Transcript AtomicStructureTheory_ejp

The History of Atomic
Theory
• Try to place yourself in the timeframe of these scientists…
• Remember, our current technologies
were not yet available
• Pay attention to the chronology of
these discoveries (i.e. the pace of
discovery)
• “Excuse me... how can you
discover a particle so small that
nobody has ever seen one?”
--J.J. Thomson
Ancient Greece, 400 B.C.
• Democritus
• Through philosophical
thought process, he
rationalized that there
must be some smallest
particle upon which
nature was founded
• Called this particle
Atomos
• Other philosophers such
as Plato, Sophocles, and
Aristotle developed ideas
of their own regarding the
nature of matter, however,
Democritus is generally
credited
England, 1800 A.D.
• John Dalton
• Performing exacting
experiments with
gases and how they
combine, Dalton
developed Four
Postulates describing
the results of his work.
Picture courtesy of http://atoms.hypermart.net/dalton/
Dalton’s Four Postulates
1. Elements are composed of atoms (submicroscopic, indivisible particles).
2. Atoms of an element are similar to each other,
but different than other element atoms.
3. Elements can mix together, or chemically
combine to form compounds, but only in whole
number ratios.
4. Atoms can rearrange, but an element cannot be
changed into another element as a result of a
chemical reaction.
Dalton’s Model (solid sphere)
Germany, 1886
• Eugen Goldstein
• Expanding on the
work of Crookes and
his vacuum tubes,
Goldstein combines
cunning logic and a
“simple” experiment
to discover the proton
• Goldstein reasoned
that if a neutral
particle gives off a
negatively charge
particle in the presence
of an electric field,
then surely it must
also produce a
positively charged
particle as well.
Goldstein’s Apparatus
Picture courtesy of http://dbhs.wvusd.k12.ca.us
England, 1897
• Sir Joseph John Thomson
• Thomson expanded upon the
work of other scientists such
as:
• Thomas Edison who had
already constructed a
complete electric circuit
(circa 1883)
• William Crookes who
developed the original
gas vacuum tube, called a
Crookes Tube (circa
1870)
Picture courtesy of www.aip.org
Thomson’s Experiment
• Thomson utilized a
sealed gas in a
chamber/tube with two
electrodes, called a
Cathode Ray Tube.
• Thomson discovered that regardless
of the gas present in the tube, or the
type of metal used for the
electrodes, the eerie beam of light
was always present.
• He concluded that a small particle,
• The electrodes were
connected to a high
voltage source, at
which point a glowing
beam of light formed.
which he called a “corpuscle” and
was later known as an electron,
must be a part of the atoms of all
elements.
http://micro.magnet.fsu.edu/electromag/java/crookestube/index.html
Thomson’s Apparatus
Animation & diagram courtesy of www.aip.org
Plum Pudding Model
Diagram courtesy of Addison Wesley Publishers
America, 1916
• Robert A. Millikan
• Calculated the charge on an
electron in his very famous “Oil
Drop” experiment
• He was also able to calculate the
mass of an electron from the
“Oil Drop” experiment
• http://www.bun.falkenberg.se/gymnas
ium/amnen/fysik/millikaneng.html
Picture courtesy of http://www.nobel.se
England, 1932
• Sir James Chadwick
• The existence of an electrically
neutral particle had already
been theorized, but as yet no
one had documented proof of
its existence.
• Expanding on his own work
with radioactive Beryllium
(and other elements) and yradiation, Chadwick reasoned
that the energy emitted from
Beryllium could not be solely
y-radiation.
• Chadwick then
hypothesized the presence
of another particle, the
neutron, and included this
particle in his
calculations.
• With the addition of the
neutron into the equation,
Chadwick’s observations
of the emitted energy
agreed well his own
calculations.
The History of Atomic
Structure
• Now that we are familiar with
the history behind protons,
neutrons and electrons, we
can ask ourselves the
question:
• How did we come to our
current model of the
structure of the atom?
Manchester, England 1911
• Ernest Rutherford
• Rutherford poses the
following questions:
• How are the neutrons and
protons arranged in the
atom?
