Molecular Geometry | General Chemistry 1

A Lewis structure shows how atoms are connected in a covalent compound but gives no information about the orientation of the bonds in space and the molecular geometry. Most molecules are not planar, and their shapes explain some of their properties (dipole moment, chirality, angles between bonds ...). The VSEPR theory is a model to predict the geometry of molecules
 

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

VSEPR model: 

A model that takes into account the repulsion of electron pairs in the valence shell of an atom (VSEPR = Valence-Shell Electron-Pair Repulsion). This model predicts the shape of molecules. The molecular shape is related to the total number of 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 to minimize their repulsive interactions

VSEPR notation:

BeH₂: AX₂,   H₂O: AX₂E₂

NH₃: AX₃E₁,   CO₂: AX₂

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

lp-lp > lp-bp > bp-bp

Lone pairs have the greatest repelling effect because they are closer to the nucleus of the central atom than the bonding pairs and the unshared electrons can spread out more. Therefore, lone pairs repel other electron domains more and will occupy the locations with the least amount of interaction (i.e. the location farthest from other electron domains)
 

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:

Electron geometry vs. molecular geometry

Electron geometry (or electron-domain geometry): the arrangement of electron domains around the central atom (lone pairs and bonds)
Molecular geometry: the arrangement of bonded atoms (only bonds)
 

Prediction of molecular geometry

• 2 electron domains: electron geometry is linear (angle = 180°)
  1 possible molecular shape ⇒ linear (AX₂)
• 3 electron domains: electron geometry is trigonal planar (angle = 120°)
  2 possible molecular shapes ⇒ trigonal planar (AX₃) & bent (AX₂E)
• 4 electron domains: electron geometry is tetrahedral (angle = 109.5°)
  3 possible molecular shapes ⇒ tetrahedral (AX₄), trigonal pyramidal (AX₃E) & bent (AX₂E₂)
• 5 electron domains: electron 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 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 geometry according to the VSEPR theory
4. Determine the molecular geometry by considering only the positions of the atoms

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

Overall dipole moment:

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

Conditions for a molecule to be polar:

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

It is possible for a molecule to contain polar bonds, but not be 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 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  that have the same molecular formula but differ in the way the atoms are bonded together
 

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

Stereoisomers:

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

• Geometric isomers: isomers with a different geometric arrangement
• Optical isomers: isomers that cannot be superimposed on their mirror image