Molecular Geometry
“Molecular geometry” refers to the positions of the atoms in space about the
central atom.
The overall molecular shape of a molecule is determined by its bond angles in three
dimensions.
• .
Lewis structures show how atoms are connected, not the angles or
lengths of bonds in 3D.
VSEPR allows the translation of a Lewis structure into a configuration
about a central atom by following the rules of VSEPR theory.
Valence Shell Electron Pair Repulsion
(VSEPR) Theory
It is a method for predicting the shape of a molecule from the
knowledge of the groups of lone pairs and bonds around a central
atom.
Repulsion strength
The nonbonding electron pairs are as important as bonding electron pairs in
determining the structure.
Lone pairs of electrons take up more space in the valence shell than the
bonding electrons.
If one or more of the electron pairs are lone pairs, the distribution of electron
pair and the geometrical shape of the molecule must be different.
The bond angles decrease as the number of nonbonding electron pairs
increases
Repulsion strengths
lone pair -lone pair  lone pair -bond pair  bond pair-bond pair
Applying the VSEPR theory.
• Draw a Lewis structure of the molecule .
• Determine the number of lone pairs and numbers of bonds
around the central atom .
• Establish the geometrical orientation as- linear, trigonal planar,
tetrahedral, trigonal bipyramid or octahedral
Electron pairs assume orientations about an atom to minimize repulsions.
The five basic molecular geometries for the five basic electronic
geometries about a central atom.
SN +2
SN = 5
8
SN = 3
SN = 4
SN = 6
Electron pairs (bonding and nonbonding electrons) repel one
another, as a result , the electron pairs remain as far apart as
possible from another as possible to minimize the repulsion.
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•
•
•
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Two electron pairs in the valence orbital are arranged linearly
Three electron pairs are organized in a trigonal planar arrangement
Four electron pairs are organized in a tetrahedral arrangement
Five electron pairs are arranged in a trigonal bipyramid
Six electron pairs are organized in an octahedral arrangement
The repulsion of lone pair electrons is grater than the repulsion of bond pair electrons
question: predict the shape of CO2 HCN CH4, NH3 SO2 PCl5,
SF6 and H2O by using VSEPR theory.
question: predict the shape of CO2 HCN CH4, NH3 SO2 PCl5,
SF6 and H2O by using VSEPR theory.
Pauline scale of
electronegativity
• .
Electronegativity and Pauline
scale
• By using electronegativities of each element you can predict what
kind of bond will form based on the difference between the
electronegativities (ΔEN).
Dipole
• A polar molecule will have a positive
region and a negative region on the
outside of the molecule. This is called a
dipole (or two poles).
Practise
• Use the electronegativity values to label
the atoms in each of the following as + or
–.
• Q.13 pg.122
Naming simple molecules
• RULES: if there is only
one of the first atom than
don’t use a prefix,
otherwise use a prefix.
• Ex: CO = carbon
monoxide
• Ex: P2O4 = diphosphorous
tetroxide
Prefix
Mono
Number
1
Di
Tri
Tetra
2
3
4
Penta
Hexa
Hepta
Octa
5
6
7
8
Nona
Deca
9
10
Although bonds may be polar in a molecule, the dipole moments
associated with the polar bonds may cancel, leading to a non-polar
molecule with polar bonds.
21
Polar or nonpolar
• Linear, trigonal planar and tetrahedral
molecules are nonpolar if the outer atoms
are identical.
• Pyramidal molecules are always polar.
• Bent molecules are always polar because
of the shape of the molecule.
• Q.16 pg.123