Atomic Theory
Past and Present
I. Early Atomic Theory
A. Democritus (400 BCE)
http://www.timelineindex.com/content/view/1228
Matter is made up of particles
called “atomos” or atoms
B. John Dalton’s Atomic Model
(1808)
Oxygen
Hydrogen
•1) Atoms are hard spheres which
cannot be further broken down.
•2) Atoms of one element are
exactly alike, but different from
atoms of another element.
•3)Law of Multiple Proportions
H2O
Water
2 grams H + 16 grams O
H2O2
Hydrogen Peroxide
2 grams H + 32 grams O
•Compounds always break down into
simple ratios of elements by mass.
C. Thomson’s Atomic Model
“Plum Pudding” (1897)
http://www.aip.org/history/electron/jjcrooke.htm
•JJ Thomson discovered all elements
contain ELECTRONS by using the
cathode ray tube; Electrons are small,
negatively charged particles.
D. Eugene Goldstein (1900)
• Discovered the proton using the
cathode ray tube. The “canal rays”
traveled in the opposite direction of the
electron beam.
• Protons are positively charged as
compared to electrons but protons are
much more massive.
E. Rutherford’s Atomic Model
“Planetary” (1911)
•Rutherford discovered that all atoms
contain a dense, positively-charged
nucleus.
•Most of the atom is empty space.
Gold Foil Experiment
http://www.regentsprep.org/Regents/physics/phys05/catomodel/ruther.htm
F. Bohr’s Atomic Model
“Quantized Electrons” (1913)
For now, the
electron
pattern is
2: 8: 8: 2
•Electrons are only allowed certain,
specific energies.
•Electrons can be on the rings but
not in-between.
Emission and Absorption
Spectra
H
Hg
Ne
H
http://www.cbu.edu/~jvarrian/252/emspex.jpg
G. Sir James Chadwick (1932)
Discovered the neutron
• Chadwick found that the particles given
off by polonium increased the mass of
the atoms but not their charge—
evidence of the neutron (same mass as
a proton but neutral)
IN SUMMARY
• Atoms contain a dense nucleus
with protons and neutrons.
• Atoms are mostly empty space.
• The nucleus has electrons orbiting
around the nucleus in discrete
energy levels.
• Electron-small mass, negative
• Proton-much larger mass, positive
• Neutron-about same mass as
proton, no charge
If this is your vision of an atom,
you are back in the 1920’s!!
II. Atomic Model moves from
the planetary view into
Quantum Theory (1920’s)
• Quantum Theory is based on
waves (electrons behave like
both particles and waves) and
probabilities
Light as Waves
http://ircamera.as.arizona.edu/NatSci102/lectures/light.htm
Several scientists have
contributed to Quantum
Theory
A. Max Planck (1900)
• Light had been thought of as WAVES.
• Higher energy means higher frequency.
• Light is composed of tiny packets
(photons) of quantized energy.
• Light can also act like particles!
B. Louis de Broglie (1923)
• Quantum numbers could describe BOTH
the orbits of electrons and the spectral
lines produced.
• Particles can act like waves
C. Werner Heisenberg (1925)
• Discovered that we can’t know exactly
both speed and position of electrons—
the “Uncertainty Principle”—because
our methods of measuring atoms
disturbs them.
D. Erwin Schrödinger (1926)
• Did “The Math” and put it all together
• He treated electrons mathematically as waves
• We can’t know exactly where an electron is,
but we can give a location of high probability
E. Max Born (1926)
Electron Probability
Born introduced the idea of probability in describing
the atomic world—we don’t know exactly where the
electron is but we have a pretty good idea where it
probably is.
http://www.regentsprep.org/Regents/physics/phys05/catomodel/ruther.htm
Modern Atomic Model
Summary—“Quantum Mechanics”
•Electrons are like waves.
•We can’t know exactly where electrons
are, only where they are most likely.
Orbital Quantum Number
•Gives the shape of the orbital cloud
(within 90% probability)
s orbital—
spherical
Orbital Quantum Number
p orbitals—”dumbbell” shaped
or similar to a “figure 8”
Orbital Quantum Number
d orbitals—shape has 4 “lobes”
Orbital Quantum Number
f orbitals—some people describe a
“daisy-like”
http://www.d.umn.edu/~pkiprof/ChemWebV2/AOs/i
ndex.html
www.chem.ous.ac.jp
III. What makes up protons
and neutrons? Quarks!
• During the 1960’s various
physicists discovered fundamental
particles, known as quarks
• Protons and neutrons are made up
of quarks
• Electrons are also fundamental
particles
IV. What is the most recent
theory? String Theory (late
20th Century to present)
• All fundamental particles are made
of vibrating strings/strands of
energy
http://www.columbia.edu/cu/record/23/18/14.html