VSEPR
What shape are your molecules in?
Background you need…
 Lewis structures
 How many bonds do each element make?
 What can expand?
 Bonding (covalent)
 Polarity
 Electronegativity and determining bond type
 Resonance v. Isomers
 Formal charge
Let’s review now…..
Lewis Structures
 Remember that Lewis structures want a
full outer shell
 Remember that for a given Lewis
structure, the number of electrons around
the atoms must equal the total number of
electrons individually assigned.
 Ex: C has 4, H has 1, so CH4 must have 8
total
Resonance Structures
 Have the same
alignment of
atoms, but
different bonding
(electrons ONLY
are moved, both
in bonds and
lone pairs)
Determining formal
charge
Formal charge can be determined by:
Normal number of electrons in outer shell
[(1/2 the number of bonded electrons)
+
lone electrons]
_____________________________________
= formal charge
Example: N in NH4
FC =5- [(1/2 of 8)+ 0]= +1
Formal charge and
stability
 The most “happy” molecules tend to have
no formal charges
 However, molecules may be “happy” if
they have not NET charge on them (if
there is 1+ and 1-, so a net of +1 + (1)=0)
 Resonance structures that are the best
have a minimal formal charge and a full
octet around each atom
What is VSEPR?
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Valence
Shell
Electron
Pair
Repulsion
Theory
Why?
 The shape of molecules influences their
characteristics:
 Things like polarity which influence things
like
 boiling point, melting point, which dictate their
nature (solid, liquid or gas at room temperature)
The parent geometries: all others come from these
Linear
 AX2
Trigonal planar
 AX3
Tetrahedral
 AX4
Pyramidal (Trigonal or
tetrahedral)
 Tetrahedral parent
shape
 1 lone pair of
electrons
 AX3E
Bent
 Tetrahedral
parent shape
 2 lone pair of
electrons
 AX2E2
When determining
polarity it is important to
look at the dipole
moments- do they cancel
out?