Chapter 6 Chemical bonding and molecular shapes

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Chapter 6 Chemical bonding and molecular shapes

• How atoms connect to one another?

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6.1 An atomic model is needed to understand how
atoms bond

• Electrons in the outermost occupied shell of any
  atom are responsible for the atoms chemical
  properties.
• The electrons that participate in chemical
  bonding are called valence electrons (???),the
  shell they occupy is called the valence shell
  (??) of an atom.
• Valence electrons can be conveniently represented
  as a series of dots surrounding an atomic symbol.
  (Electron-dot structure or Lewis dot symbol).
• Valance electrons can be either paired or
  unpaired. Paired electrons usually do not form
  chemical bonds with other elements.

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Unpaired electrons
Unpaired electron


Paired electrons
C



Chlorine
Carbon
Question Why Carbon atom has four (not two)
unpaired electrons?
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6.2 Atoms can lose or gain electrons to become
ions (??)

• When the number of protons in the nucleus of an
  atom equals to the number of electrons in the
  atom, the charges balance and the atom is
  electrically neutral. If one or more electrons
  are lost or gained, the atom takes on a net
  electric charge. Any atom having a net electric
  charge is called an ion. If the electrons are
  lost, the ions net charge is positive. If the
  electrons are gained, the ions net charge is
  negative.

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• Shell model can be used to deduce the type of ion
  an atom tends to form Atoms tend to lose or gain
  electrons so that they end up with an outermost
  occupied shell that is filled to capacity.

1 2 3 4- 3- 2- 1- 0
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6.3 Ionic bonds result from a transfer of
electrons

• When an atom that tends to lose electrons is
  placed in contact with an atom that tends to gain
  them, the result is that an electron transfer and
  the formation of two oppositely charged ions. The
  two oppositely charged ions are thus attracted to
  each other by the electric force, which holds
  them together. This electric force of attraction
  between two oppositely charged ions is called an
  ionic bonds (???). All compounds containing ions
  are referred to as ionic compounds (?????).

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How to judge the composition of an ionic
compound? Balance of electric charge
Ruby
Sapphire
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6.4 Covalent bonds result from a sharing of
electrons

• The electrical attraction in which atoms are hold
  together by their mutual attraction for shared
  electrons is called an covalent bond (???). A
  compound composed of atoms held together by
  covalent bonds is a covalent compound (?????).

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• More than one electron can be shared in an atom
  double bond (??) and triple bond (??).

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• Question Whats in common for ionic bonds and
  covalent bonds and what are the differences?

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6.5 Valence electrons determine molecular shape

• How to determine the three dimensional structure
  of a covalent compound?
• Valence shell electron-pair repulsion (VESPR)
  any given pair of valence-shell electrons strives
  to get as far away as possible from all other
  electron pairs in the shell.

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6.6 Polar covalent bonds result from an uneven
sharing of electrons

• What is the distribution of a shared pair of
  electrons in a covalent bond?
• In HF the shared electrons are drawn more close
  to F atom, so the fluorine side of bond is
  electrically negative while the hydrogen side of
  bond is electrically positive. This kind of
  charge separation is called a dipole (??).

In H2, evenly shared
in HF, unevenly shared
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The ability to draw bonding electrons is decided
by the atoms electronegativity (???).
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Polar bond and nonpolar bond?

• When the two atoms have the same
  electronegativity, no dipole is formed and the
  bond is called a nonpolar bond (????).
• When the two atoms have the different
  electronegativity, a dipole is formed and the
  bond is called a polar bond (???).

electron rich region
electron poor region
C C
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6.7 Molecular polarity results from an uneven
distribution of electrons

• If all bonds in a molecule are nonpolar, the
  molecules as a whole is also nonpolar. (H2, O2)
• If a molecule consists of polar bonds, the
  polarity of the molecule is decided by the
  polarity of the bonds and also the shape of the
  molecule.

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Molecular shape of H2O and NH3

• H2O

nonpolar
H O H
H H
..
polar
O
..
O sp3 hybrid orbitals, two paired electrons, two
unpaired electrons to form covalent bonds with
two H atoms
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The molecular polarity decides a lot of the
physical properties of substances
Substance Boiling Point(?)
Polar
Water,H2O 100
Ammonia,NH3 -33
Nonpolar
Oxygen,O2 -183
Hydrogen,H2 -253
Nitrogen,N2 -196
Carbon dioxide,CO2 -79
Boiling Points of some polar and nonpolar
substance
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Chapter 7 Molecular Mixing

• Water that has been fully aerated at room
  temperature, contains only 1 oxygen molecule for
  every 200,000 water molecules. Why? The
  attractive forces between water molecules and
  oxygen molecules are very weak.