• How are electrons so
easily stripped from an
atom to form ions?
Picture courtesy of www.aip.org
Gold Foil Experiment
• Rutherford decided to
test the “Plum
Pudding” model of
atomic theory…
• He devised the
following apparatus
utilizing radioactive
alpha particles…
Diagram courtesy of Addison Wesley Publishers
Rutherford con’t
http://chemmovies.unl.edu/ChemAnime/RUTHERFD/RU
THERFD.html
setup explanation
http://www.micro.magnet.fsu.edu/electromag/java/rutherf
ord/
gold foil scattering
http://www.phys.virginia.edu/classes/109N/more_stuff/A
pplets/rutherford/rutherford2.html
Thomson scattering
http://www.phys.virginia.edu/classes/109N/more_stuff/A
pplets/rutherford/rutherford.html
Rutherford scattering
The Nucleus
• Rutherford’s new proposal for the structure of an atom…
• If the nucleus were the size of a penny, then the entire
atom would be the size of the metrodome!
Diagram courtesy of Addison Wesley Publishers
Diagram courtesy of www.britannica.com
The story of the atom is to be
continued next semster…
Coming up:
Quantum Theory of the early 1900’s!
Planck
Einstein
Bohr
Heisenberg
Schroedinger
England, 1900
• Max Planck
• Studying radiation
emission from hot objects
(think of a hot stove
burner) relative to
temperature, intensity and
wavelength
• The current belief was that
a continuous spectrum of
energy emission was
possible
Picture courtesy of www.nobel.se
Quite the Conundrum
• The prevailing law of
physics predicted
continuous light
spectrums…
Diagram courtesy of http://csep10.phys.utk.edu
• Experimental results,
however, showed a
different picture…
Diagram courtesy of http://spacesciencegroup.nsula.edu
Climbing the Ladder
• Planck made a
revolutionary statement:
• Energy can be released or
absorbed by atoms only
in fixed “chunks” or
quantum…
• Energy of quantum is
proportional to frequency
Diagrams courtesy of http://csep10.phys.utk.edu
Germany to America, 1905
• Albert Einstein
• Expanding on Plank’s
work, Einstein studied
the unusual phenomenon
known as the
photoelectric effect…
Picture courtesy of www.chembio.uoguelph.ca
Photoelectric Effect
• When light shines on a clean metal surface,
sometimes the surface emits electrons…
• For each metal, there is a minimum frequency
of light below which no electrons are
emitted…
What a Genius…
• Einstein assumed that
• Simplified
energy (light) striking the
metal surface must behave
like a stream of tiny
energy packets, called
photons…
• Each photon MUST have
a quantized energy
proportional to the
frequency of the light…
Photoelectric Effect
Animation
• http://faculty.ucc.edu/c
hemistrypankuch/Photoelectric/
PE3.html
Denmark, 1913
• Niels Bohr
• Bohr posed the question:
• If the electron is
negatively charged, and
the proton is positively
charged…why don’t
orbiting electrons crash
into the nucleus and
collapse the atom?
Picture courtesy of www-groups.dcs.st-and.ac.uk
Look to the Heavens
• Bohr looked at our
solar system for
inspiration in creating
his atomic model…
• He proposed that
electrons orbit the
nucleus in fixed orbits
corresponding to
quantized energy
Diagram courtesy of www.howstuffworks.com
Germany, 1920’s
• Werner Heisenberg
• Knowing that matter
behaves both as a
particle and as a wave,
Heisenberg extrapolated
the Uncertainty
Principle…
• It is inherently impossible
to simultaneously know
both the exact momentum
and location in space
Picture courtesy of www. scienceworld.wolfram.com
Austria, 1926
• Erwin Schrodinger
• Combined Heisenberg’s
Uncertainty Principle
with Bohr’s atomic
model (and a whole lot of
Super-duper complex
mathematics) to create…
Picture courtesy of www.nobel.se/physics/laureates/
Atomic Orbitals & Quantum
Mechanics
Pictures courtesy of www.orbitals.com