Atomic Theory

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Atomic Theory

• Past and Present

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A. Democritus (400 BCE)
I. Early Atomic Theory
http//www.timelineindex.com/content/view/1228

• Matter is made up of particles called atomos or
  atoms

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B. John Daltons 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.
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3)Law of Multiple Proportions
H2O Water
H2O2 Hydrogen Peroxide
2 grams H 16 grams O
2 grams H 32 grams O

• Compounds always break down into simple ratios of
  elements by mass.

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C. Thomsons Atomic ModelPlum 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.

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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.

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E. Rutherfords Atomic ModelPlanetary (1911)
Rutherford discovered that all atoms contain a
dense, positively-charged nucleus. Most of the
atom is empty space.
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Gold Foil Experiment
http//www.regentsprep.org/Regents/physics/phys05/
catomodel/ruther.htm
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F. Bohrs Atomic ModelQuantized 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.
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Emission and Absorption Spectra
H Hg Ne H
http//www.cbu.edu/jvarrian/252/emspex.jpg
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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 chargeevidence of the neutron (same mass
  as a proton but neutral)

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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 1920s!!
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II. Atomic Model moves from the planetary view
into Quantum Theory (1920s)

• Quantum Theory is based on waves (electrons
  behave like both particles and waves) and
  probabilities

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Light as Waves
http//ircamera.as.arizona.edu/NatSci102/lectures/
light.htm
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Several scientists have contributed to Quantum
Theory
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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!

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B. Louis de Broglie (1923)

• Quantum numbers could describe BOTH the orbits of
  electrons and the spectral lines produced.
• Particles can act like waves

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C. Werner Heisenberg (1925)

• Discovered that we cant know exactly both speed
  and position of electronsthe Uncertainty
  Principlebecause our methods of measuring atoms
  disturbs them.

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D. Erwin Schrödinger (1926)

• Did The Math and put it all together
• He treated electrons mathematically as waves
• We cant know exactly where an electron is, but
  we can give a location of high probability

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E. Max Born (1926)Electron Probability
Born introduced the idea of probability in
describing the atomic worldwe dont 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
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Modern Atomic ModelSummaryQuantum Mechanics
Electrons are like waves. We cant know exactly
where electrons are, only where they are most
likely.
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Orbital Quantum Number

• Gives the shape of the orbital cloud (within 90
  probability)
• s orbital
• spherical

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Orbital Quantum Number
p orbitalsdumbbell shaped or similar to a
figure 8
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Orbital Quantum Number
d orbitalsshape has 4 lobes
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Orbital Quantum Number
f orbitalssome people describe a daisy-like
http//www.d.umn.edu/pkiprof/ChemWebV2/AOs/index.
html
www.chem.ous.ac.jp
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III. What makes up protons and neutrons? Quarks!

• During the 1960s various physicists discovered
  fundamental particles, known as quarks
• Protons and neutrons are made up of quarks
• Electrons are also fundamental particles

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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