Molecular Geometry | General Chemistry 1

Molecular geometries are studied in this chapter: VSEPR theory, VSEPR notation, lone pair effect, electron-domain geometry vs. molecular geometry, prediction of molecular geometry, deviation from ideal bond angles, polarity of molecules, isomers

VSEPR Theory

A Lewis structure shows how atoms are connected in a covalent compound, but does not give information about the orientation of bonds in space and molecular geometry. Most molecules are not planar and their shapes explain some of their properties (dipole moment, chirality, angles between bonds ...)
 

The tetrahedral shape explains why dichloromethane (CH2Cl2) has only one isomer:

 

VSEPR model: 

A model that accounts for electron pairs in the valence shell of an atom repelling one another (VSEPR = Valence-Shell Electron-Pair Repulsion). This model predicts the shapes of molecules. The molecular shape is related to the total number of the electron domains (lone pair or bond regardless of the multiplicity) on the central atom: they will arrange themselves to be as far apart as possible in order to minimize their repulsive interactions
 

VSEPR notation:

AXnEm

n = number of atoms bonded to the central atom
m = number of lone pairs on the central atom

 

BeH2: AX2,   H2O: AX2E2

NH3: AX3E1,   CO2: AX2

Lone Pair Effect

Typical repulsions between electron domains (lp = lone pair; bp = bonding pair):
 

lp-lp > lp-bp > bp-bp


Lone pairs in the valence shell affect the shapes of molecules: unshared electrons can spread out more than a bonding pair of electrons ⇒ lone pairs must occupy the locations that have the least repulsive interactions
 

Trigonal bipyramidal geometry has 2 types of positions: axial or equatorial. The axial position has 3 strong repulsions (with an angle of 90°) while the equatorial position has only 2. The lone pair will be in the equatorial position:

Prediction of Molecular Geometry

Electron-domain geometry vs. molecular geometry

Electron-domain geometry: the arrangement of electron domains around the central atom
Molecular geometry: the arrangement of bonded atoms
 

Prediction of molecular geometry

  • 2 electron domains: electron-domain geometry is linear (angle = 180°)
    1 possible molecular shape ⇒ linear (AX2)
  • 3 electron domains: electron-domain geometry is trigonal planar (angle = 120°)
    2 possible molecular shapes ⇒ trigonal planar (AX3) & bent (AX2E)
  • 4 electron domains: electron-domain geometry is tetrahedral (angle = 109.5°)
    3 possible molecular shapes ⇒ tetrahedral (AX4), trigonal pyramidal (AX3E) & bent (AX2E2)
  • 5 electron domains: electron-domain geometry is trigonal bipyramidal (angles = 90° and 120°)
    4 possible molecular shapes ⇒ trigonal bipyramidal (AX5), seesaw (AX4E), T-shaped (AX3E2) & linear (AX2E3)
  • 6 electron domains: electron-domain geometry is octahedral (angle = 90°)
    3 possible molecular shapes ⇒ octahedral (AX6), square pyramidal (AX5E) & square planar (AX4E2)
     

 

How to determine the molecular geometry:

  1.  Draw the Lewis structure of the compound
  2. Count the number of electron domains on the central atom
  3. Determine the electron-domain geometry according to the VSEPR theory
  4. Determine the molecular geometry by considering the positions of the atoms only

Deviation from Ideal Bond Angles

Lone pairs effect

Lone pairs have more freedom to spread out than a bond and therefore have a greater capacity to repel other electron domains. The angle between a lone pair and a single bond will be greater than the angle between 2 single bonds
 

Tetrahedral formed by CCl4 has equivalent angles (109.5°) while that formed by NH3 has two different types of angle:

 

Multiple bonds effect

Multiple bonds contain more electron density and therefore repel more strongly than single bonds. The angle between a multiple bond and a single bond will be greater than the angle between 2 single bonds

Molecular Geometry and Polarity

Overall dipole moment:

The dipole moment of a molecule determined by vector addition of the individual bond dipoles

 

Conditions for a molecule to be polar:

  • It must contain polar bonds (bond dipoles)
  • Molecular geometry must not cancel out the effect of polar bonds (by vector addition)

It is possible that a molecule contains polar bonds, but is not polar. Its overall dipole moment is equal to 0
 

CO2 is linear
⇒ bond dipoles in CO2 cancel each other
⇒ overall dipole moment of CO2 = 0

H2O is bent
⇒ bond dipoles do not cancel each other
⇒ overall dipole moment of H2O ≠ 0
⇒ water is a polar molecule

 

Isomers

Isomers are molecules that have the same number and type of atoms but differ in the way their atoms are arranged. They have different chemical and physical properties
 

Structural (or constitutional) isomers:

Chemical species with the same molecular formula but which differ in the way atoms are connected to each other
 

The two following molecules are structural isomers: they have the same chemical formula (C4H10) but different connectivity:

 

Stereoisomers:

Isomers with the same molecular formula, the same connections but which differ in the way atoms are oriented in space (different 3D geometry). Hashed-wedged line structures are used to depict the 3D arrangement

  • Geometric isomers ⇒ different geometric arrangement
  • Optical isomers ⇒ nonsuperimposable on its mirror image