Download Molecular Geometry and Bonding Theories In this chapter

Molecular
Geometry
and
Bonding
Theories
Chapter 9 Part
1
November 11th,
2004
1
In this chapter
• Molecular shapes
• Valence Shell Electron Pair Repulsion model.
• Molecular shapes and polarity.
• Valence bond theory.
• Hybridization.
• Molecular orbitals, multiple bonds.
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Molecular Shapes
• Lewis structures give atomic connectivity: they tell us
which atoms are physically connected to which.
• The shape of a molecule is determined by its bond
angles.
• Consider CCl4: experimentally we find all Cl-C-Cl
bond angles are 109.5°.
– Therefore, the molecule cannot be planar.
– All Cl atoms are located at the vertices of a tetrahedron
with the C at its center.
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Molecular Shapes
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Molecular Shapes
• The Valence Shell Electron Pair Repulsion (VSEPR)
theory proposed by Gillespie and Nyholm is a
reliable method for predicting shapes of covalent
molecules and polyatomic ions.
• The VSEPR theory is based on the idea that bond
and lone pair electrons in the valence shell of an
element repel each other and move as far apart as
possible.
• VSEPR is extremely successful in predicting the
structures of molecules and ions of the main group
elements, but is generally useless for transition
metal containing compounds.
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There are five fundamental geometries for molecular shape:
AB2
AB3
AB5
AB4
AB6
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Predicting the Shape
• Predict the shape of CH2Cl2
• CH2Cl2 has one central atom and 4 other atoms
around it. Draw the Lewis structure.
• Referring to the possible arrangements we can
have for such a molecule the shape can be
predicted.
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Molecular Shapes
• When considering the geometry about the central
atom, we consider all electrons (lone pairs and
bonding pairs).
• When naming the molecular geometry, we focus
only on the positions of the atoms.
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Molecular Shapes
• To determine the shape of a molecule, need to
distinguish between lone pairs (or non-bonding
pairs, those not in a bond) of electrons and bonding
pairs (those found between two atoms).
• The electron domain geometry is defined by the
positions in 3D space of ALL electron pairs (bonding
+ non-bonding). This becomes especially important
when the central atom has lone pairs.
• The electrons adopt an arrangement in space to
minimize e--e- repulsion.
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Lone pairs on Central Atom
• Consider NH3: The Lewis structure of
NH3 is:
• The molecular geometry of NH3 is
described as trigonal pyramidal with the
N atom at the apex and the 3 H atoms
forming the base of the triangle.
• The electron-pair geometry is described
as tetrahedral.
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Lone Pairs & Bond Angles
• The electron pair geometry is decided only by
looking at the positions of the electrons.
• The molecular geometry is decided only by
the positions of atoms, lone pairs are ignored
in the molecular geometry.
• Lone pairs are considered to be “fat”, while
bonding pairs are considered “skinny”.
• The increased volume of lone pairs makes
bond pairs squeeze together, reducing bond
angles.
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Lone Pairs & Bond Angles
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Lone Pairs & Bond Angles
• Experimentally, the H-X-H bond angle decreases on
moving from C to N to O:
H
H C H
H
109.5O
H N H
H
107O
O
H
H
104.5O
• Since the electrons in a bonding pair are attracted by
two nuclei they do not repel as much as lone pairs.
• The bond angle decreases as the number of lone pairs
on the central atom increases.
• Typically the relative strengths of repulsion is:
Lone pair-lone pair > lone pair-bond pair > bond
pair-bond pair
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Multiple Bonds & Bond Angles
• Similarly, electrons in multiple bonds repel more
than electrons in single bonds.
• A multiple bond is counted as one electron domain.
• The Lewis structure of COCl2 (carbonyl chloride)
indicates the molecular and the electron pair
geometry to be trigonal-planar.
Cl
111.4o
Cl
C O
124.3o
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Molecules with Expanded Valence
Shells
• Atoms that have expanded octets have AB5 (trigonal
bipyramidal) or AB6 (octahedral) electron pair
geometries.
• For trigonal bipyramidal structures there is a plane
containing three electrons pairs. The fourth and fifth
electron pairs are located above and below this
plane.
• For octahedral structures, there is a plane containing
four electron pairs. Similarly, the fifth and sixth
electron pairs are located above and below this
plane.
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