Origin of the Chemical Elements

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Origin of the Chemical Elements
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Formation of First Atoms

• Following the Big Bang, nuclei of atoms and
  electrons existed as a gas (plasma), primarily
  H, He2, and e-
• When Universe cooled sufficiently, 300000-500000
  years, e- became associated with nuclei to form
  atoms of H and He having electrons moving in
  circles about nucleus

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Galaxies and Stars

• Gravity brought atoms of H and He together to
  form galaxies of protostars
• Gravity initiated nuclear reactions in stars
  which produced heavier nuclei
• When resulting plasma cooled, the neutral atoms
  of the heavier elements were formed

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Origin of the Chemical Elements
Galaxies
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Stabilities of Nuclei

• Particles making up the nucleus are called
  nucleons sum of the protons and neutrons Mass
  Number, A
• Mass of nucleus lt total mass of nucleons making
  it up
• Difference is called mass defect total mass of
  nucleons in nucleus - actual mass of nucleus

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Nuclear Binding Energy

• Mass defect is the mass which is converted to
  energy, E mc2
• This energy holds nucleus together nuclear
  binding energy, NBE
• NBE/A nuclear binding energy per nucleon
  serves to measure stability
• The larger NBE/A the more stable the nucleus

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http//www.courses.fas.harvard.edu/chem15/lecture
s/1_nuclear.pdf
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http//www.courses.fas.harvard.edu/chem15/lecture
s/1_nuclear.pdf
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http//www.courses.fas.harvard.edu/chem15/lecture
s/1_nuclear.pdf
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Stability of nucleus is measured by NBE/A 2.37
x 10-11 J /19 nucleons 12 x 10-13 J/nucleon
http//www.courses.fas.harvard.edu/chem15/lecture
s/1_nuclear.pdf
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Graph of Nuclear Binding Energy per Nucleon
http//www.courses.fas.harvard.edu/chem15/lecture
s/1_nuclear.pdf
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Stabilty of Nuclei and Fusion Reactions
Stability Increasing
Jones, "Chemistry, Man and Society", Saunders,
1972, 623
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Synthesis of the Chemical Elements in Stars
Olmstead, Chemistry, Mosby, St. Louis, MO,
1994, p. 960.
Olmstead, Chemistry, Mosby, St. Louis, MO,
1994, p. 960.
Olmstead, Chemistry, Mosby, St. Louis, MO,
1994, p. 960.
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Birk, ChemistryHoughton Mifflin, Boston,
MA,1994,p 880.
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Star Composition at Supernova
Cox, The Elements, Oxford, New York, NY, 1989,
p. 80
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Collapse of Star and Supernova
As Fe builds in core, core temperature drops
since Fe does not undergo fusion. The unbalanced
gravitational force causes star to collapse.
http//wine1.sb.fsu.edu/chm1046/notes/Nucleo/Nucle
o.htm
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Cosmic Dust
Coalescing
H-Burning
He-Burning
C-Burning
Silberberg, Chemistry Mosby,St. Louis, MO,
1996, p 965
Supernova
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Formation of Nuclides Heavier Than Iron

• Supernovas produce large amounts of neutrons
• Neutrons can react with atoms one at a time and
  release electrons to produce elements heavier
  than iron
• Many neutrons can also react with one atom and
  release electrons to produce elements heavier
  than iron

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Reactions that Produce Nuclides Heavier than Iron
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Abundances of the Chemical Elements in Universe
Heavier elements formed only in a supernova are
less abundant due to short time frame for
collisions with neutrons - only a small fraction
of lighter atoms converted in each supernova
Radel/Navidi, "Chemistry", West, NY,1990, 987
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Debris from supernova is recycled in a new star
making it possible to form heavier and heavier
nuclei but in decreasing amounts
http//www.howstuffworks.com/star5.htm
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Formation of Atoms of Heavier Elements

• As the debris from a supernova cools, the nuclei
  will pick up electrons to form atoms
• Electrons repel each other sufficiently to cause
  groups of electrons to occupy shells at
  ever-increasing distances from nucleus

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Electron Structures of Atoms

• Electrons move about the nucleus in spherical
  shells designated as K,L,M... or 1,2,3 starting
  nearest nucleus
• Capacities of shells given by 2n2 where n shell
  number (2,8,18, 32 )
• Order of filling of shells is in order of
  increasing n except outermost shell (valence
  shell) never exceeds 8e

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Elements 1-18
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Elements 19-20
When outermost shell reaches 8e, a new shell is
added which is occupied by the next two electrons
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Elements 21-30
After valence shell acquires 2e, the next ten
electrons are added to the third shell until its
reaches capacity of 18e
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Elements 31-36
Once 3rd shell is filled, the next electrons are
added to valence shell until it reaches 8e when
a new shell is added and pattern repeats
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Valence Electrons
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Periods and Families
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Main Group Transition Elements
Electrons add to inner shell
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Electron Structures of Atoms
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Metals and Non-Metals
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Relative Sizes of Atoms
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Periodic Table

• Periodic table provides the link between the real
  world and the imaginary world
• Elements in same family are observed to have
  similar properties in real world
• Explanation atoms of elements in the same family
  have the same valence shell electron structure in
  the imaginary world

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Valence Electrons and Chemical Reactions

• Valence shells of approaching atoms come in
  contact first
• Atoms having same numbers of e in valence shell
  have similar reactions
• Chemical reactions involve a rearrangement of
  valence electrons

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A-Families of Periodic Table
1A 2A 3A 4A 5A 6A 7A 8A
1 2 3 4 5 6
7 8 Valence Electrons
http//www.chemicalelements.com/index.html
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Reactions of Selected 1A Elements with Water
Real World all 1A elements react with water
Imaginary World all 1A elements have 1 valence e
Real World speeds of reactions of 1A elements
with water are different
Imaginary World The larger the 1A atom, the
faster the reaction
http//www.chemicalelements.com/index.html
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Periodic Table(Symbolic World)
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B-Families of Periodic Table
3B 4B 5B 6B 7B 8
8 8 1B 2B
For Chem 83, disregard irregularities and assume
all have 2e in their valence shells
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