons were ejected and then the protons would be detected and counted by a
Geiger counter. He determined that the radiation was neutral as it was not affected by magnetism and
pinpointed the mass of the neutral particle by massing everything in the reaction, alpha particles and protons,
and then found that there was a missing piece, which he determined to be the mass of the neutron. He concluded
that there was a neutral particle that was 1.001x as massive as the proton in the nucleus.
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Louis De Broglie
De Broglie's greatest contribution to quantum mechanics was the wave-particle duality theorum. He
believed that like light, all things could be both particles (matter) and waves (energy); at least under certain
circumstances. The electron was one particle that frequently acted as both a particle and a wave, explaining
many previously unanswered questions about the atom.
The wave-particle duality theorum changed how we viewed matter.
Werner Heisenberg
Heisenberg was known for The Uncertainty Principle, which says that subatomic particles (such as
electrons) travel incredibly fast, and because of this high speed, we can’t measure both their speed (energy) and
exact position at the same time. We can only know how much energy it has, or where it is, but not both.
The Uncertainty Principle changed how we thought about how electrons can be measured.
Erwin Schrödinger
Schrödinger gained his fame in the quantum mechanics world from his Wave Equation (HΨ=EΨ). The
equation can calculate behaviors (orbitals) and energies of subatomic particles (electrons), and can determine
electron density probability. The Wave Equation changed the way we viewed the electrons inside of atoms.