History of the Atom White Board Presentations

1. History of the AtomWhite Board Presentations John Dalton • English schoolteacher, 1766-1844 • Problem w/ previous model: No logical/scientific structure or rules • Atomic Theory Of Matter: • 5 postulates (next slide) • Model: 

2. Atomic Theory of Matter • Each element is composed of atoms • All atoms of a given element are identical • All atoms of different elements are different (#2 and #3 often combined) • Atoms are neither created nor destroyed • Atoms combine to form compounds and a given compound has the same relative numbers and kinds of atoms

4. History of the AtomWhite Board Presentations JJ Thomson • English physicist, 1856-1940 • Problem w/ previous model: no subatomic particles • Experimental Evidence: cathode ray tube experiment where particles smaller than atoms are shown to have negative charge (electrons) with a small mass to charge ratio • Model: Plum Pudding Model:

6. History of the AtomWhite Board Presentations Robert Millikan • American physicist, 1869-1953 • Problem w/ previous model: no known amount of charge of an electron • Experimental evidence: oil drop experiment where (-) charged oil drops pass between (+) plates and rate of fall is measured. All were multiples of 1.60 x 10-19C (one e-). • He also calculated the mass of an e- based on the charge and the known mass:charge ratio. • Model: (looks the same as Thomson’s)

8. History of the AtomWhite Board Presentations Ernest Rutherford • Scientist from New Zealand, 1871-1937 • Problem w/ previous model: does not account for  particles bouncing back • Experimental evidence: gold foil experiment where he shot (+) charged  particles at very thin gold foil and saw evidence for a very small, dense, positively charged core to the atom. • Model: Nuclear atom

10. History of the AtomWhite Board Presentations Henry Moseley • English scientist, 1887-1915 (Rutherford’s student) • Problem w/ previous model: does not account for elemental differences • Experimental evidence: X-Ray diffraction patterns of metals showed that each element has a unique positive charge in the nucleus that determines the atoms identity (atomic number) • Model: (looks the same as Rutherford’s)

12. History of the AtomWhite Board Presentations Sir James Chadwick • English physicist, 1891-1974 • Problem w/ previous model: mass of protons did not account for mass of nucleus and a nucleus of all protons (+) would repel and fall apart • Experimental evidence: He bombarded metals (beryllium) with  particles, causing a radiation that was both neutral and a mass similar to a proton • Model: (looks the same as Rutherford's, but with neutrons in the nucleus as well)

14. History of the AtomWhite Board Presentations Niels Bohr • Danish physicist, 1885-1962 • Problem w/ previous model: did not account for specific energy levels of e- seen in the light spectrum analysis • Experimental evidence: The energy spectrum of hydrogen shows discrete “quanta” of energy involved in e- configurations • Model: (Bohr model)