Atomic Structure and Function 25 September 2003 Professor Taylor

1
Atomic Structure and Function25 September
2003Professor Taylor

• Physics
• Chemistry
• Astronomy
• Geology
• Biology
• Ecology

2
Atomic Structure and FunctionOutline

• Early history in 18th and 19th centuries
• Discovery of the electron (1897)
• Discovery of the nucleus (1911)
• Atomic structure consternation
• Atomic structure the Bohr atom (1913)
• Atomic Structure Theory of Quantum Mechanics
  (1930s)
• Applications of our understanding
• Discovery of the elements

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Atomic Structure and FunctionTake Home Message

• Science as a evolutionary, incremental process
  over decades (e.g., exploring the structure of
  atom)
• Role of hypothesis development and testing in the
  sciences scientific method
• Demystification of physics and chemistry
  structure of the atom is a simple concept
• Understanding of atomic structure underpins
  future course topics (e.g., chemical reactions,
  geology, biochemistry, ecology, environmental
  sciences)
• Applications some examples

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Early History in 18th and 19th Century

• Atomic Theory of Dalton
• Atoms as indivisible marbles
• Element (shape and size the same for a given
  element)
• Molecules/compounds (e.g., water or H2O) combine
  in predictable ways and ratios
• Atoms as basis for understanding chemistry but
  without electrons, protons and neutrons (analogy
  to a marble)
• Elements 60 known (e.g., hydrogen, gold, sodium)

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Atomic Structure and FunctionOutline

• Early history in 18th and 19th centuries
• Discovery of the electron (1897)
• Discovery of the nucleus (1911)
• Atomic structure with consternation
• Atomic structure the Bohr atom (1913)
• Atomic Structure Theory of Quantum Mechanics
  (1930s)
• Applications of our understanding

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Thompsons Discovery of the Electron

• Idea that there may be some smaller components
  that comprise atoms
• Hypothesis Atoms consist of subcomponents, one
  of which is negatively charged and very small in
  mass
• Experiment to test hypothesis

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Thompsons Experiment
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Thompsons Experiment
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Thompsons Discovery of the Electron

• Idea that there may be some smaller components
  that comprise atoms
• Hypothesis Atoms consist of some subcomponents,
  one of which is negatively charged and very small
  in mass
• Hypothesis accept the hypothesis

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Atomic Structure and FunctionOutline

• Early history in 18th and 19th centuries
• Discovery of the electron (1897)
• Discovery of the nucleus (1911)
• Atomic structure with consternation
• Atomic structure the Bohr atom (1913)
• Atomic Structure Theory of Quantum Mechanics
  (1930s)
• Applications of our understanding

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Rutherfords Discovery of a Nucleus

• Idea that there may be some smaller components
  that comprise atoms, including negatively and
  positively charges components
• Hypothesis Atoms consist of multiple
  subcomponents, some negative and some positive
• Experiment to test hypothesis
• Alpha particles (), gold foil, bullets, and
  tracks

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Rutherfords Experiment
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Explanation
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Observations, Hypothesis and Conclusion

• Observations
• Almost all alphas passed through foil unaffected
• Very small number of alphas deflected _at_ small
  angle
• 1/1000 deflected at large angle (struck head
  on)
• Hypothesis Atoms consist of some positive
  subcomponents
• Accept the hypothesis

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New Model of Atom

• Small, dense center, positively charged called
  the nucleus (later to be called the proton)
• Small, light (w/o mass), negative charged
  subcomponent circles the nucleus (electron)
• Later additions (1932)
• Neutrons
• Protons
• Charge (neutrality calls for and charges)

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Atomic Structure and FunctionOutline

• Early history in 18th and 19th centuries
• Discovery of the electron (1897)
• Discovery of the nucleus (1911)
• Atomic structure with consternation
• Atomic structure the Bohr atom (1913)
• Atomic Structure Theory of Quantum Mechanics
  (1930s)
• Applications of our understanding

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Atomic Structure with Consternation

• Consternation with Rutherford Model
• Electrons moving, so must give off energy
• 2nd Law of Thermodynamics
• Electrons should collapse
• Yet atoms are billions of years old
• Conclusions model violated fundamental laws of
  physics

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Atomic Structure and FunctionOutline

• Early history in 18th and 19th centuries
• Discovery of the electron (1897)
• Discovery of the nucleus (1911)
• Atomic structure with consternation
• Atomic structure the Bohr atom (1913)
• Atomic Structure Theory of Quantum Mechanics
  (1930s)
• Applications of our understanding

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Atomic Structure Bohr Atom

• Observations (1913)
• Heat hydrogen gas and light is emitted as a
  discrete wavelength (not continuous spectrum)
• Other gases behave the same in producing discrete
  wavelength, but each gas unique in wavelengths
  produced

