Formation of Earth's Oceans and Atmospheres

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Formation of Earth's Oceans and Atmospheres

• Components are molecular covalent substances
  which can exist as a gas or liquid at low T

2
Molecular Covalent Substances

• Covalent bonds holding atoms together in a single
  molecule (intramolecular force) are very strong
• Force of attraction between molecules
  (intermolecular force) is weak
• Molecules with low intermolecular forces became
  components of the atmosphere of primitive Earth

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Continued...

• The molecule with the strongest intermolecular
  force (water) became the component of the oceans
  (hydrosphere)
• To understand atmospheres and oceans, need to
  understand the factors that determine the
  strengths of intermolecular forces

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Electronegativities of Atoms

• Ability of an atom to attract electrons in a bond
  is called the electronegativity (EN) of the
  element
• EN of Main Group elements (A-families) generally
  increase in going upward in a family or across a
  period from left to right

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http//wine1.sb.fsu.edu/chm1045/notes/Bonding/Pola
rity/Bond05.htm
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Electronegativity and Bonding

• Ionic bonding occurs between atoms having widely
  differing EN (metal/non-metal) ?EN 2
• Metallic bonding occurs between atoms having low
  EN (metals) ?EN 0 to 0.4
• Covalent bonding occurs between atoms having high
  EN (non-metals) ?EN 0 to 2

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Types of Covalent Bonds Based on Electronegativity

• Two kinds of covalent bonds possible depending on
  the differences in the EN of the bonding atoms
• Non-polar bond difference in EN is less than 0.4
• Polar bond difference in EN is between 0.4 and
  2.

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Predicting Polarities of Covalent Bonds
If the difference in electronegativities between
the two bonded atoms is lt 0.4, the bond is
non-polar covalent
Bonding pair of electrons is equally shared by
two F atoms
http//wine1.sb.fsu.edu/chm1045/notes/Bonding/Pola
rity/Bond05.htm
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If the difference in electronegativities between
the two bonded atoms is between 0.4 and 2.0, the
bond is polar covalent
Atom having higher EN has a small negative charge
while atom with lower EN, a small positive
charge
http//wine1.sb.fsu.edu/chm1045/notes/Bonding/Pola
rity/Bond05.htm
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Polarities of Molecules

• A molecule is non-polar if it contains no polar
  bonds or if it contains identical polar bonds in
  geometries that cancel their polarities (linear,
  trigonal planar or tetrahedral)
• A molecule is polar if it contains non-identical
  polar bonds or identical polar bonds in
  geometries that do not cancel bond polarities
  (bent, trigonal pyramidal)

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Geometries and Polarities of Molecules

• Molecules having one central atom and two
  terminal atoms are linear if there are no lone
  pairs on central atom or bent if there are lone
  pair(s) on central atom

Polar
http//wine1.sb.fsu.edu/chm1045/notes/Geometry/VSE
PR/Geom02.htm
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Molecules having one central atom and 3 terminal
atoms are trigonal planar if there are no lone
pairs on the central atom or trigonal pyramidal
if there is a lone pair on the central atom
Non-polar for AB3
Polar
http//wine1.sb.fsu.edu/chm1045/notes/Geometry/VSE
PR/Geom02.htm
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• Simple molecules having one central atom and four
  terminal atoms are tetrahedral

Non-polar for AB4
http//wine1.sb.fsu.edu/chm1045/notes/Geometry/VSE
PR/Geom02.htm
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Polarity and Intermolecular Forces

• Forces between molecules are called
  intermolecular forces
• Intermolecular forces depend, in part, on the
  polarities of molecules
• Three types of intermolecular forces
  Dipole-Dipole Force, London Dispersion Force, and
  Hydrogen Bonding

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Dipole-Dipole Force
Force of attraction between opposite charges of
permanent dipoles of adjacent polar molecules
Most readily observed in properties of molecules
having same size (molecular wt.)
http//wine1.sb.fsu.edu/chm1045/notes/Forces/inter
mol/Forces02.htm
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Dipole Forces
Masterton/Hurley, Chemistry, 3rd, Saunders, 1997.
240
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London Dispersion Force (LDF)

• Weakest cohesive force present between all
  molecules (polar and non-polar)
• Results from the force of attraction between
  temporary fluctuating dipoles consisting of an
  instantaneous dipole and induced dipoles

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London Dispersion Force (LDF)
Force of attraction between a temporary
instantaneous dipole and an induced dipole
http//wine1.sb.fsu.edu/chm1045/notes/Forces/inter
mol/Forces02.htm
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London Dispersion Force (LDF)
LDF increases with increasing size (molecular
weight) for similar molecules
Tro, 295
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Hydrogen Bonding

• Polar molecules containing H bonded to either F,
  O, or N exhibit LDF, DDF, and an additional
  intermolecular force called H-bonding
• Force of attraction between H in one polar
  molecule and a lone pair of electrons on O, N, or
  F in an adjacent molecule

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Hydrogen Bonding
Each water molecule can form H-bonds with 4 other
water molecules
http//users.rcn.com/jkimball.ma.ultranet/BiologyP
ages/H/HydrogenBonds.html
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Effect of H-Bonding
As size decreases, LDF decreases, and B.P.
decreases. However, the smallest molecule, water,
has the highest B.P. due to a strong additional
force H-bonding
Ebbing/Gammon, General Chemistry,6th,Houghton
Mifflin, 1999, 461
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Effect of H-Bonding
HF and NH3 exhibit H-bonding while CH4 does not
Ebbing/Gammon, General Chemistry,6th,Houghton
Mifflin, 1999, 461
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Primitive Atmospheres of Earth

• Atmosphere-1, when Earth formed 4.5 billion years
  ago, was H2 and He. When solar fusion started
  these light gases escaped earths gravity
• Atmosphere-2 came from outgassing, "Big Burp" and
  volcanoes N2, H2, H2O, NH3, and CH4 and from
  collisions with comets, which consist primarily
  of ice

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Formation of Oceans

• Of the molecules in the second atmosphere of
  primitive Earth, water was most abundant and had
  the strongest intermolecular forces (LDF, DDF,
  and H-bonding)
• As Earth cooled, water was the first to liquify
  and it came down as rain which formed the
  hydrosphere - oceans, etc

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Formation of Earth Related to Chemical Bonding
LDF London Dispersion Force DDF
Dipole-Dipole Force