AP Chemistry Unit 4 – Molecular Structure

1
AP ChemistryUnit 4 Molecular Structure

• Lesson 3 Lewis Structures for Molecular
  Compounds
• Book Section 8.3-8.5, 8.7

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Covalent Bonding

• In covalent bonds atoms share electrons.
• There are several electrostatic interactions in
  these bonds
• Attractions between electrons and nuclei
• Repulsions between electrons
• Repulsions between nuclei

3
Polar Covalent Bonds

• Though atoms often form compounds by sharing
  electrons, the electrons are not always shared
  equally.

• Fluorine pulls harder on the electrons it shares
  with hydrogen than hydrogen does.
• Therefore, the fluorine end of the molecule has
  more electron density than the hydrogen end.

4
Polar Covalent Bonds

• When two atoms share electrons unequally, a bond
  dipole results.
• The dipole moment, µ, produced by two equal but
  opposite charges separated by a distance r, is
  calculated
• µ Qr
• It is measured in debyes (D).

5
Polar Covalent Bonds

• The greater the difference in electronegativity,
  the more polar is the bond.

6
1999 MC 40

• Of the following molecules, which has the largest
  dipole moment?
• CO
• CO2
• O2
• HF
• F2

7
1999 MC 40

• Of the following molecules, which has the largest
  dipole moment?
• CO
• CO2
• O2
• HF 54 correct, medium
• F2

8
Lewis Structures

• Lewis structures are representations of molecules
  showing all electrons, bonding and nonbonding.

9
Writing Lewis Structures

• 1) Find the sum of valence electrons of all atoms
  in the polyatomic ion or molecule.
• If it is an anion, add one electron for each
  negative charge.
• If it is a cation, subtract one electron for each
  positive charge.

PCl3
5 3(7) 26
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Writing Lewis Structures

• 2) The central atom is the least electronegative
  element that isnt hydrogen. Connect the outer
  atoms to it by single bonds.
• Keep track of the electrons (1 bond 2
  electrons)
• 26 6 20 e-

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Writing Lewis Structures

• 3) Fill the octets of the outer atoms.
• Keep track of the electrons
• 26 6 20 e-
• 20 18 2 e-

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Writing Lewis Structures

• 4) Fill the octet of the central atom.
• Keep track of the electrons
• 26 6 20 e-
• 20 18 2 e-
• 2 2 0 e-

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Writing Lewis Structures

• 4) Fill the octet of the central atom.
• Keep track of the electrons
• 26 6 20 e-
• 20 18 2 e-
• 2 2 0 e-

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Writing Lewis Structures

• 5) If you run out of electrons before the central
  atom has an octet
• form multiple bonds until it does.
• Double and triple bonds are shorter than single
  bonds.

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Writing Lewis Structures

• Then assign formal charges.
• For each atom, count the electrons in lone pairs
  and half the electrons it shares with other
  atoms.
• Subtract that from the number of valence
  electrons for that atom the difference is the
  formal charge.

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Writing Lewis Structures

• The best Lewis structure
• is the one with the fewest charges.
• puts a negative charge on the most
  electronegative atom.
• Ions get put in brackets with the charge in the
  upper-right.

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1994 MC 62

• The electron-dot structures (Lewis structure) for
  which of the following molecules would have two
  unshared pairs of electrons around the central
  atom?
• A) H2S
• B) NH3
• C) CH4
• D) HCN
• E) CO2

18
1994 MC 62

• The electron-dot structures (Lewis structure) for
  which of the following molecules would have two
  unshared pairs of electrons around the central
  atom?
• A) H2S 64 correct, easy
• B) NH3
• C) CH4
• D) HCN
• E) CO2

19
1989 MC 17

• The Lewis dot structure of which of the following
  molecules shows only one unshared pair of valence
  electrons?
• Cl2
• N2
• NH3
• CCl4
• H2O2

20
1989 MC 17

• The Lewis dot structure of which of the following
  molecules shows only one unshared pair of valence
  electrons?
• Cl2
• N2
• NH3 - 76 correct, easy
• CCl4
• H2O2

21
1984 MC 41

• Which of the following molecules has the shortest
  bond length?
• N2
• O2
• Cl2
• Br2
• I2

22
1984 MC 41

• Which of the following molecules has the shortest
  bond length?
• N2 49 correct, medium
• O2
• Cl2
• Br2
• I2

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Homework 8.30, 8.32, 8.34, 8.40, 8.42, 8.44,
8.46, 8.60, 8.62

• Next week
• Monday Resonance Structures (8.7)
• Tuesday VSEPR (9.1-9.2), Stoichiometry Lab Due
• Wednesday Polarity (9.3)
• Thursday Hybridization (9.4-9.5)
• Friday No Class (Early Dismissal)