Prediction of Molecular Geometries | General Chemistry 1

Molecular geometries are studied in this chapter: molecular shape, VSEPR theory and prediction of molecular geometry, lone pair effects, the relation between structure and dipole moment, optical isomers and chirality.

Molecular Shape

Lewis structures:
- show how atoms are connected in a covalent molecule or ion
- do not show the shapes of molecules 

but Lewis structures can be used to predict molecular geometries

 

Tetrahedron: four equivalent vertices.
Tetrahedral shape: one of the common molecular shapes.

 

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

VSEPR Theory

VSEPR Theory = Valence-Shell Electron-Pair Repulsion Theory 


This theory is used to predict the shapes of molecules:
molecular shape is related to the total number of bonds and lone pairs in the valence shell of the central atom ⇒ Objective: minimizing the mutual repulsion between electron groups.

Electron group: electron pair, lone pair, single unpaired electron, double bond or triple bond on the center atom


2 different ways to talk about the shape of a molecule:
- Electron Pair Geometry: determined by the number of electron groups
- Molecular shape: determined by the number of electron groups and the number of lone pairs

VSEPR and Prediction of Molecular Geometry

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

 


 

2 electron groups:
Electron pair geometry = linear (angle = 180°)
1 molecular shape ⇒ linear (AX2)


3 electron groups:
Electron pair geometry = trigonal planar (angle = 120°)
2 molecular shapes ⇒ trigonal planar (AX3) + bent (AX2E)


4 electron groups:
Electron pair geometry = tetrahedral (angle = 109.5°)
3 molecular shapes ⇒ tetrahedral (AX4) + trigonal pyramidal (AX3E) + bent (AX2E2)


5 electron groups:
Electron pair geometry = trigonal bipyramidal (angles = 90° and 120°)
4 molecular shapes ⇒ trigonal bipyramidal (AX5) + seesaw (AX4E) + T-shaped (AX3E2) + linear (AX2E3)


6 electron groups:
Electron pair geometry = octahedral (angle = 90°)
3 molecular shapes ⇒ octahedral (AX6) + square pyramidal (AX5E) + square planar (AX4E2)


 


Wedge and Dash notation: 
wedge: bond projecting toward you; dash: bond going away from you

Lone Pair Effects

Typical repulsions (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 ⇒ lone pairs must go in locations that produce the least repulsive interactions.

 

The trigonal bipyramidal electron pair geometry has 2 different position:
the axial and the equatorial position.

The axial position has 3 strong repulsions (with an angle of 90°) while equatorial position has only 2.
The lone pair will be in equatorial position:
 

 


Lone pairs also lead to bond angle anomalies:
tetrahedral formed by CCl4 has equivalent angles (109.5°) while that formed by NH3 has two different types of angle:


 

Structure and Dipole Moment

Conditions for a molecule to be polar (have a dipole moment):

- it must contain polar bonds (bond dipoles)
- the molecular geometry must not cancel out the effect of the polar bonds (through vector addition)

It is possible for a molecule to contain polar bonds, but not be polar overall.


 

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 and Chirality

Isomers: same number and type of atoms but differ in the way the atoms are arranged ⇒ different properties.


Structural (or constitutional) isomers: same chemical formula but different connectivity (different Lewis structures).

 

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


 

 

Stereoisomers: same chemical formula, same atom connectivity but different spatial arrangement:
- geometric isomers ⇒ different geometric arrangement
- optical isomers ⇒ nonsuperimposable on its mirror image


 

Optical isomers occur when we have a central atom bonded to 4 different substituents.
Chiral molecule: molecule that exhibit optical isomerism