Atoms and Elements 29 September 2005

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Atoms and Elements 29 September 2005

• Physics
• Chemistry
• Astronomy
• Geology
• Biology

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Topics

• Discovery of atom structure and function
• Models of atoms
• Bohr atom (1913)
• Quantum mechanics (1930s)
• Elements and atoms
• Electrons and orbits/shells
• Valence electrons

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Scientific Method
Observations
Rejection/Acceptance
Hypothesis
Testing Hypothesis
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Models in the Sciences

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

Protons and Neutron
Electrons in orbits
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Crystals Structure and Color
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Compounds to Subatomic Particles A Hierarchy

• Compound
• Element
• Atom
• Subatomic Particle

Emergent Properties
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Thompsons Discovery of the Electron

• Idea that there are smaller components that
  comprise atoms
• Hypothesis Atoms consist of subcomponents, one
  of which is negatively charged with a very small
  mass
• Testing the hypothesis

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Thompsons Experiment
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Rutherfords Discovery of a Nucleus

• Idea that there are other smaller components of
  atoms, including some that are positively charged
• Hypothesis Atoms consist of multiple
  subcomponents, some negative and some positive
• Testing the 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 (reflected back)
• Hypothesis Atom consists of a positive
  subcomponent that is very small in volume in
  comparison to atom itself
• Accept/reject?

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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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Hypothesis and Model
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Model 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 known facts
• Prediction of new properties yet to be discovered

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Questions

• Is a model in the sciences a form of an
  hypothesis?
• All of these models (when proven correct)
  resulted in scientific revolutions in physics.
  How are revolutions in the sciences analogous to
  revolutions in society?

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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
pompous protons
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Question
The existence of a tiny, massive, and positively
charged atomic nucleus was deduced from the
observation that ___. A. fast, massive, and
positively charged alpha particles all
move in straight lines through gold foil B.
alpha particles were deflected by a magnetic
field C. some alpha particles were deflected by
metal foil D. all of the above
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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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Topics

• Discovery of atom structure and function
• Models of atoms
• Bohr atom (1913)
• Quantum mechanics (1930s)
• Elements and atoms
• Electrons and orbits/shells
• Valence electrons

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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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Compounds to Subatomic Particles A Hierarchy

• Compound
• Element
• Atom
• Subatomic Particle

Emergent Properties
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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 have the same number of
  subatomic particles, we abbreviate this 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 may
  be difficult

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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

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Electrons

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

What are shells and what are orbits?
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Example of Electrons in Shells

• As electrons move among 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
  orbit (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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Periodic Table of Elements

• Concept most stable state for an atom is one in
  which the outermost shell is filled with the
  maximum number of electrons
• 1st Shell (1 orbit 2 electrons)
• Hydrogen (11H 1 electron stable ?)
• Helium (24He 2 electrons stable ?)
• Periodic Tables 1st Row
• Hydrogen and Helium

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Periodic Table of Elements

• 2nd shell has 4 orbits with 2 electrons (maximum)
  per orbit (total of 8 electrons/shell)
• Most stable configuration is
• 1st shell filled with 2 electrons
• 2nd shell filled with 8 electrons
• Total of 10 electrons (1020Ne)
• 2nd row of Periodic Table
• 8 elements

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Periodic Table of Elements

• 3nd shell has 4 orbits with 2 electrons maximum
  per orbit (total of 8 electrons/shell)
• Most stable configuration is the following
• 1st shell filled with 2 electrons
• 2nd shell filled with 8 electrons
• 3rd shell filled with 8 electrons
• Total of ___ electrons (1840Ar)
• 3nd row of Periodic Table
• 8 elements (list and relate to the above)

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Periodic Table of Elements

• Number of elements in a row is not chance but
  reflects the maximum number of electrons in the
  outermost shell
• Row 1 2
• Row 2 8
• Row 3 8
• Row 4 18
• etc

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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 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

• In many ways, the Periodic Table is a model to
  help scientists understand patterns in the
  natural world. List ways in which this model
  helps scientists to understand the natural world?
• How does this model compare with that of
  Rutherfords model of the atom or the quantum
  mechanics model of the atom?