Atoms and Elements 30 September 2004

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Atoms and Elements 30 September 2004

• Physics
• Chemistry
• Astronomy
• Geology
• Biology

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Topics

• Discovery of what an atom is composed
• Models of atoms
• Bohr atom (1913)
• Quantum mechanics (1930s)
• Elements and atoms
• Electrons and shells/orbits
• Valence electrons

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Synopsis (1st lecture)

• Principles of physics, astronomy, chemistry,
  geology, and biology
• Dominant theories and laws that underlie how the
  natural world operates
• Theories and laws are relevant in every day life
• Underlying themes across all disciplines (e.g.,
  emergent properties, models, scientific method)
• Cardinal features of scientific investigations
  (e.g., scientific method, data visualization,
  models, data collection), through a combination
  of computer simulations and hands-on experiments
  in lab

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Models in the Sciences

• Conceptual models based on observations
• Example Dynamics of family interactions
• Example Structure of the atom (protons, neutrons
  and electrons
• Models are an abstraction and are meant to be
  challenged

Protons and Neutron
Electrons in orbits
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Scientific Method
Observations
Rejection/Acceptance of Hypothesis
Proposal/Hypothesis
Testing Hypothesis
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Crystals Structure and Color
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Topics

• Discovery of what an atom is composed
• Models of atoms
• Bohr atom (1913)
• Quantum mechanics (1930s)
• Elements and atoms
• Electrons and shells/orbits
• Valence electrons

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Compounds to Subatomic Particles A Hierarchy

• Compound
• Element
• Atom
• Subatomic Particle

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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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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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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 Atom consists of a positive
  subcomponent that is very small in volume in
  comparison to atom itself
• Accept the hypothesis

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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 is unique in wavelengths
  emitted

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Bohr Atom
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A Model of Quantum Mechanics
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Theory of Quantum Mechanics

• Observations
• Behavior of electrons in heated hydrogen gas
  consistent with Bohr Model (orbits, etc.)
• Behavior of other heavier gases not 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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Models in the Sciences

• Conceptual models based on observations
• Example Dynamics of family interactions
• Example Structure of the atom (protons, neutrons
  and electrons
• Models are an abstraction and are meant to be
  challenged

Protons and Neutron
Electrons in orbits
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Elements

• Patterns
• 92 naturally occurring elements (e.g., hydrogen,
  gold, helium)
• Total of 113
• 25 of 92 are essential to life (e.g., what are
  they?)
• Key points
• any element is the same in its chemical structure
  and physical properties (stable over time)
• All elements have origin in either the big bang
  (hydrogen and helium) or the subsequent evolution
  of the universe

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Elements

• Compound
• Elements combine in very precise ways that are
  recurrent and predictable
• Sodium Chlorine Sodium Chloride
• Na Cl NaCl
• metal gas solid
• Key points
• Atoms of sodium (Na) and chlorine (Cl) remain
  atoms of each
• Emergent property emergence of new properties
  in a compound not be explained by the summation
  of the two elements (hierarchy theory)

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Particles

• Atoms are composed of particles (subatomic
  particles)
• Most stable particles
• Neutrons
• Protons
• Electrons
• Other less stable particles (quarks, neutrinos,
  etc.)
• Relationship among the more stable particles
• Neutron Proton Electron
• Charge neutral positive negative
• Mass 2x10-24 g 2x10-24 g
  5x10-28 g

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Atomic Structure

• Atoms of the same element (e.g., hydrogen,
  helium, gold) have the same number of subatomic
  particles and by convention we abbreviate as
  follows
• 2 of protons
• Helium He Abbreviation of element
• 4 Atomic mass (g/mole)
• 1
• Hydrogen H
• 1
• Mass Protons Neutrons

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Electrons

• Background of quantum mechanics
• Energy barons of the atom (motion)
• Energy ability to do work
• Potential energy energy stored due to position
  or location
• Charge is negative (-) and particle is always in
  motion
• Capturing an atom and its orbiting electron

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Key to Electron Structure

• Count the number of electrons (and compare with
  abbreviation of element)
• Electrons are negative in charge and in constant
  motion
• Electrons are in orbits around the nucleus
• Chemists sometimes refer to orbits as shells

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Electrons

• Example of Sulfur (1632S)
• (16 electrons in 3 orbits)

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Example of Electrons in Shells

• As electrons move between shells, they change
  potential energy
• Hot summer day, bright sun and car top
• Light absorption by pigments and electrons jump
  to higher shell (potential energy) give off
  energy when they drop back (kinetic energy)
• Banana, orange juice or bagel this AM?
• Excited electron captured by chlorophyll in
  leaf and shuttled to a sugar molecule in its
  excited state (potential energy) until you
  release the energy via digestion/respiration,
  allowing the electron to drop back to a lower
  level (kinetic energy)

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Some keys to Electron Structure

• Electrons reside in shells as a function of
  quantum mechanics (1-4 orbits per shell)
• Never more than two electrons per orbit (Paulis
  Exclusion Principle)
• Distribution of electrons is key to understanding
  why elements and atoms behave the way they do
• Outermost electrons are called valence electrons
  and they have a very special significance in
  chemistry

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Question
According to the Rutherford model of the atom,
the volume of any atom is largely
_______. A. protons and neutrons B. electrons
C. empty space D. covertly sequenced nuons
surrounded by proton
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Question
According to the Bohr model of the atom, an
electron gains or looses energy only by
______. A. jumping from one atom to
another B. speeding up or slowing down in its
orbit C. jumping from one orbit to
another D. being removed from the atom
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Question
According to the Rutherford model of the atom,
the volume of any atom is largely
_______. A. protons and neutrons B. electrons
C. empty space D. covertly sequenced nuons
surrounded by proton
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Question
According to the Rutherford model of the atom,
the volume of any atom is largely
_______. A. protons and neutrons B. electrons
C. empty space D. covertly sequenced nuons
surrounded by proton
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Discussion Question

• As you scan the night sky, you see multiple
  objects, and you question whether these objects
  are similar to or different from Earth.
• Suppose the person next to you says that she has
  an instrument that can identify the occurrence of
  specific atoms (e.g., sodium, hydrogen, sulfur)
  based on the energy patterns/signatures of
  electrons (PE and KE) in atoms on that planet.
  Could she be right?

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Discussion Question

• Models are one of the key investigative tools in
  the scientific method. Models of our
  understanding of the atom are a classic example
  of the role that models play in the sciences.
• List five features of models not the specifics
  of any given model, but rather how models are
  constructed, how they serve to help promote our
  understanding of natural systems, and their
  fallibility.

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Discussion Question

• You are in the atomic world, having won a free
  all day ride on an electron nested within a
  carbon atom. Your peers are jealous (hang with
  me on this).
• You are seated on an electron, duly belted down
  with a safety harness and off you go.
• Twenty-four hours later you return to life at
  GMU. List the cardinal features of your ride.