Chem 40, Spring 2014
Section 1 Handout
1.) Lewis Dot Structures
Though we will be discussing the shortcomings of Lewis dot structures and VSEPR theory in detail, they
are nevertheless very useful ideas; with practice, drawing molecules will become second nature.
Drawing a Lewis structure:
1. Count the total number of valence electrons (don’t forget charges!)
2. Place the largest and/or most electropositive element in the center
3. Surround the central atom with the more electronegative atoms
4. Draw single bonds between the central atom and each peripheral atom (2 e- each)
5. Place electrons (lone pairs) around the outside atoms to fill their octets
6. Any remaining electrons are placed on the central atom
7. Use the lone pairs on the peripheral atoms to make double bonds if the central atom
has an incomplete octet or a formal positive charge
8. Draw resonance structures
Things to keep in mind when drawing Lewis structures:
Second-row elements (Li-Ne) CANNOT have expanded octets. (Why?)
Atoms from columns 1, 2, and 13 often have incomplete octets, and that’s OK!
A complete Lewis dot structure includes ALL contributing resonance structures.
Formal charge = (# valence e- in a free atom) – (bonds) – (non-bonding e-)
2.) VSEPR: Predicting molecular geometry
VSEPR gives us a good “first guess” of molecular geometry. The “VSEPR geometries” depend only on the
number of atoms (X) and lone pairs (or radicals) (E) surrounding an atom, and are summarized in the
table below. “Steric number” refers to the sum of the number of atoms and the lone pairs (or radicals).
Steric number
2
0 lone pairs
1 lone pair
2 lone pairs
3 lone pairs
linear
3
trigonal planar
bent
tetrahedral
trigonal
pyramidal
trigonal
bipyramidal
see-saw
octahedral
square pyramidal
pentagonal
bipyramidal
pentagonal
pyramidal
4
bent
5
T-shaped
6
7
8
square
antiprismatic
Lone pairs always occupy the “largest” site.
square planar
linear
3.) Symmetry and Point Groups
A symmetry element (E, Cn, , i, Sn) is different from a symmetry operation (E, Cnm, , i, Snm).
Identify all the symmetry elements present in the following molecules, and assign the point group of
each.
Cl
3+
HN
HN
OC
NH H
N
Co
N
NH H
CO
Fe
OC
CO
Cl
D4h
E, C4, C2, 2C2, 2C2, i, S4, h, 2v, 2d
D3
E, C3, 3C2
F
F
Ph
Me
P
Me
Sb
P
Me
C2
E, C2
Cl
Fe
Cl
F
Ph
Ni
Me
C2h
E, C2, i, h
F
F
D3h
E, C3, 3C2, i, S3, h(xy), 3v