Prediction of Molecular Geometries | General Chemistry 1
- 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 = 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
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