1 for ions we must adjust the number of electrons available, a: add one e - to a for each negative...
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For ions we must adjust the number of electrons available, A: Add one e- to A for each negative charge Subtract one e- from A for each positive charge
The Octet Rule: Examples
NH4+
BF4–
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Example: CO32-
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Resonance
There are three possible structures for CO32-
The double bond can be placed in one of three places
These are called equivalent resonance structures
The real structure of the CO32- anion is an
average of these three resonance structures
COO
O
COO
O
COO
O
2- 2- 2-
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Resonance
There are no single or double bonds in CO32-
All three bonds are equivalent
They are intermediate between the single and double bond
C
OO
O
2-
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Resonance: Other Examples SO3
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Resonance: Other Examples NO3
–
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Resonance: Other Examples SO4
2–
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Exceptions to the Octet Rule
In those cases where the octet rule does not apply, the substituents attached to the central atom nearly always attain noble gas configurations
The central atom does not have a noble gas configuration but may have fewer than 8 or more than 8 electrons
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Examples
BBr3 AsF5
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Assignments & Reminders
Go through the recent lecture notes
Read Chapter 7 completely, except for Sections 7-7 & 7-8
Homework #4 due by Oct. 16 @ 3 p.m.
Review Session @ 5:15 p.m. on Sunday
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CHAPTER 8
Molecular Structure & Covalent Bonding Theories
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Stereochemistry
The study of the three-dimensional shapes of molecules
With the knowledge acquired so far we will be able to predict the shapes of molecules and ions
Our instrument – Valence Shell Electron Pair Repulsion theory (VSEPR theory – R. J. Gillespie)
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VSEPR Theory
In any molecule or ion there are regions of high electron density: Bonds (shared electron pairs) Lone pairs (unshared electrons)
Due to electron-electron repulsion, these regions are arranged as far apart as possible
Such arrangement results in the minimum energy for the system
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BeCl2
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BBr3
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CH4
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PCl5
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SF6
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Five Basic Geometries
LinearTrigonal
Octahedral
Trigonal bipyramidal
Tetrahedral
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SiF4
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NH3
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Electronic geometryElectronic geometry Distribution of regions of high
electron density around the central atom
Molecular geometryMolecular geometry Arrangement of atoms around the
central atom
Electronic Geometry and Molecular Geometry
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H2O