Download The VSEPR Theory and Hybridization

The VSEPR Theory
Advanced Chemistry
Ms. 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 shape
AXmEn
A - central atom
X – surrounding atom
E – non-bonding valence electron group
m 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 Angles
Double Bonds
Non-Bonding (Lone) Pairs
The Single Molecular Shape of Linear
Electron-Group Arrangement
• AX2
• Examples
▫ CS2, HCN, BeF2
X
A
X
The 2 Molecular Shapes of Trigonal
Planar Electron-Group Arrangement
Trigonal Planar
Bent
• AX3
• Examples
▫ SO3, BF3
• AX2E
• Examples
▫ SO2
X
X
E
A
A
X
X
X
The 3 Molecular Shapes of the Tetrahedral
Electron-Group Arrangement
Tetrahedral
• AX4
• Examples
▫ CH4, SiCl4,
SO42-, ClO4-
X
Trigonal Pyramidal
• AX3E
• Examples
▫ NH3, PF3,
ClO3, H3O+
Bent
• AX2E2
• Examples
▫ H2O, OF2, SCl2
X
E
E
A X
X
A
A
X
X
X
X
E
X
The 4 Molecular Shapes of the Trigonal
Bipyramidal Electron-Group Arrangement
Trigonal
Bipyramidal
• AX5
• Examples
▫ PCl5, PF5,
AsF5, SOF4
See-Saw
T-Shaped
Linear
• AX4E
• Examples
• AX3E2
• Examples
• AX2E3
• Examples
▫ SF4, XeO2F2,
IF4+, IO2F2-
▫ ClF3, BrF3
▫ XeF2, I3-,
IF2-
The 3 Molecular Shapes of the Octahedral
Electron-Group Arrangement
Octahedral
• AX6
• Examples
▫ SF6, IOF5
Square Pyramidal
• AX5E
• Examples
▫ BrF5, XeOF4,
TeF5-
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!
Electronegativity
• 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
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