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_standard.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): 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
How does this compare to a physical mixture of iron and 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
How does this example show the existence of atoms (See my notes under ppt slide #12)?
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 these two compounds of sulfur and oxygen, what is the small whole-number ratio described by the
law of multiple proportions?
GoTo:
http://higheredbcs.wiley.com/legacy/college/olmsted/0471478113/dialogue/dialogues/info_dalton.swf
http://antoine.frostburg.edu/chem/senese/101/atoms/dalton.shtml
http://library.thinkquest.org/28582/history/crayexp.htm
Dalton’s Atomic Theory (John Dalton, 1803)
What are the five main ideas of Dalton's theory? What idea compromised the acceptance of his work?
The Historical Development of the Atomic Model
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…
GoTo:
http://micro.magnet.fsu.edu/electromag/java/crookestube/
http://www.wwnorton.com/college/chemistry/gilbert2/tutorials/interface.asp?chapter=chapter_02&folder=c
athode_ray
http://library.thinkquest.org/28582/history/crayexp.htm
William Crookes (1870’s): English physicist
Unknowingly , Crookes had discovered
. Crookes tubes (Geissler tubes) are now
called cathode __________ _________and are used as (See my ppt slide #18) __________________.
GoTo:
http://www-outreach.phy.cam.ac.uk/camphy/electron/electron_index.htm
http://www.aip.org/history/electron/jj1897.htm
http://www.aip.org/history/electron/jjelectr.htm
http://web.lemoyne.edu/~giunta/JTHOMSON.HTML
J.J. Thomson (1897): English scientist
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.
GoTo:
http://g.web.umkc.edu/gounevt/Animations/Animations211/MillikanOilDropExp.swf
http://library.thinkquest.org/28582/history/crayexp.htm
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?
Go To:
http://www.wwnorton.com/college/chemistry/gilbert2/tutorials/interface.asp?chapter=chapter_02&folder=r
utherford_experiment
http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/ruther14.swf
The Historical Development of the Atomic Model
http://www.iun.edu/~cpanhd/C101webnotes/modern-atomic-theory/rutherford-model.html
http://micro.magnet.fsu.edu/electromag/java/rutherford/
Ernest Rutherford (1906): Gold Leaf Experiment
What three conclusions did Rutherford draw from this evidence (slide 46 in ppt)?
Go to the micro.magnet.fsu site.
What do you observe when the slit width is adjusted to 4.0 nm?
How did Rutherford’s experiment disprove Thomson’s model of the atom?
What new model did Rutherford propose?
Draw Democritus’s model of the atom→
http://www.chemcases.com/2003version/nuclear/nc-01.htm
James Chadwick
Which subatomic particle did he discover? 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.
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
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.