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The Atomic Model Through Time
(see p. 54-55 in text)
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Aristotle Aristotle (460 B.C. – 370 B.C.)(460 B.C. – 370 B.C.)
• emphasized that nature consisted of four elements: air, earth, fire, and water.
• he did not believe in discontinuous or separate atoms, but felt that matter was continuous
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Democritus (460 B.C. – 370 B.C.)Democritus (460 B.C. – 370 B.C.)
• greek philosopher• first to suggest the
existence of “atoms” (Greek word “atomos” = indivisible)
• atoms indivisible and indestructible
• no experimental support
http://www.stenudd.com/myth/Greek/images/democritus_1628_Brugghen.jpg
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John Dalton (1766-1844)John Dalton (1766-1844)
• english chemist and schoolteacher• used scientific method to test Democritus’s
ideas• Dalton’s atomic theory
1. elements composed of atoms2. atoms of the same element are alike3. different atoms can combine in ratios to form
compounds4. chemical reactions can occur when atoms are
separated, joined, or rearranged (but atoms are not created nor destroyed)
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J.J. Thompson (1856-1940)J.J. Thompson (1856-1940)
• English physicist• discovered the
electron• thought atom was
negative charges stuck in a positive charged lump– referred to as the
“plumb pudding model”
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We still use Thompson’s “cathode ray tube” in TV and
other applications
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Robert A. Millikan (1868-1953)Robert A. Millikan (1868-1953)
• American physicist that continued Thompson’s work on electrons
• Found the quantity of charge carried by an electron (one unit of negative charge)
• Calculated the mass of an electron (1/1840th the mass of a hydrogen atom)
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Ernest Rutherford (1871-1937)Ernest Rutherford (1871-1937)
• New Zealand physicist and former student of Thompson
• proposed that the atom is mostly empty space
• positive charges and almost of the mass are in a small, centralized region called the nucleus
“Like howitzer shells bouncing off of tissue paper!”
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Evidence for Rutherford’s Evidence for Rutherford’s ConclusionsConclusions
http://www.cartage.org.lb/en/themes/Sciences/Chemistry/Generalchemistry/Atomic/BasicStructure/ErnestRutherford/ruther.gif
Rutherford Flash Animation
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Try it Yourself!Try it Yourself!In the following pictures, there is a target hidden by a cloud. To figure out the shape of the target, we shot some beams into the cloud and recorded where the beams came out. Can you figure out the shape of the target?
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The AnswersThe Answers
Target #1 Target #2
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Niels Bohr (1855-1962)Niels Bohr (1855-1962)
• former student of Rutherford• electrons found only in
specific circular paths (orbits) around the nucleus
• based on information about how the energy of an atom changes when it absorbs and emits light
• called these fixed energies “energy levels”
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Erwin Schrodinger (1926)Erwin Schrodinger (1926)
• Austrian physicist • quantum
mechanical model – probability of
electron locations around the nucleus
– not an exact orbit• led to the electron
cloud model
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Werner Heisenberg (1927)Werner Heisenberg (1927)
• German physicist• Heisenberg
Uncertainty Principle – impossible to know the exact position and velocity of a particle (electron) at the same time
• “the observer affects the observed”
http://www.deutsches-museum-bonn.de/ausstellungen/heisenberg/bilder/heisenberg_2.jpg
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