Trigonal Planar: triangular, planar with 120° bond angles
Example: BF3 (see pg. 382)
Tetrahedral structure: tetrahedron
Example: methane
Four identical triangular faces
Figure 12.12: Molecular structure of methane.
Molecular Structure: The VSEPR Model
Structure very important to molecular properties
Determines taste
Biological molecules – structure change can convert cell from normal to cancerous
Experimental methods exist for determining 3-D structure
Useful to predict approximate structure
VSEPR Model
Valence Shell Electron Pair Repulsion (VSEPR) model
Used to predict molecular structure of molecules formed from nonmetals
The structure around a given atom is determined by minimizing repulsions between electron pairs
Bonding & non-bonding electron pairs around an atom are positioned as far apart as possible
VSEPR Model Rules (see ex pp 384-386)
Two pairs of electrons on a central atom of a molecule are always placed at an angle of 180° to each other to give a linear arrangement
Three pairs of electrons on a central atom in a molecule are always placed 120° apart in the same plane as the central atom – trigonal planar
Four pairs of electrons on a central atom are always placed 109.5° apart - tetrahedral
When every pair of electrons on central atom is shared, the molecular structure has same name as the arrangement of electron pairs
2 = linear, 3 = trigonal planar, 4 = tetrahedral
When one or more of the electron pairs around the central atom are unshared, the name of the structure is different from that for the arrangement of electron pairs (see table 12.4 # 4 & 5)
