Structure Determines Properties

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Structure Determines Properties

• Chapter 1

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Chapter 1 Structure Determines Properties

• Lewis Dot Structures
• Octet Rule
• Formal Charge on Atom
• Resonance Structures
• Representation of Organic Molecules
• Structural, Chemical, Condensed Structural, and
  Line Formulas

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Chapter 1 Structure Determines Properties

• Constitutional (Structural) Isomers
• Molecular Structure
• Polarity of Molecules
• Hybridization Valence Bond Theory
• Bond Strength and Length Trend
• Molecular Orbital Theory

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Chemistry of compounds primarily made of carbon
(C) and hydrogen (H). The compounds may also
contain N, O, F, Cl, S, Br, and I.

• Organic Chemistry

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Students Should Review

• Atomic Structure
• Electronic Structure
• s and p orbitals
• Hybrid orbitals
• Lewis Dot Structures
• Valence Bond Theory

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Lewis Dot Structures
Octet Rule

• When Nonmetal Atoms Share Valence Electrons to
  Form a Covalent Bond
• It Will Be to Have the Same Number of Valence
  Electrons as the Closest Noble Gas
• H ? 2 electrons
• Everything Else ? 8 Electrons

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Writing Lewis Structures for Molecular Compounds

• Draw skeletal structure of compound showing what
  atoms are bonded to each other.
• Count total number of valence e-. Add 1 for each
  negative charge. Subtract 1 for each positive
  charge.
• Connect surrounding atoms to central atom with
  single bonds. Add remaining electrons (2 at a
  time) such that surrounding atoms follow octet
  rule (H -2 electrons). Stop adding electrons
  once the number exceeds value calculated in step
  2.

9.6
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Writing Lewis Structures(cont)

• 4. Make sure that every atom satisfies octet
  rule and the total number of valence electrons in
  Lewis Structure is correct.
• 5. If a surrounding atom does not have an
  octet move lone pair to bonding position from an
  adjacent atom to form a double or triple bond.

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Check Lewis Dot Structures

• Correct Number of Valence Electrons in Structure
• Every Atom Follows the Octet Rule
• Every Atom Follows Generality About Bonds and
  Lone Pairs UNLESS it Follows One of the Exceptions

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Exception to Generality of Bonds and Lone Pairs
for Atom

• Resonance Structures
• Polyatomic Ions
• Exceptions to the Octet Rule
• Incomplete Octet
• Expanded Octet
• Odd Electrons

If an atom does not follow generality it will
have a nonzero formal charge
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An atoms formal charge is the difference between
the number of valence electrons in an isolated
atom and the number of electrons assigned to that
atom in a Lewis structure.
The sum of the formal charges of the atoms in a
molecule or ion must equal the charge on the
molecule or ion.
9.7
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Example

• Ozone (O3)
• Lewis structure of ozone shows one double bond
  and one single bond

Expect one short bond and one long
bond Reality bonds are of equal length (128 pm)
O
O
O




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Resonance Structures

• Resonance Structures (RS) are imaginary the real
  structure is intermediate the resonance
  structures
• The RS differ only in the position of p electrons
  or lone pairs atoms positions do not change.
• The electron delocalization that occurs due to
  resonance stabilizes the molecule

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Resonance Structures (cont)

• Equivalent RS are of equal importances
• Rules for determining the importance of
  non-equivalent RS
• RS with minimum of non-zero formal charges on
  atoms is most prevelant
• If same of non-zero formal charges negative
  formal charge on more electronegative atom is
  more prevelant.

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Using Formal Charges to Asses Lewis Structures
more stable Lewis structure
less stable Lewis structure
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Rules for Writing Electron Shifts in Resonance
Structures

• Arrows are used to show electron movement
• Foot of arrow electron rich
• Head of arrow electron poor
• Only p electrons or lone pair may be shifted and
  only to adjacent positions
• From p bond to adjacent position
• from p bond to adjacent atom
• From lone pair to adjacent bond

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Rules for Writing Electron Shifts in Resonance
Structures
a)From p bond to adjacent position
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Rules for Writing Electron Shifts in Resonance
Structures
b) From p bond to adjacent atom c) From lone pair
to adjacent bond

more stable Lewis structure
less stable Lewis structure
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Rules for Writing Electron Shifts in Resonance
Structures
b) From p bond to adjacent atom c) From lone pair
to adjacent bond
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Types of Formulas

• Chemical Formula State number and type of each
  atom
• Structural Formula Shows atom connectivity as
  well as number and type of each atom

C4H10
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Condensed structural formulas
Lewis structures in which many (or all) covalent
bonds and electron pairs are omitted.
can be condensed to
CH3-CH2-CH2-CH3
and be condensed further to
CH3-(CH2)2-CH3
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Condensed structural formulas

• Lewis structures in which many (or all) covalent
  bonds and electron pairs are omitted.

can be condensed to
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Bond-line formulas

• Omit atom symbols. Represent structure by
  showing bonds between carbons and atoms other
  than hydrogen.
• Atoms other than carbon and hydrogen are called
  heteroatoms.

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Bond-line formulas
is shown as

• Omit atom symbols. Represent structure by
  showing bonds between carbons and atoms other
  than hydrogen.
• Atoms other than carbon and hydrogen are called
  heteroatoms.

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Constitution

• The order in which the atoms of a molecule are
  connected is called its constitution or
  connectivity.
• The constitution of a molecule must be determined
  in order to write a Lewis structure.

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Constitutional Isomers

• Isomers are different compounds that have the
  same molecular formula.
• Constitutional isomers are isomers that differ
  in the order in which the atoms are connected.
• An older term for constitutional isomers is
  structural isomers.

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Constitutional (Structural) Isomers of C4H10
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NH4OCN
Urea
Ammonium cyanate

• Ammonium cyanate and urea are constitutional
  isomers of CH4N2O.

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Examples of constitutional isomers
..
H

O

H
N
C



O
H
..
Nitromethane
Methyl nitrite

• Both have the molecular formula CH3NO2 but the
  atoms are connected in a different order.