History of the AtomNotes

Dalton pg 88-89 Zumdahl Thomson pg 91 Figure 4.3 Zumdahl

pg 97 & 98 LeMay Millikan pg 98 LeMay Rutherford pg 100-101 LeMay Figure 3-16 pg 91 –93 Zumdahl Moseley pg 104 LeMay Chadwick pg 94 Zumdahl Bohr pg 287-288 Zumdahl Figure 10-

17 pg 284 –287 Zumdahl

History of the AtomWhite Board Presentations

Democritus (example) Greek philosopher, around 400 BCEProblem with previous model: no

system or explanation, just “magic”Model: “atomos” compose the world- tiny,

indivisible particles that retain the chemical identity of the element

History of the AtomWhite Board Presentations

John DaltonEnglish schoolteacher, 1766-1844Problem w/ previous model: No

logical/scientific structure or rules Atomic Theory Of Matter:

5 postulates (next slide)

Model:

Atomic Theory of Matter

1. Each element is composed of atoms2. All atoms of a given element are

identical3. All atoms of different elements are

different (#2 and #3 often combined)4. Atoms are neither created nor destroyed5. Atoms combine to form compounds and

a given compound has the same relative numbers and kinds of atoms

History of the AtomWhite Board PresentationsJJ ThomsonEnglish physicist, 1856-1940Problem w/ previous model: no

subatomic particlesExperimental Evidence: cathode ray

tube experiment where particles smaller than atoms are shown to have negative charge (electrons) with a small massto charge ratio

Model: Plum Pudding Model:

History of the AtomWhite Board PresentationsRobert MillikanAmerican physicist, 1869-1953Problem w/ previous model: no known

amount of charge of an electronExperimental 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)

History of the AtomWhite Board PresentationsErnest RutherfordScientist from New Zealand, 1871-1937Problem w/ previous model: does not

account for particles bouncing backExperimental 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

History of the AtomWhite Board Presentations

Henry MoseleyEnglish scientist, 1887-1915 (Rutherford’s

student)Problem w/ previous model: does not account

for elemental differencesExperimental 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)

History of the AtomWhite Board Presentations

Sir James ChadwickEnglish physicist, 1891-1974Problem 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)

History of the AtomWhite Board Presentations

Niels BohrDanish physicist, 1885-1962Problem 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)

Info on Experimentation: Thomson's Experiment:

http://www.aip.org/history/electron/jj1897.htm Cathode Ray tube- http://www.youtube.com/watch?

v=O9Goyscbazk Millikan's Experiment:

http://webphysics.davidson.edu/applets/pqp_preview/contents/pqp_errata/cd_errata_fixes/section4_5.html

Oil Drop- http://www.youtube.com/watch?v=XMfYHag7Liw

Here’s what the drops would actually look like… don’t watch the whole thing.  It’s hypnotic- http://www.youtube.com/watch?v=SxcxN_UVuFI

Ruthorford's Experiment: http://www.rsc.org/chemsoc/timeline/pages/1911.html

Gold Foil- http://www.youtube.com/watch?v=XBqHkraf8iE