Download - History of the Atom
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History of the Atom
Scientists who shaped the modern atomic theory
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4th Century Greece
Democritus first suggested
matter was made of atoms
there was no way to test these theories
Aristotle suggested matter
was continuous, not made of atoms
this was believed
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Alchemists
Transmutation: tried to turn all metals into gold
brought observation and experimentation to science
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Antoine Lavoisier
In ordinary chemical reactions in a closed system, mass remained constant.
Law of conservation of mass• matter can not be created or
destroyed
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John Dalton and the Atomic Theory
In the early 1800s, Dalton wrote explanations of the
atom based on experiments by Lavoisier and Proust.
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Dalton’s Atomic Theory Atoms are indivisible. Elements are made of atoms. Atoms of the same element are
identical. Atoms of different elements are different.
Atoms combine in simple, whole number ratios.
Reactions occur when atoms are separated or combined.
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J.J. Thomson
• 1897• cathode rays are composed of electrons• determined the charge of an electron• “plum pudding model”
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In the early 1900s...
Robert Millikan (American)• oil drop experiment• accurately measured the charge of an
electron
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Then a ray of light??
1896 Henri Becquerel
discovered that uranium exposes photographic film.
Radioactivity
Early 1900s Marie and Pierre
Curie discovered that rays are given off by uranium and radium.
Atoms will split into smaller units
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Albert Einstein
Explained the origin of the energy produced in nuclear reactions with E = mc2.
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Then the modern theory began to develop
Lord Rutherford 1913
Proved Thomsonwrong
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Rutherford Model
Also called the planetary model
Most of the atom is empty space
Consists of a central nucleus surrounded by electrons in orbits
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The “final” particle is discovered...
1932 James Chadwick
• found particle with the same mass as a proton, but no charge
• now known as a neutron
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So what did this all mean to the atomic theory?
Dalton’s theory had to be revised to include subatomic particles!
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Bohr Model
Calculated specific energy levels for electrons
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Quantum mechanical model(Schrodinger, de Broglie, &
Heisenberg) Current model Electron cloud
model Electrons do not
have fixed positions, but exist somewhere in the electron cloud