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7.1 Submicroscopic particles electrically attract
one another

• Four types of interaction between ions, molecules
  and atoms (submicroscopic particles)
•  
• Attraction Relative strength
• Ion-dipole Strongest
• Dipole-dipole
• Dipole-induced dipole (????)
• Induced dipole-induced dipole weakest

Question Whats in common for these four
interactions?
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The molecular-molecular interaction determines
many of properties of substance

• Ion- dipole interaction table salt dissolved in
  water

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Dipole-dipole interaction

• An unusually strong dipole-dipole attraction
  hydrogen bond (??) (occurs between hydrogen atom
  covalently bonded to a highly electronegative
  atom, usually nitrogen, oxygen and fluorine)
• The hydrogen bond is responsible for many of the
  unusual properties of water and many of
  biomolecules, such as DNA.

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Hydrogen Bond
Question The role of hydrogen bonds in DNA
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Hydrogen bonding play important role in
self-assembly

• Self-assembly is the only scientific issues in
  the 25 most important issues in 21st century
  claimed by Science

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Dipole-induced dipole interaction

• oxygen dissolved in water, carbon dioxide in
  water

Isolated oxygen molecule
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Induced dipole-induced dipole interaction
(dispersion force ???)

Iodine,I2,a solid at room temperature
Fluorine,F2,a gas at room temperature
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Fluorine-containing molecules usually have very
small induced dipole, and thus have very weak
interaction with other molecules

Question Why fluorine-containing molecules
usually have very small induced dipole?
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• Question Other applications for
  fluorine-containing molecules?

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7.2 A solution is a single-phase homogeneous
mixture

• Solution can be solid, liquid and gaseous
• Solid ruby (red chromium compounds in aluminum
  oxide)
• Blue sapphire (green iron compounds and blue
  titanium compounds in aluminum oxide)
• Metal alloys
•  
• Gaseous air (breath in 78 nitrogen, 21
  oxygen, 1 others)
• (exhale 75 nitrogen, 15 oxygen, 5
  carbon dioxide, 6 water vapor)

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• Solvent (??), Solute (??), Dissolving (??)
•  
• How much a given solute can dissolve in a given
  solvent?
• Unsaturated solution (?????), Saturated solution
  (????)
• Concentration (??) amount of solute/amount of
  solvent
• Concentration can be weight and molar
•  

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7.3 Solubility (???) is a measure of how well a
solute dissolves

• Solubility depends on attractions between
  solute/solvent particles.
• When the molecule-to-molecule attractions among
  solute molecules are comparable to the
  molecule-to-molecule attractions among solvent
  molecules, the result can be no practical point
  of saturation, as in the case of ethanol/water.
  Ethanol and water can mix homogeneously in any
  proportion. (infinitely soluble)

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Solubility changes with temperature
100?
20?
93g of NaNO3 precipitates out of solution
180g NaNO3 in 100ml water
87g NaNO3 in 100ml water
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• Gases are more soluble at low temperature and
  high pressure
• Nonpolar gases readily dissolve in
  perfluorocarbons Oxygen has much higher
  solubility in perfluorocarbons than in air. This
  has many applications.

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• ????????????????????(????),TEFLON??????????

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7.4 Soap works by being both polar and nonpolar
grime dirt grease Grime is difficult to
remove from hands or clothing using just water,
because grime is nonpolar and water is polar.
Grime can be dissolved and washed away by
nonpolar substances such as trichloroethane.   Gri
me can also be washed away by soap and water,
such soap is a substance having both polar and
nonpolar properties.
Soup molecules play as a coupling agent.
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• Detergents
• Calcium and magnesium ions in hard water can
  replace the sodium ions in soap to form insoluble
  substances. Sodium carbonate is usually added to
  detergent as a water-softening agent.

Treat with NaOH
Glycerol molecule
Fat molecule
Three fatty acid soap molecules
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• Question Apart from coupling, any other
  technique to stick two materials with different
  properties together?

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