Dalton was wrong about all elements of the same type being identical



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Dalton was wrong about all elements of the same type being identical

  • Dalton was wrong about all elements of the same type being identical

  • Atoms of the same element can have different numbers of neutrons.

  • Thus, different mass numbers.

  • These are called isotopes.



  • Frederick Soddy (1877-1956) proposed the idea of isotopes in 1912 (note this was close to 30 years after Dalton’s original idea)

  • Isotopes are atoms of the same element having different masses, due to varying numbers of neutrons.

  • Soddy won the Nobel Prize in Chemistry in 1921 for his work with isotopes and radioactive materials.



proposed by John Dalton in 1804

  • proposed by John Dalton in 1804

  • this theory proposed that matter was composed of small, spherical particles

  • but evidence was later gathered that matter was composed of even smaller bits



during the 1900s evidence was discovered regarding charges:

  • during the 1900s evidence was discovered regarding charges:

    • atoms have positive (Rutherford’s contribution) and negative (Thomson’s contribution) parts
    • charges interact:
  • as a result, revisions to Dalton’s model had to be made



using available data on the atom, J.J. Thomson came up with the idea of having charges embedded with Dalton’s Billiard Balls

  • using available data on the atom, J.J. Thomson came up with the idea of having charges embedded with Dalton’s Billiard Balls

  • Also used cathode ray experiment to discover the existance of the electron









Ernest Rutherford discovered a huge flaw in the previous concept of the atom during his now famous gold foil experiment

  • Ernest Rutherford discovered a huge flaw in the previous concept of the atom during his now famous gold foil experiment



Discovered the Nucleus and the Positive Protons

  • Discovered the Nucleus and the Positive Protons

  • Surmised atoms are made of mostly empty space

  • Didn’t know about the Neutrons

  • Famous Gold Foil Experiment

  • Click on me: http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/ruther14.swf





If previous models were correct alpha particles would have passed straight through the the gold

  • If previous models were correct alpha particles would have passed straight through the the gold







Rutherford found that most (99%) of the alpha particles that he shot at the gold went straight through

  • Rutherford found that most (99%) of the alpha particles that he shot at the gold went straight through

  • From these experiments Rutherford concluded that the atom had a dense positive core, with the rest composed of mostly empty space with the occasional negatively charged electron





Discovered that electrons exist in several distinct layers or levels

  • Discovered that electrons exist in several distinct layers or levels

  • “Jimmy Neutron Model”

  • Travel around nucleus like planets travel around sun

  • Electrons Orbit

  • Electrons can jump between levels with energy being added/released



Niels Bohr proposed that electrons revolve around the central positive nucleus (like planets in the solar system)

  • Niels Bohr proposed that electrons revolve around the central positive nucleus (like planets in the solar system)



Bohr also suggested that the electrons can only revolve in certain orbits, or at certain energy levels (ie, the energy levels are quantized)

  • Bohr also suggested that the electrons can only revolve in certain orbits, or at certain energy levels (ie, the energy levels are quantized)



Found that Electrons live in fuzzy regions or “clouds” not distinct orbits

  • Found that Electrons live in fuzzy regions or “clouds” not distinct orbits

  • Improved on Bohr’s findings

  • Electron location can not be predicted

  • Quantum Mechanical Model



the current understanding of the atom is based on Quantum Mechanics

  • the current understanding of the atom is based on Quantum Mechanics

  • this model sees the electrons not as individual particles, but as behaving like a cloud - the electron can be “anywhere” in a certain energy level

  • Remember back to CPE with electrons behaving like bees in a beehive





most things we do can be explained using Dalton’s and/or Bohr’s model

  • most things we do can be explained using Dalton’s and/or Bohr’s model

  • the Quantum Mechanical model, although most accurate, is complex even at a university level (conceptually and mathematically)

  • Click on me or copy and paste into browser: http://www.hyperhistory.com/online_n2/people_n2/science_n2/atomic_theory.html

  • to learn more about quarks & hadrons (its located near the bottom of the page)



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