Molecular Shapes
VSEPR Theory Valence Shell Electron Pair Repulsion Theory VSEPR Theory is one method that chemists use to predict the shapes of molecules. This theory predicts that electron pairs, whether involved in bonds or as non-bonding pairs, will adopt a geometry in which they maximize the distance from one another in order to minimize repulsions.
Regions of “Electron Density” Electron Density = Bonding Area or Paired Electron Site. Li Be B C N O F Ne 1 2 3 4 4 4 4 4 Bonding area can be a single, double or triple bond
One Region of “Electron Density” Electron Density = Bonding Area or Paired Electron Site. Bonding area can be a single, double or triple bond Any time there are just two atoms creating a molecule, the geometry is LINEAR
Two Regions of “Electron Density” Electron Density = Bonding Area or Paired Electron Site. Bonding area can be a single, double or triple bond The maximum distance two regions of electron density can get away from affords a geometry called linear.
Two Regions of “Electron density” Bonding Sites Atoms
Linear Example O C
Linear Example O C CO2
Linear Examples O2 O O S Si S SiS2
Three Regions of Electron Density The maximum distance three regions of electron density can get away from affords a geometry called trigonal planar.
Three Regions of Electron Density
Trigonal Planar F BF3 B F F
Four Regions of Electron Density The maximum distance four regions of electron density can get away from affords a geometry called tetrahedral. Three possible shapes Tetrahedral Pyramidal Bent
Four Regions of Electron Density Non-bonding electron pairs
Tetrahedral H CH4 C H H H
Pyramidal NH3 N H H H
Bent H2O O H H
One More Time…
Linear Type Bond Angle Example Lewis Structure AX2: Linear 180o BeH2
Trigonal Planar AX3: Trigonal Planar 120o AlH3 Type Bond Angle Example Lewis Structure AX3: Trigonal Planar 120o AlH3
Tetrahedral AX4: Tetrahedral 109.5o CH4 Type Bond Angle Example Lewis Structure AX4: Tetrahedral 109.5o CH4
Pyramidal AX3E: Pyramidal 109.5o NH3 Type Bond Angle Example Lewis Structure AX3E: Pyramidal 109.5o NH3
Bent Type Bond Angle Example Lewis Structure 109.5o H2O AX2E2: Bent