the vsepr theory advanced chemistry ms. grobsky. determining molecular geometries in order to...
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The VSEPR TheoryAdvanced ChemistryMs. Grobsky
Determining Molecular Geometries• In order to predict molecular shape, we use the
Valence Shell Electron Pair Repulsion (VSEPR) theory
•This theory proposes that the geometric arrangement of groups of atoms about a central atom in a covalent compound is determined solely by the repulsions between electron pairs present in the valence shell of the central atom▫The molecule adopts whichever 3-D geometry minimizes the repulsion between valence electrons
Determining Molecular Geometries• To determine the shape of a molecule, we distinguish
between:▫ Lone pairs (non-bonding pairs)▫ Bonding pairs (those found between two atoms)
Multiple bonds are considered as ONE bonding pair even though in reality, they have multiple pairs of electrons
• All electrons are considered when determining 3-D shapeAXmEn
A - central atomX – surrounding atom
E – non-bonding valence electron groupm and n - integers
Electron Group Repulsions and the Five Basic Molecular Shapes
Factors Affecting Electron Repulsion(And therefore, Bond Angles!)• Two factors that affect the amount of electron repulsion
around an atom:▫ Multiple bonds
Exert a greater repulsive force on adjacent electron pairs than do single bonds
Result of higher electron density Distorts basic geometry!
▫ Non-bonding (lone) pairs Lone pairs repel bonding pairs more strongly than bonding
pairs repel each other
The Effect of Non-Bonding Electrons on Bond Angles
• Remember, electron pairs of bonding atoms are shared by two atoms, whereas the nonbonding electron pairs (lone pairs) are attracted to a single nucleus▫ As a result, lone pairs can
be thought of as having a somewhat larger electron cloud near the parent atom
• This “crowds” the bonding pairs and the geometry is distorted!▫ Bond angles change!
Factors Affecting Bond AnglesDouble Bonds Non-Bonding (Lone) Pairs
The Single Molecular Shape of Linear Electron-Group Arrangement• AX2• Examples
▫ CS2, HCN, BeF2
A XX
The 2 Molecular Shapes of Trigonal Planar Electron-Group Arrangement Trigonal Planar Bent
• AX3
• Examples▫ SO3, BF3
• AX2E
• Examples▫ SO2
A
X
X
X X
X AE
The 3 Molecular Shapes of the Tetrahedral Electron-Group ArrangementTetrahedral
• AX4
• Examples▫ CH4, SiCl4,
SO42-, ClO4
-
• AX3E
• Examples▫ NH3, PF3,
ClO3, H3O+
Trigonal Pyramidal Bent
• AX2E2
• Examples▫ H2O, OF2,
SCl2
A
X
XX
XA
XX
X
E
A
E
EXX
The 4 Molecular Shapes of the Trigonal Bipyramidal Electron-Group Arrangement
• AX5
• Examples▫ PCl5, PF5,
AsF5, SOF4
Trigonal Bipyramidal See-Saw T-Shaped Linear
• AX4E
• Examples▫ SF4,
XeO2F2, IF4+, IO2F2-
• AX3E2
• Examples▫ ClF3, BrF3
• AX2E3
• Examples▫ XeF2, I3-,
IF2-
The 3 Molecular Shapes of the Octahedral Electron-Group ArrangementOctahedral
• AX6
• Examples▫ SF6, IOF5
• AX5E
• Examples▫ BrF5, XeOF4,
TeF5-
Square Pyramidal Square Planar
• AX4E2
• Examples▫ XeF4, ICl4-
What You Need to Know From All of This• Five BASIC geometries of covalent compounds and
their bond angles (ideal bond angles)▫ Linear (AX2)▫ Trigonal planar (AX3)▫ Tetrahedral (AX4)▫ Trigonal bipyramidal (AX5)▫ Octahedral (AX6)
• The following “special” geometries of covalent compounds with lone pairs▫ AX2E▫ AX3E▫ AX2E2
Steps in Determining a Molecular Shape•Refer to front of Page 237!
• Electronegativities determine polarity since it measures a nucleus’ attraction or “pull” on the bonded electron pair
▫ When two nuclei are the same, sharing is equal Non-polar
▫ When 2 nuclei are different, the electrons are not shared equally
Polar▫ When electrons are shared unequally to a greater extent,
IONIC• Bonds can be polar while the entire molecule is not
▫ Determined by geometry! More on this later!
• Dipole moment▫ Separation of the charge in a molecule (slightly
positive/slightly negative poles)▫ IF octet rule is obeyed AND all the surrounding bonds are the
same (even if they’re very polar), then the molecule is NONPOLAR
Example: CCl4
Electronegativity
VSEPR and Polarity• Knowing the geometry of a molecule allows one to predict
whether it is polar or nonpolar▫ A bond between unlike atoms is usually polar with a positive
end and a negative end• The symmetry of the molecule determines polarity
▫ A diatomic molecule containing two different atoms is polar HF, CO
▫ A diatomic molecule containing the same two atoms is nonpolar N2, O2
▫ A polyatomic molecule may be nonpolar even if it contains polar bonds because, in such cases, the polar bonds are counteracting each other CO2, CH4 = nonpolar
VSEPR Symmetry and Molecular Polarity
VSEPR Symmetry and Molecular Polarity