History of the Atom Part I

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History of the AtomPart I

• CPS Chemistry - CH. 3

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Democritus Ancient Greeks

• (400 B.C.) Democritus of Adbera (northern Greece)
  asserted that all material things are composed of
  extremely small irreducible particles called
  atoms
• Atmos is the Greek word for indivisible

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Dalton - Modern Atomic Theory

• (1803) John Dalton (England) formulated the
  modern version of the atomic theory. In his
  model all atoms in a given chemical element are
  exactly alike, while the atoms of different
  elements differ by atomic weight.

Dalton believed that atoms were the fundamental
building blocks of nature and could not be split.
In chemical reactions, the atoms would rearrange
themselves and combine with other atoms in new
ways.
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Daltons idea of the atom
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Daltons Atomic Theory

• John Dalton in 1808
• Proposed first modern version of atomic theory
• 1.) All matter is composed of tiny particles
  called atoms
• 2.) Atoms of a given element are identical each
  element is different
• 3.) Atoms cannot be created or destroyed
• 4.) Atoms combine in definite ratios
• 5.) In chemical reactions atoms are combined and
  rearranged

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J.J. Thompson - Electron

• (1898) J.J. Thompson (England) discovered the
  electron, the component of the atom with negative
  charge. His model of the atom had the negatively
  charged electron evenly distributed throughout a
  sphere of positively charged material. This is
  known as the plum pudding model of the atom.

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J.J.Thompsons idea of the atom
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J.J. Thompsons Experiment
Thomson did experiments on the beams of particles
in his tube. They were attracted to a positive
charge, so Thomson correctly concluded that they
must be negatively charged themselves.
Other experiments showed that it would take about
2000 electrons to weigh the same as the lightest
atom, hydrogen. He called the tiny, negatively
charged particles electrons
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Robert Millikan

• In 1909 Robert Millikan (1868-1953) of the
  University of Chicago succeeded in measuring the
  charge of an electron by performing an experiment
  known as the "Millikan oil-drop experiment."
• He then calculated the mass of the electron by
  using his value for the charge,
• 1.60 x 10 -19 C, and the mass to be
  9.1x10-31kilograms

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The charge mass of an electron

• Once the charge-to-mass ratio of the electron was
  known, a scientist who could measure either the
  charge or the mass of an electron could easily
  calculate the other quantity.

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Ernest Rutherford

• In 1911, Ernest Rutherford interpreted these
  results and suggested a new model for the atom.
  He said that Thomson's model could not be right.
  The positive charge must be concentrated in a
  tiny volume at the center of

the atom, otherwise the heavy alpha particles
fired at the foil could never be repelled back
towards their source. On this model, the
electrons orbited around the dense nucleus
(center of the atom).
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Rutherfords Experiment

• 1 in 8000 alpha particles were reflected back to
  the source

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What was happening in the gold foil
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Rutherfords idea of the atom


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Bohr the atom

• (1913) Neils Bohr (Denmark) developed the first
  successful model of the atom. Since we still use
  Bohrs model to explain many aspects of physical
  phenomena such as the appearance of light

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Bohr cont.

• Bohrs model of the atom builds on Rutherfords
  basic conception. In detail, the nucleus contains
  a number of relatively high mass particles with
  positive charge called protons along (sometimes,
  not always) with some neutral particles of about
  the same mass called neutrons

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James Chadwick - Neutron

• in 1935
• Bombarded atoms with alpha rays
• Discovered particle with same mass as proton but
  neutral charge - Neutron

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Bohrs idea of the atom

• Orbiting the nucleus, much like planets orbiting
  the sun, are the electrons.

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Quantum Mechanics

• Until 20th century, universe was describe by
  classical mechanics
• Bohr, Schrodinger, and Heisenberg
• Formulated quantum theory
• Electron dont actually orbit
• Electron clouds are more accurate
• Electrons are in multiple places at once

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Heisenbergs Uncertainty Principle

• Electrons are in a superposition of states
• Impossible to know the location, spin, speed all
  at once different from classical physics
• Can only know probabilities

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The Elements

• Each element represents a type of atom
• of protons determine which element
• Oxygen 8 protons, Helium 2 protons
• Neutrons are the largest particle
• 1.675 x 10-27 kg
• Protons, 1.673 x 10-27 kg
• Electrons, 9.12 x 10-31 kg

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Nuclear Force

• How do atoms stay together?
• Protons Electrons -
• attractive electromagnetic forces
• Nucleus? Protons together?
• Nuclear force

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Atomic Number Mass Number

• Atomic Number
• of protons
• Mass Number
• of proton neutrons
• Electrons same as
• protons in a neutral atom

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Isotopes

• Varieties of the same element with different
  numbers of neutrons
• Carbon 12 Carbon 14
• Hydrogen
• Deuterium, Tritium

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Ions

• Atoms are normally neutral balanced
• Gain or lose electrons, get a charge
• Gain negative
• Lose positive
• Results in bonding chemical reactions
• Positive and negative ions attract