Atomic models 2011.notebook 1 September 16, 2011 Sep 112:00 AM It all started in Greece about 2500 years ago • Democritus • matter is discontinuous • indivisible • like grains of sand • differences in arrangement account for all elements Sep 112:03 AM Along came Aristotle about 100 years later... • matter is continuous • made of earth, wind, water, fire • a magic knife could cut it forever • particles divisible Sep 112:08 AM John Dalton 1800's • all matter made of atoms (indivisible) • atoms of same element are identical • atoms of different elements are different • during chemical reactions atoms combine in definite proportions to form products Sep 112:12 AM J.J. Thompson 1906 • discovered the electron, a negative () particle that makes up part of the atom. • They can detach themselves easily (static electricity) • The atom is a positively (+) charged ball with electrons () imbedded • Atoms are neutral + = • Atoms are divisible Sep 112:15 AM Rutherford and the Gold Foil Experiment (McGill 1911) • Radioactive substances can give off alpha (+), beta () and gamma (neutral) radiation • Rutherford bombarded gold foil with alpha (+) particles Sep 112:17 AM Rutherford and the Gold Foil Experiment (Con’t)
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Atomic models 2011.notebook
1
September 16, 2011
Sep 112:00 AM
It all started in Greece about 2500 years ago
• Democritus • matter is discontinuous• indivisible• like grains of sand• differences in arrangement account for all elements
Sep 112:03 AM
Along came Aristotle about 100 years later...
• matter is continuous• made of earth, wind, water, fire• a magic knife could cut it forever• particles divisible
Sep 112:08 AM
John Dalton 1800's
• all matter made of atoms (indivisible)
• atoms of same element are identical
• atoms of different elements are different
• during chemical reactions atoms combine in definite proportions to form products
Sep 112:12 AM
J.J. Thompson 1906
• discovered the electron, a negative () particle that makes up part of the atom.
• They can detach themselves easily (static electricity)
• The atom is a positively (+) charged ball with electrons () imbedded
• Atoms are neutral + =
• Atoms are divisible
Sep 112:15 AM
Rutherford and the Gold Foil Experiment(McGill 1911)
• Radioactive substances can give off alpha (+), beta () and gamma (neutral) radiation
• Rutherford bombarded gold foil with alpha (+) particles
Sep 112:17 AM
Rutherford and the Gold Foil Experiment (Con’t)
Atomic models 2011.notebook
2
September 16, 2011
Sep 112:18 AM
Rutherford and the Gold Foil Experiment (Con’t)
Results:• Most alpha (+) particle passed through
• A few hit the gold foil and bounced back
Conclusions:
• The atom is mostly empty
• The (+) alpha particles hit a concentrated (+) area called a nucleus
• The nucleus is Positively (+) charged
Sep 112:22 AM
Rutherford and the Gold Foil Experiment (Con’t)
Sep 112:24 AM
Rutherford’s Atomic Model
• The nucleus contains positively (+) charged particles called Protons
• There has to be an equal number of negatively () charged Electrons circling the nucleus to cancel each other out
Sep 112:25 AM
The Problem with Rutherford’s Model
• Opposite charges attract each other; the atom should collapse.
• What prevents the () electrons from crashing into the (+) protons ?
Sep 112:27 AM
The RutherfordBohr Model
Bohr’s Contribution • White light is broken into the electromagnetic spectrum
Sep 112:28 AM
• Bohr concluded electrons could occupy specific orbits (energy levels, shells)
• Electrons could jump orbits when energy (heat or electricity) is supplied
• When an electron falls back to it’s original orbit it gives off the energy in the form of colored light