The Atomic Model Through
Time
(see p. 54-55 in text)
Aristotle (460 B.C. – 370 B.C.)
• emphasized that nature
consisted of four
elements: air, earth, fire,
and water.
• he did not believe in
discontinuous or separate
atoms, but felt that
matter was continuous
Democritus (460 B.C. – 370 B.C.)
• greek philosopher
• first to suggest the
existence of “atoms”
(Greek word “atomos”
= indivisible)
• atoms indivisible
and indestructible
• no experimental
support
http://www.stenudd.com/myth/Greek/images/democritus_1628_Brugghen.jpg
John Dalton (1766-1844)
•
•
•
english chemist and schoolteacher
used scientific method to test
Democritus’s ideas
Dalton’s atomic theory
1.
2.
3.
4.
elements composed of atoms
atoms of the same element are alike
different atoms can combine in ratios to form
compounds
chemical reactions can occur when atoms are
separated, joined, or rearranged (but atoms
are not created nor destroyed)
J.J. Thompson (1856-1940)
• English physicist
• discovered the
electron
• thought atom was
negative charges
stuck in a positive
charged lump
– referred to as the
“plumb pudding
model”
We still use Thompson’s
“cathode ray tube” in TV and
other applications
Robert A. Millikan (1868-1953)
• American physicist that
continued Thompson’s
work on electrons
• Found the quantity of
charge carried by an
electron (one unit of
negative charge)
• Calculated the mass of an
electron (1/1840th the mass
of a hydrogen atom)
Ernest Rutherford (1871-1937)
• New Zealand physicist
and former student of
Thompson
• proposed that the atom
is mostly empty space
• positive charges and
almost of the mass are
in a small, centralized
region called the
nucleus
“Like howitzer
shells bouncing
off of tissue
paper!”
Evidence for Rutherford’s
Conclusions
http://www.cartage.org.lb/en/themes/Sciences/Chemistry/Generalchemistry/Atomic/BasicSt
ructure/ErnestRutherford/ruther.gif
Rutherford Flash Animation
Try it Yourself!
In the following pictures, there is a target hidden by a cloud.
To figure out the shape of the target, we shot some beams into
the cloud and recorded where the beams came out. Can you
figure out the shape of the target?
The Answers
Target #1
Target #2
Niels Bohr (1855-1962)
• former student of Rutherford
• electrons found only in
specific circular paths (orbits)
around the nucleus
• based on information about
how the energy of an atom
changes when it absorbs and
emits light
• called these fixed energies
“energy levels”
Erwin Schrodinger (1926)
• Austrian physicist
• quantum
mechanical model
– probability of
electron locations
around the
nucleus
– not an exact orbit
• led to the electron
cloud model
Werner Heisenberg (1927)
• German physicist
• Heisenberg
Uncertainty Principle
– impossible to know
the exact position
and velocity of a
particle (electron) at
the same time
• “the observer affects
the observed”
http://www.deutsches-museumbonn.de/ausstellungen/heisenberg/bilder/heisenb
erg_2.jpg