Unit 4 - MoLECULES

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Chemical Bonds Continued

• Unit 4 - MoLECULES

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NaCl vs. CO2

• What do you already know?
• Imagine!
• Close your eyes and picture a piece of salt. Now
  zoom in..what does it look like at the atomic
  level?
• Now imagine carbon dioxide what does it look
  like at the atomic level?
• How are these two compounds different?

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How do non-metals bond with each other?

• Recall non-metals have______(high or low)
  ionization energy when compared to metals.
• Reason
• Result
• What is electronegativity? Why does it exhibit a
  distinct periodic trend?
• How do nonmetals bond?

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Electronegativity - Linus Pauling
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Covalent bonding

• Atoms are bonded because both nuclei () are
  attracted to the same electrons (-).

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Covalent vs Ionic
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Using electronegativities

• Electronegativity Difference /Bond Character
• gt 1.7 and above / ionic
• 0.4 - 1.7 / polar covalent
• 0 -0.4 /nonpolar covalent

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What is a molecule?

• Two or more atoms covalently bonded to make a
  neutral particle is a unit called a molecule.
• A polyatomic ion (i.e. NO3-) is very similar to a
  molecule, except that it has a charge.
• Covalent compounds AND most non-metal elements
  are composed of molecules.
• H2O CO2 O2 P4

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MORE KEY TERMS

• All diatomic molecules are NONPOLAR COVALENT
• Practice lewis dot structure
• Electronegativity tendency of an atom to
  attract shared electrons to itself! (greatest
  attraction for electrons)

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What is a Lewis symbol?

• Lewis symbols
• are simple pictures of atoms
• are used to represents covalent bonds

Lewis Symbol
Ne
Each dot is a valence electron
Gilbert N. Lewis
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Molecular Representations
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Diatomic nonpolar molecules
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Polar covalent vs. nonpolar covalent
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What determines the structure of molecule?

• Octet rule - atoms usually form covalent bonds
  with each other resulting in a total of 8 valence
  electrons around each atom.
• What is special about eight e-?
• Why usually but not always 8?
• Whats the actual REASON that non-metals share
  electrons?

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Drawing Lewis Dot Structure

• 1. Write symbols of the elements.
• - if 3 or more elements figure out which one is
  the central one ( typically C , N, P, S
  sometimes O) If all of these are present
  usually C is the central one.
• 2. Determine total of valence electrons
• Dont forget about adding or subtracting an
  electron/s if you are dealing with the polyatomic
  ions
• 3. Use a single bond to connect each atom
  together
• -gt then fill in the remaining electrons
  around the atoms to complete the octet rule
• - if total of electrons wont fulfill octet
  rule double or triple bonds are
  necessary

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Example Lewis structure for water

• Formula of water is H2O
• Total valence electrons
• Lewis structure to obey octet rule
• Practice on your own PCl3 and SF2

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Lewis structures have limitations

• What does a Lewis structure show us about a
  molecule?
• What does it NOT show?

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CF4
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CF4
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NH3
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NH3
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What about the ammonium ion? NH4
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N2
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N2
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nomenclature for molecules

• Rules for naming covalent molecules
• 1. Use prefixes to indicate the of atoms
  present
• EXCEPTION
• Never use mono for the first element in the
  molecule
• 2. Same rules for ide ending for the last
  element in molecule
• Prefixes
• Mono -1 hexa -6
• Di- 2 hepta -7
• Tri -3 octa -8
• Tetra- 4 nona -9
• Penta- 5 deca -10

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Practice!

• Name these compounds
• PCl3
• H2O
• N2O4
• SF6
• Write formulas for these compounds
• Diphosphorus pentoxide
• Carbon tetrachloride

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VSEPR theory
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Linear shape (2/0)
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Trigonal planar 3/0
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Tetrahedral 4/0
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Trigonal Pyramidal(3/1)
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Bent 2/2 or 2/1
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Bonds to central atom lone pairs on central atom Molecular geometry Bond Angle example
2 0 Linear 180
2 1 Bent 120
2 2 Bent 104.5
3 0 Trigonal Planar 120
3 1 Trigonal Pyramidal 107
4 0 Tetrahedral 109.5
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Molecular Structures and the Periodic table
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Summary of 4 major structural units of C.
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VSEPR video

• http//www.youtube.com/watch?vi3FCHVlSZc4NR1

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Intramolecular Forcesvs. Intermolecular

• Intermolecular Forces
• Hydrogen bonding
• Dipole forces
• London forces (dispersion forces)

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Intra vs Inter

• Intramolecular forces (within a compound)
• Covalent and ionic bonds
• Intermolecular forces (between compounds)
• IMFs Intermolecular Forces
• Dipole Forces
• Hydrogen Bonding
• LDFs London Dispersion Forces

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Bond Polarity vs. Molecule Polarity

• Bond Polarity results from unequal sharing of
  the electrons in the covalent bond. Use the
  electronegativity differences to figure out how
  polar the bond is.
• Molecule Polarity a molecule is polar if it has
  1 or more polar bonds and its shape does not
  cancel out the polarity.
• Nonpolar covalent molecules (nonpolar)
• Ex All diatomics such as N2, H2, etc
• Polar covalent molecules (polar)
• Ex water, carbon monoxide, etc

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Intramolecular ForceTutorial Video Advanced,
But Slow and Methodical
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Using electronegativities

• Electronegativity Difference /Bond Character
• gt 1.7 and above / ionic
• 0.41 - 1.7 / polar covalent
• 0 -0.4 /nonpolar covalent
• Dipole molecule -

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Polar Molecule notations/representations
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Intermolecular Forces Hydrogen bonding
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Hydrogen bonding
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Hydrogen bonding characteristics

• 1. Must involve a hydrogen that is bonded to
  highly electronegative element (O, F, N)
• 2. The slight positive on the hydrogen is
  attracted to a neighboring molecules nonbonding
  electron pair.

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Dipole Forces (Vander wall)

• Dipoles molecules that have separate centers of
  partial negative and partial positive charges.
• Note dipole forces are only 1 as strong as an
  ionic bond attraction.

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London dispersion forces (LDFs)

• They are small, transient , attractive forces
  between NONPOLAR molecules
• Larger or heavier atoms typically exhibit
  stronger dispersion forces than smaller, lighter
  molecules
• http//www.youtube.com/watch?v3t1Jn_jrsQk
• http//dl.clackamas.edu/ch104/lesson9molecular_pol
  arity.html

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Ionic vs molecular compounds

• Melting point
• Conductivity

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Intermolecular forces and boiling points!

• The GREATER The force of attraction between
  molecules the higher the melting or evaporation
  point will be.
• Think about Ionic bonding..
• Which would melt the quickest when heated and
  why?
• Aluminum nitride
• Sodium chloride
• Calcium oxide
• Now what about covalent compounds..

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Boiling points of ionic vs. molecular

• Melting Points and Boiling Points of Substances
  with Similar Formula Weights Substance FW (g/mol)
  

Covalent molecules b.p. (Celsius) Ionic compounds b.p. (Celsius)
CS2 46.0 NaF 1695
CH4O 64.7 CaCO3 825
propane -42.1 NaCl 801
CO2 -57 MgSO4 1125
ethanol 78.5 TiO 1750
Glucose 146
H2O 100
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