9.1 Molecular Shapes
The overall shape of a molecule is determined by its bond angles, the angles made by the lines joining the nuclei of the atoms in the molecule.
The possible shapes of Abn molecules depend on the value of “n.”
Examples: AB2 molecule must be either linear or bent
AB3 molecule must be either trigonal planar pr trigonal pyramidal
9.2- The VSEPR Model
A bonding pair of electrons thus defines a region in which the elec-trons will more likely be found. (The region is referred to as electron domain)
Nonbonding pair (lone pair) of electrons defines an electron domain that is located principally on one atom.
THE VSEPR model is based on the idea that electron domains are negatively charged & therefore, repel each other.
-The VSEPR predicts that the best arrangement of electron domains is the one that minimizes the repulsions among them.
Electron domain geometry- The arrangement
Of electron domains about the central atom of a
Molecule or ion.
*Molecular geometry is the arrangement of only
the atoms in a molecule or ion.
By looking at the VSEPR model, we can predict
the electron-domain geometry.
-From knowing how many domains are due to
Nonbonding pairs, we can then predict the
Molecular geometry of a molecule or ion from
Its electron-domain geometry.
Molecular Geometry
TrigonalBipyra-midal5 electron do-
mains
Seesaw
TrigonalBipyrami-
dal
T-shaped
Lin-ear
PCl5 SF4
XeF2
ClF3
Molecular Geometry
Octahe-dral
6 electron do-mains
OctahedralSquare
Pyramidal
Square Pla-nar
SF6
BrF5
XeF4
9.3 Molecular Shape & Molecular Polar-ity
For a molecule that consists of more than two atoms, the dipole moment depends on both the polarities of the individual bonds and the geometry of the molecule.
H ClPo-lar
CO ONon-Po-lar
9.5 Hybrid Orbitals
Atomic orbitals on an atom mix to form new orbitals called hybrid or-bitals.
The process of mixing atomic orbitals is called hybridization.
Provide a convenient model for using valence-bond theory to describe the covalent bonds in molecules with geometries that conform to the electron domain geometries predicted by the VSEPR model.
Steps that allow us to predict the hybrid orbitals used by an atom in the bonding:
1. Draw the lewis structure for the molecule or ion.
2. Determine the electron-domain geometry using the VSEPR model.
3. Specify the hybrid orbitals needed to accommodate the electron pairs based on their geometric arrangement.
Example:
NH3
Find: molecular formula, electron domain ge-ometry, molecular geometry, number of elec-tron domains(bonding/nonbonding), polarity, bond angles, hybridization
Phosporus hexaflouride (negative one ion)
PF6