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Atomic Structure Bohr Atom

• Hypothesis
• Electrons circle the nucleus at specific
  distances from the nucleus in defined orbits
• Diagram

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Atomic Structure Bohr Atom

• Explanation of light in discrete wavelengths
• As excited/heated electrons drop back from
  orbit, a packet of energy is lost as a function
  of distance from the nucleus
• Packet of energy is called photon (light)

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Atomic Structure Bohr Atom

• Key aspects of the Bohr atom
• No space between the orbits for electrons to be
  so jump has to be all or none quantum
  levels of electrons
• Excited electrons in outer orbit has 2 options
• Drop down to next lower orbit and emit a packet
  of energy (light) unique for that distance
• Drop back to even lower level (ground state) and
  emit packet of energy (light) unique for that
  that distance
• Energy release is unique for each element and is
  a window on atomic structure (lab exercise)
• Ideas verified two decades later using quantum
  mechanics

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Bohr Atom
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Atomic Structure and FunctionOutline

• Early history in 18th and 19th centuries
• Discovery of the electron (1897)
• Discovery of the nucleus (1911)
• Atomic structure consternation
• Atomic structure the Bohr atom (1913)
• Atomic Structure
• Consternation with the Bohr Atom (theory)
• Theory of Quantum Mechanics (1930s)
• Applications of our understanding
• Discovery of the elements

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Theory of Quantum Mechanics

• Observations
• Behavior of electrons in heated hydrogen gas were
  consistent with Bohr Model (orbits, etc.)
• Behavior of other heavier gases could not be
  explained by Bohr Model
• Investigators (Heisenberg, Shrodinger)
  wave-particle duality of light
• Key Integrated mechanisms of waves and
  particles, focusing on fuzzy electron
  clouds/waves
• Hypothesis new model of atomic structure and
  function quantum mechanics theory of the atom
• Incorporation of know facts
• Prediction of new properties yet to be discovered

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Quantum Mechanics
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Quantum Mechanics
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Quantum Mechanics Model

• Key Principles
• Electron behaves as a wave not a particle (Bohr)
• Energy of electron behaves as a spread-out wave
  in three dimensions - not discrete bundle of
  energy (Bohr)
• Description is based on quantum numbers
• Distance from nucleus
• Energy level and sublevel
• Orientation of electrons in space
• Direction of electron spin

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Quantum Mechanics Model Principle Energy Levels

• Main energy level of electrons
• Distance from nucleus
• n 1 (lowest energy level)
• n 2
• n 3
• n 4 (highest energy level)
• etc.
• Higher the number, the higher the energy

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Quantum Mechanics Model Sub-Energy Levels

• Electrons in fuzzy probability distribution
  called an orbital
• e.g., n 1 has 1 sub-energy level/orbital (2
  electron orientations)
• Heisenberg Uncertainty Principle
• Electrons in an orbital can only exist in pairs
  (2) and the pair spins in opposite directions
• Pauli Exclusion Principle

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Paulis Exclusion Principle
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Quantum Mechanics Model Electron Configuration

• Electrons always adopt most energetically stable
  energy levels/orbitals consistent with Laws of
  Thermodynamics and previous conditions (above)
• If n 1, two electrons in 1st orbital (s
  orbital)
• If n 2, six electrons in 3 orbitals (p orbitals)

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Electron Configuration in p Orbital
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Atomic Structure and FunctionOutline

• Early history in 18th and 19th centuries
• Discovery of the electron (1897)
• Discovery of the nucleus (1911)
• Atomic structure with consternation
• Atomic structure the Bohr atom (1913)
• Atomic Structure Theory of Quantum Mechanics
  (1930s)
• Applications of our understanding

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Applications of Our Understanding of Atomic
Structure

• Spectroscopy quantum differences in light
  emitted or absorbed result in unique signature
  for each element (fingerprint)
• LASER (Light Amplification by Stimulated Emission
  of Radiation)
• Binding of elements to create compounds
• Neurotransmitters in the brain
• Allergy reactions
• Etc.

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Atomic Structure and FunctionTake Home Message

• Science as a evolutionary, incremental process
  over decades (e.g., exploring the structure of
  atom)
• Role of hypothesis development and testing in the
  sciences scientific method and progress
• Consternation, rejection, new models new
  hypotheses
• Demystification of physics and chemistry
  structure of the atom is a simple concept
• Understanding of atomic structure underpins
  future course topics (e.g., chemical reactions,
  geology, biochemistry, ecology, environmental
  sciences)
• Applications some examples

37
Atomic Structure
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Atomic Structure and FunctionOutline

• Early history in 18th and 19th centuries
• Discovery of the electron (1897)
• Discovery of the nucleus (1911)
• Atomic structure consternation
• Atomic structure the Bohr atom (1913)
• Atomic Structure Theory of Quantum Mechanics
  (1930s)
• Applications of our understanding
• Discovery of the elements