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History of the Atomic Model
Greeks (300 B.C)
• Hypothesized that matter cut into smaller and smaller pieces would eventually reach the atom (literally meaning indivisible)• Thought there were 4 elements: Air, Fire, Earth,
Water• Findings based on reason, not empirical evidence (experimentation)
Dalton’s “Billiard Ball” Model (1808)• Atoms are small indestructible, indivisible spheres• All matter is made of atoms or
combinations of atoms (in specific proportions/ratios) • All atoms of one element are identical•Matter cannot be created or destroyed,
only changed in form
Dalton’s Laws
• Law of definite composition: elements combine in a characteristic mass ratio• Law of multiple proportions: there may be
more than one mass ratio• Law of conservation of mass: total mass
remains constant
Dalton’s Shortfall
• Dalton’s model did not explain the behaviour of substances (why atoms combined in ratios)
Thomson’s “Raisin Bun” or “Plum Pudding” Model (1897)• Discovered a negatively charged particle, the electron
using cathode ray experiments• Atoms consisted of a positively charged sphere
embedded with negatively charged electrons• Overall charge of the atom is neutral• Charged particles could exist if electrons were added or removed
http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::100%::100%::/sites/dl/free/0072512644/117354/01_Cathode_Ray_Tube.swf::Cathode%20Ray%20Tube
Rutherford’s “Electron Cloud” Model (1911)
• Discovered the proton by using cathode ray tubes and found the smallest possible positive charge from ionized hydrogen• Gold Foil Experiment:• Aimed alpha particles at thin metal foil• 1/8000 alpha particles were deflected significantly• Inconsistent with Thomson’s Model
• Conclusion:• Atoms are made up mainly of empty space, with
small concentrated charge around the center
Gold Foil Experiment
• http://www.mhhe.com/physsci/chemistry/animations/chang_2e/rutherfords_experiment.swf
Rutherford’s Limitations• A nucleus made of entirely positive charges should fly
apart• Did not explain the total mass of the atom • Solution: In 1932 the neutron was discovered by
Chadwick by bombarding nuclei with alpha particles and noticing a difference in their mass and that of the number of protons they contained
• Orbiting electrons should emit light and constantly lose energy and then collapse• However, matter is very stable and this does not
happen
Bohr’s “Planetary” Model• Electrons have specific, allowable energy levels• Electrons moving within an energy level (shell)
do not lose energy• An electron changes energy levels by emitting
or absorbing a specific quantity (quantum) of energy in the form of light (photons)•When electrons in an atom are in the lowest
possible energy levels, they are in its ground state
Bohr’s “Planetary” Model (cont.)•When an electron absorbs energy in the form
of photons, the atom becomes excited• If enough energy is absorbed, the electron can
leap to the next energy level•When the electron returns back to it’s original
state, it releases energy in the form of a photon, which we see as visible light• The energy of the photon determines its
wavelength or colour• Each element has its own frequencies of color
http://www.visionlearning.com/library/flash_viewer.php?oid=1347&mid=51
Summary of Atomic Models1) Dalton’s “Billiard ball” model (1800-1900)
Atoms are solid and indivisible.
2) Thomson’s “Raisin bun” model (1900)Negative electrons in a positive framework.
3) Rutherford’s “Nuclear” model (~1910)Atoms are mostly empty space.Negative electrons orbit a positive nucleus.
4) Bohr’s “Planetary” model (~1920)Negative electrons orbit a positive nucleus.Quantized energy shells
5) Quantum Mechanical model (~1930)Electron probabilities (orbitals)
Song: http://www.youtube.com/watch?v=vUzTQWn-wfE