Download Chem 40, Spring 2014 Section 1 Handout 1.) Lewis Dot Structures

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:
First-row elements (Li-Ne) CANNOT have expanded octets. (Why?)
Atoms from the left half of the periodic table often have incomplete octets, and that’s OK!
For the purposes of this class, you ONLY need to draw no-bond resonance structures for fluorides
that have central atoms with exceeded octets.
A complete Lewis dot structure includes ALL contributing resonance structures.
Formal charge = (# valence e- in a free atom) – (bonds) – (non-bonding e-)
If you need additional help with drawing Lewis dot structures, consult Appendix D of your textbook.
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).
There are several conventions that we will use when naming symmetry elements:
1.) The principal axis of rotation (Cn) is always taken as the Cartesian z-axis. The xz plane
contains the most atoms possible.
2.) When performing rotation operations, the directionality of the rotation does matter for a
few applications (such as rotational spectroscopy). We will use the counterclockwise rotation
convention used by the textbook for problem sets and exams.
3.) Indistinguishable symmetry elements (for example, the C2 axes of borane or the
indistinguishable v planes of PtCl42-) share the exact same label and are represented by a
coefficient in front of the symmetry element (3C2 or 2v). If there are different, distinguishable
symmetry elements of the same type (for example, there are three types of C2 axes in PtCl42- -one is the z-axis, one set goes through the chlorines, one set bisects the chlorines), we
differentiate them with primes (C2, C2’, C2’’). The single-prime symmetry elements contain
more atoms than the double-prime elements.
4.) The superscripts h, v, and d differentiate -planes. Horizontal planes (h) are by definition
the xy-plane; there cannot be multiple h-planes. Vertical planes (v) contain the z-axis.
Dihedral planes (d) bisect C2 axes, and often (but not necessarily) contain the z-axis.
Flow Chart for assigning point groups
4.) Practice Problems
Draw complete Lewis dot structures for the following molecules and polyatomic ions, including all
resonance contributors, lone pairs, and formal charges. Indicate the expected geometry of the molecule
or ion.
NO3-
RnCl22-
SbCl5
NCO-
GeF2
PCl3
ICl3
SbF52-
Identify all the symmetry elements present in the following molecules, and assign the point group of
each.
Cl
3+
OC
NH2
H2N
H2
H2N Co N
N
NH2 H2
CO
Fe
OC
CO
Cl
F
F
Ph
Me
P
Me
F
Sb
P
Ni
Me
Me
Cl
Fe
Cl
F
Ph
F