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Atomic Theory and Structure of the Atom
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• first to suggest the existence of atoms
• believed atoms were small indivisible particles
• Atom: smallest particle of an element that retains all properties of the element.
A. Democritus (460 B.C. – 370 B.C)
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B. Dalton• Atomic theory composed of 4
postulates– Elements are composed of atoms– Atoms of the same element are
identical, each element is unique– Compounds are composed of atoms
of more than 1 element, atoms combine in fixed ratios to form compounds
– Chemical rxn involves separation, combination, or rearrangement of atoms
Name of theoryBillard Ball
Model
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C. Electron1. Symbol: e−
– Charge −1
**after Dalton, there was a big hiatus in the theory of the atom. Everyone accepted Daltons theory and went on to study radiation for a while, and this is when the electron was discovered
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D. J.J. Thomson• An atom is thought of as a uniform positive
sphere with electrons evenly distributed through out (like plum pudding or a chocolate chip cookie)
Name of theoryPlum Pudding
Model
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Cathode Ray Tube
Battery
Evacuated tube
CathodeAnode
− +
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E. MillikanMillikan Oil Drop
• Found the mass of an
electron to be 9.10×10−28 g
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F. Rutherford• Conducted gold foil experiment
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What did Rutherford expect?
Gold Foil Experiment Animation and Discussion
Click here for video!
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What did Rutherford actually see?
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F. Rutherford
• Concluded all positive charge and mass is concentrated in small region called nucleus and the electrons are outside the nucleus
• Proposed atom is mostly empty space
Name of theoryNuclear Model
Proton– p+
– Charge: +1– Mass:1.673×10−24 g
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G. Bohr• Expanded on the nuclear atom• Electrons travel in circular orbits around
the nucleus
Name of theoryPlanetary Model
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H. Schrödinger• model allowed the electron to occupy
three-dimensional space like an electron “cloud”
Name of theoryQuantum Mechanical
Model or Electron Cloud Model
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I. ChadwickThe NeutronSymbol: n0
Charge: noneMass:1.675×10−24 g
**The neutron was found by comparing the mass of hydrogen to helium, since He increased by 1proton, the mass should be double, but it was four times, leading to a subatomic particle that had mass but no charge.
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Fun Facts
• The volume of a hydrogen nucleus is a trillion times smaller than the volume of a hydrogen atom, yet the nucleus contains most of the mass.
• If the nucleus (proton) of a hydrogen atom were as large as the width of a human thumb, the electron would be on the average about one kilometer away in a great expanse of empty space.