The Historical Development of the Atomic Model
Directions:
Use the power point presentation or supplemental notes posted on our class web site:
http://www.broadneck.org/wohlfarth1/atom_honors_2013.html
to guide you through most of the sections below. Some sections will require you to use some external
sites. The addresses (and hyperlinks) are included as needed.
Continuous Theory of Matter = the idea that all matter can be divided into smaller and smaller pieces
.
Discontinuous (Particle) Theory of Matter = (~400 B.C., Democritus, Leucippus ) matter is made up of
particles so small and indestructible that they cannot be divided into anything smaller.
“Atom” comes from the Greek word atomos, meaning?
.
Draw Democritus’s model of the atom→
law of conservation of mass (1770’s, Antoine Lavoisier): (slide 7 in ppt) provided the first experimental
evidence that (explain)…
law of definite proportions (Joseph Proust, 1799) the proportion by mass of the elements in a pure
compound is always the same
Examples:
all samples of water (H2O) contain a ratio of 8 g oxygen to 1 g hydrogen
all samples of iron sulfide (FeS) contain a ratio of 7 g iron to 4 g sulfur
law of multiple proportions = when a pair of elements can form 2 or more compounds, the masses of
one element that combine with a fixed mass of the other element form simple, whole-number ratios
Example A: 2 compounds of hydrogen and oxygen, H2O and H2O2
2 H2O  2 H2 + O2
2 H2O2  2 H2O + O2
Example B: Sulfur and oxygen can form 2 compounds.
Sulfur dioxide samples show a ratio of 2 g S to 2 g O.
Sulfur trioxide samples show a ratio of 2 g S to 3 g O.
For alternative resources to my notes go to:
http://higheredbcs.wiley.com/legacy/college/olmsted/0471478113/dialogue/dialogues/info_dalton.swf
http://antoine.frostburg.edu/chem/senese/101/atoms/dalton.shtml
Dalton’s Atomic Theory (John Dalton, 1803)
(slide 11 in ppt) What are the main ideas of Dalton's theory?
Which of the basic ideas of Dalton's Atomic Theory is an extension of what the Greeks believed about
atoms?
Draw Dalton’s model of the atom→
Dalton’s ideas are still useful today, but modifications to his theory have been made…
The Historical Development of the Atomic Model
For alternative resources to my notes go to:
http://www-outreach.phy.cam.ac.uk/camphy/electron/electron_index.htm
http://www.aip.org/history/electron/jj1897.htm
J.J. Thomson (1897): English scientist (slide 26 in ppt)
What conclusion did Thomson draw from his observations?
How did Thomson’s findings revise Dalton’s Atomic Theory?
Draw the plum pudding model →
Using a mass spectrometer, Thomson was able to calculate the charge to mass ratio (e/m), of an
electron.
For alternative resources to my notes go to:
http://g.web.umkc.edu/gounevt/Animations/Animations211/MillikanOilDropExp.swf
http://library.thinkquest.org/28582/history/crayexp.htm
HONORS ONLY: Robert Millikan determined the charge on an electron in his oil drop experiment.
Based on the interaction between the oil drops and the electric field, what did Millikan conclude?
For great visuals & tutorials of Rutherford’s Gold Foil experiment go to:
http://www.wwnorton.com/college/chemistry/chemistry3/ch/02/chemtours.aspx (click on Rutherford
Experiment)
http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/ruther14.swf
http://www.iun.edu/~cpanhd/C101webnotes/modern-atomic-theory/rutherford-model.html
Ernest Rutherford (1906): Gold Leaf Experiment
What three conclusions did Rutherford draw from this evidence (slide 46 in ppt)?
How did Rutherford’s experiment disprove Thomson’s model of the atom?
What new model did Rutherford propose?
Draw Rutherford’s model of the atom→
The Historical Development of the Atomic Model
For alternative resources to my notes go to:
http://www.chemcases.com/2003version/nuclear/nc-01.htm
James Chadwick
Which subatomic particle did he discover (slide 47 in ppt)? Compare its mass to that of a proton.
Niels Bohr (1913): Danish physicist. Proposed that electrons can only possess certain amounts of
energy, called quanta. (slide 51 in ppt)
What does this mean in terms of the location of electrons?
Draw Bohr’s model of the atom→
Electron Energy Levels in the Bohr Model
 energy levels = the possible electron orbits of an atom
 ground state = exists when an atom is energetically stable
 excited state = exists when electrons absorb energy, are moved to higher levels, and the atom
become energetically unstable
Charge-Cloud Model, or Quantum Mechanical Model
Quantum Mechanics = the idea that energy is quantized
(slide 57 in ppt) In an atom, where are the electrons, according to the quantum mechanical model?
Draw the Modern view of the Atom→
Summary of the Atomic Model
The atomic model has changed over time, and continues to change as we learn more.