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
Isomers
 Same formula, different arrangement of
atoms
 Physically break bonds and MOVE
atoms
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
Steric Number
 The number of “things” sprouting off of an
atom
 These can be either
 Bonds
 Of any order (1, 2, or 3)
Or
 Lone pairs of electrons
Steric Number Examples
 Ex #1: CH4
 There are 4 H’s branching
off , so the steric number
is 4
 SN=4
 Ex #2: H2O
 SN= 4
 Explain why
 Ex #3: CO2
 SN= 2
 Explain why
General Formulas
 All molecules with a shared general
formula have a shared geometry
 we use them to help note shape
 Formulas are typically written with A’s,
X’s, and E’s
The letters stand for:
 A= the central atom
 X *= the number of atoms attached to the central
atom
 E= the number of lone pairs of electrons attached
to the central atom
 *Some sources use A’s, B’s, and E’s
General Formula Examples
 Ex #1: CH4
 AX4
 Ex #2: H2O
 AX2E2
 Ex #3: CO2
 AX2
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?
Trigonal bipyramidal
 AX5
Seesaw
a.k.a.
Teeter-totter
 Trigonal bipyramidal
parent shape
 1 lone pair of
electrons
 AX4E
T-shaped
 Trigonal bipyramidal
parent shape
 2 lone pair of
electrons
 AX3E2
Linear
 Trigonal bipyramidal
parent shape
 3 lone pair of
electrons
 AX2E3
Octahedral
 AX6
Square pyramidal
 Octahedral parent
shape
 1 lone pair of
electrons
 AX5E
Square planar
 Octahedral parent
shape
 2 lone pair of
electrons
 AX4E2
T-shaped
 Octahedral parent
shape
 1 lone pair of
electrons
 AX3E3
Summary of shapes
ID these VSEPR shapes…
Sweet drill and practice
web site
 Given generic shapes to ID:
 http://www.chemistry-drills.com/VSEPR1.php?q=1
 Given molecules to draw out:
 Basic: http://www.chemistrydrills.com/VSEPR-1.php?q=2
 Advanced: http://www.chemistrydrills.com/VSEPR-1.php?q=3