Announcement

• LT #3

▫ 120 points: 40 items MC x 2.5 pts each + 2 PS items x 10 pts each

• March 19, Thursday

▫ 6-7:30 PM

▫ Schmitt Hall C-114

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Fractional Bond Order

2

3 electron pairs

2 bonded-atom pairs Bond order = = 1½

4 shared electron pairs

3 bonded-atom pairs Bond order = = 1⅓

Formal Charge

• When more than one Lewis structure is plausible, we apply the concept of formal charge to determine the best one.

3

Formal Charge = # valence e- - # e- assigned to atom

# valence e- 6 4 6 6 4 6

# assigned e- 6 4 6 5 4 7

Formal Charge

0 0 0 +1 0 -1

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1. The best structure is the one that minimizes total formal charge. Net charge of ion or molecule must equal total formal charge.

2. Also, the best Lewis structure places more negative charge on the more electronegative atom.

Formal Charge = # valence e- - # e- assigned to atom

Practice Exercise

• Which is the most important resonance form of the cyanate ion, NCO-?

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-2 0 +1 -1 0 0 0 0 -1

Formal Charge vs Oxidation Number

• For a formal charge, bonding electrons are shared equally by the atoms.

▫ The formal charge of an atom may change between resonance forms.

• For an oxidation number, bonding electrons are transferred to the more electronegative atom.

▫ The oxidation number of an atom is the same in all resonance forms.

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+2 0 -1 -1 0 0 0 0 -1

Formal charges

-3 +4 -2 -3 +4 -2 -3 +4 -2

Oxidation numbers

Major Exceptions to the Octet Rule

1. Incomplete Octet – often occurs with Be and B as central atoms (eg BeCl2, BF3)

2. Expanded Octet – occurs mostly with Period 3 or higher nonmetals

3. Odd-number electrons – highly reactive species called radicals that have an odd number of electrons (eg NO2)

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Practice Exercises

• Draw a Lewis structure and identify the octet-rule exception for:

▫ (a) POCl3 (b) ClO2 (c) XeF4

• Draw 3 plausible Lewis structures for the thiocyanate ion [SCN]- (atom sequence SCN). Identify the best structure among the three.

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Video: https://www.youtube.com/watch?v=Dj4sz6L9IdM

• GOAL: To understand geometry (via VSEPRT) and to relate it to a picture of covalent bonding in molecules.

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Molecular Formula

Lewis Structure

VSEPRT Geometry

Hybrid Orbitals

NH3

VSEPRT explains the shape or geometry of molecules.

• The Valence Shell Electron Pair Repulsion Theory (VSEPRT) can be used to predict the shapes of molecules.

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1. Draw the Lewis structure from the chemical formula.

2. Count all electron domains to get the AXE code.

3. Group domains into bonding and non-bonding pairs of electrons.

4. Match the number of bonding and non-bonding domains to the proper VSEPRT geometry.

AXnEm Code

• Shorthand for bonding/nonbonding info

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AXnEm A: Central atom

n: # of bonded domains

m: # of non-bonded domains

Electron and Molecular Geometry

• Electron geometry: geometry of all electron groups

• Molecular geometry: geometry of only the atoms bonded to the central atom

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2 Electron Groups: Linear Electron Geometry

and 1 Possible Molecular Geometry

Examples:

CS2, HCN, BeF2

AX2 AX2

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3 Electron Groups: Trigonal Planar Electron

Geometry and 2 Possible Molecular Geometries

AX3 AX3

Examples:

SO3, BF3, NO3–, CO3

2−

AX2E

Examples:

SO2, O3, PbCl2, SnBr2

AX2E

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Lone pair–lone pair > lone pair–bonding pair > bonding pair–bonding pair

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Examples:

CS2, HCN, BeF2

AX2

Examples:

SO2, O3, PbCl2, SnBr2

AX2E

Examples:

CH4, SiCl4,

SO42–, ClO4

–

AX4 AX4

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4 Electron Groups: Tetrahedral Electron

Geometry and 3 Possible Molecular Geometries

AX3E

Examples:

NH3, PF3

ClO3–, H3O

+

AX3E

Examples:

H2O, OF2, SCl2

AX2E2 AX2E2

Lone pair–lone pair > lone pair–bonding pair > bonding pair–bonding pair

5 Electron Groups: Trigonal Bipyramidal

Electron Geometry and 4 Possible Molecular

Geometries

Axial position

AX5 Examples:

PF5, AsF5, SOF4

AX5

Examples:

SF4, XeO2F2

IF4+, IO2F2

–

AX4E AX4E

AX3E2

Examples:

ClF3, BrF3

AX3E2 AX2E3

Examples:

XeF2, I3–, IF2

–

AX2E3

Equatorial position

17

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6 Electron Groups: Octahedral Electron

Geometry and 3 Possible Molecular Geometries

Examples:

SF6, IOF5

AX6 AX6

AX5E

Examples:

BrF5, TeF5–,

XeOF4

AX5E AX4E2

Examples:

XeF4, ICl4–

AX4E2

Electron and Molecular Geometry

• Electron geometry: geometry of all electron groups

• Molecular geometry: geometry of only the atoms bonded to the central atom

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Bond Polarity, Bond Angle, and Dipole

Moment

• Overall molecular polarity depends on both shape and bond polarity.

• A molecule is polar if

- it contains one or more polar bonds and

- the individual bond dipoles do not cancel.

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C (EN = 2.5) and O (EN = 3.5)

H (EN = 2.1) and O (EN = 3.5)

Bond Polarity, Bond Angle, and Dipole

Moment

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Molecules with the same shape may have

different polarities.

CCl4

Bonds are polar, but

individual bond

polarities cancel.

CHCl3

Bond polarities do

not cancel. This

molecule is polar.