Molecular
Geometries &
Bonding Theories
, Bonding & shapes of molecules
Molecular Geometry
Shape of a molecule plays role in reactivity
The Nitrogen has 4 To predict shape, note number of bonding & nonbonding (lone pairs) electron pairs
electron domains (ED)
Assume electron pairs are placed as far apart from each other as possible
Areas of electron density, aka electron domains, electron pairs (bonding or nonbonding)
Double/triple bonds are still considered only 1 electron domain
Prediction of molecular shapes
Molecular shapes can predicted based on:
Lewis structures & valence-shell electron-pair repulsion, VSEPR model
Valence-shell electron-pair repulsion theory: best arrangement of given number of
electron domains is the one that minimizes repulsions among them
Electron domain geometry
Arrangement of electron domains (lone pairs/bonds) around central atom of molecule/ion
Five basic geometric structures: NB! Electron-domain geometry is often NOT the shape of the molecule, molecular geometry
Linear is defined by positions of only atoms in molecules, not nonbonding pairs (lone pairs).
Trigonal planar 180°
120° 109,5°
90° 90°
90°
120°
Tetrahedral
Trigonal bipyramidal
Octahedral Linear (2 ED) Trigonal planar (3ED) Tetrahedral (4ED) Trigonal bipyramidal (5ED) Octahedral (6ED)
For each electron domain geometry, there might be more than one molecular geometry.
Each time a bond is replaced by a lone pair, a different molecular geometry is created.
Molecular geometries
Linear electron domain
Only one molecular geometry: linear 2ED
If there are only two atoms in molecule (eg, CO, C≡O),Allmolecule
bonding
is always linear
Trigonal planar electron domain Non-bonding pair
All bonding
Two molecular geometries:
Trigonal planar Bent Trigonal planar, if all electron domains are bonding
Bent, if one of the domains is a nonbonding pair
Tetrahedral electron domain
Non-bonding pair
Three molecular geometries: All bonding
Non-bonding
pairs
Tetrahedral, if all are bonding pairs
Trigonal pyramidal, if one is a nonbonding pair
Bent, if there are two nonbonding pairs Tetrahedral Trigonal pyramidal Bent
Trigonal bipyramidal electron domain
Two distinct positions: Four distinct molecular geometries: Trigonal
Axial Seesaw
90° Trigonal bipyramidal, all bonding pairs bipyramidal
120°
Equatorial Seesaw, one nonbonding pair
All bonding Non-bonding pair
T-shaped, two nonbonding pairs
Lower-energy conformations result from
Linear, three nonbonding pairs T-shaped
having non-bonding electron pairs in equatorial,
Non-bonding Linear
rather than axial, positions in this geometry (ie.
pairs
non-bonding pairs always in equatorial rather Non-bonding
than axial, less electron repulsion) pairs
Geometries &
Bonding Theories
, Bonding & shapes of molecules
Molecular Geometry
Shape of a molecule plays role in reactivity
The Nitrogen has 4 To predict shape, note number of bonding & nonbonding (lone pairs) electron pairs
electron domains (ED)
Assume electron pairs are placed as far apart from each other as possible
Areas of electron density, aka electron domains, electron pairs (bonding or nonbonding)
Double/triple bonds are still considered only 1 electron domain
Prediction of molecular shapes
Molecular shapes can predicted based on:
Lewis structures & valence-shell electron-pair repulsion, VSEPR model
Valence-shell electron-pair repulsion theory: best arrangement of given number of
electron domains is the one that minimizes repulsions among them
Electron domain geometry
Arrangement of electron domains (lone pairs/bonds) around central atom of molecule/ion
Five basic geometric structures: NB! Electron-domain geometry is often NOT the shape of the molecule, molecular geometry
Linear is defined by positions of only atoms in molecules, not nonbonding pairs (lone pairs).
Trigonal planar 180°
120° 109,5°
90° 90°
90°
120°
Tetrahedral
Trigonal bipyramidal
Octahedral Linear (2 ED) Trigonal planar (3ED) Tetrahedral (4ED) Trigonal bipyramidal (5ED) Octahedral (6ED)
For each electron domain geometry, there might be more than one molecular geometry.
Each time a bond is replaced by a lone pair, a different molecular geometry is created.
Molecular geometries
Linear electron domain
Only one molecular geometry: linear 2ED
If there are only two atoms in molecule (eg, CO, C≡O),Allmolecule
bonding
is always linear
Trigonal planar electron domain Non-bonding pair
All bonding
Two molecular geometries:
Trigonal planar Bent Trigonal planar, if all electron domains are bonding
Bent, if one of the domains is a nonbonding pair
Tetrahedral electron domain
Non-bonding pair
Three molecular geometries: All bonding
Non-bonding
pairs
Tetrahedral, if all are bonding pairs
Trigonal pyramidal, if one is a nonbonding pair
Bent, if there are two nonbonding pairs Tetrahedral Trigonal pyramidal Bent
Trigonal bipyramidal electron domain
Two distinct positions: Four distinct molecular geometries: Trigonal
Axial Seesaw
90° Trigonal bipyramidal, all bonding pairs bipyramidal
120°
Equatorial Seesaw, one nonbonding pair
All bonding Non-bonding pair
T-shaped, two nonbonding pairs
Lower-energy conformations result from
Linear, three nonbonding pairs T-shaped
having non-bonding electron pairs in equatorial,
Non-bonding Linear
rather than axial, positions in this geometry (ie.
pairs
non-bonding pairs always in equatorial rather Non-bonding
than axial, less electron repulsion) pairs
