Lewis structures
Five factors that affect the stability of a Lewis structure (not necessarily listed in order of relative
importance) are:
1) Obeying the octet rule.
2) Minimum overall charge. Molecules (uncharged species) tend to be more stable than ions.
The creation of charge separation requires energy and therefore lower charged species tend
to be more stable than higher charged species.
3) Minimal formal charges. Resonance structures with the lowest possible formal charges
tend to be the most stable and therefore dominant resonance structures. If all formal charges
being zero is possible that is often the structure that bears the greatest resemblance to the
molecule or ion itself. Otherwise, less extreme formal charge distributions are favored over
more extreme ones. Among structures with equally extreme charge distributions structures
that place more negative charge on more electronegative atoms, or stated another way, more
positive charge on less electronegative atoms, tends to be the dominant resonance structure.
4) Distribution of charge. Structures that have charges are more stable if the formal charges
can be spread out onto different atoms by having resonance structures that place the formal
charges on different atoms.
5) Minimizing Ring Strain. Skeletal structures with rings as small as three atoms are known
to exist, though rings smaller than five atoms are less stable due to the orbitals involved in
the bond formation not pointing directly at each other. This is referred to as Ring Strain.
Consider these five factors when answering the questions below.
Part I. Structures that obey the octet rule
1) Draw a valid Lewis structure that obeys the octet rule for CH4, NH3, H2O, H2S, and HF.
a. Use VSEPR theory to determine the arrangement of electron pairs (aka the electron pair
geometry) in each of the above molecules.
b. Use VSEPR theory to determine the molecular geometry of each of the above molecules.
c. Based upon their shape and the presence or absence of polar bonds classify each of the
molecules above as having no dipole moment, a small dipole moment, or a significant dipole
moment.
d. Based upon their shape and the presence or absence of polar bonds classify each of the
molecules above as being polar or non-polar.
e. What is the hybridization of each of the central atoms in CH4, NH3, H2O, H2S, and HF?
2) Draw valid Lewis structures that obey the octet rule for CH2Cl2, CHCl3, and CCl4.
a. Use VSEPR theory to determine the molecular geometry in each of the above molecules.
b. Based upon their shapes and the types of bonds present classify each of the above molecules
as polar or non-polar.
3) Draw valid Lewis structures that obey the octet rule for CH3CH3, CH2CH2, and CHCH.
a. Classify each of the above molecules as polar or non-polar.
b. What is the hybridization of each of the C atoms in CH3CH3, CH2CH2, and CHCH?
4) Draw two resonance structures that obey the octet rule for the acetate ion CH 3CO2-. The
skeletal structure has one C with three H atoms attached, and the other C has two O atoms
attached
a. Assign formal charges to all the atoms in the structures above.
b. Do the formal charges suggest that any structure is dominant over the others? If yes circle
the structure and label it as dominant.
c. Assign hybridizations to all of the atoms in both resonance structures of the acetate ion.
5) The thioacetate ion (CH3COS-) is isoelectronic with the acetate ion from the previous
problem. Draw two resonance structures that obey the octet rule for the thioacetate ion. The
skeletal structure has one C with three H atoms attached, and the other C has one O atom
and one S atom attached.
a. Assign formal charges to all the atoms in the structures above.
b. Do the formal charges suggest that any structure is dominant over the others? If yes circle
the structure and label it as dominant.
6) For each of the three molecules SiO2, SiTe2, and SiSTe, draw three resonance structures that
obey the octet rule. All three skeletal structures have silicon (Si) as the central atom and do
not have a ring. All three are isoelectronic.
a. Based upon the electronegativity of the atoms in these three structures, which structures
contain polar bonds?
b. Based upon the presence or absence of polar bonds describe each of the molecules above as
having no dipole moment, a small dipole moment, or a significant dipole moment.
c. Classify each of the molecules above as polar or non-polar.
7) Draw three resonance structures that obey the octet rule for:
a. SeCN- (skeletal structure Se-C-N) and OSiN- (skeletal structure O-Si-N).
b. Assign formal charges to all the atoms in the structures above.
c. Do the formal charges suggest that any resonance structure(s) is dominant over the others? If
yes circle the structure.
8) Draw a Lewis structure and any possible resonance structures that obey the octet rule for
cyanogen (CN)2. It has the skeletal structure N-C-C-N.
a. Assign the hybridization of each atom in cyanogen.
9) Draw a Lewis structure that obeys the octet rule for H2SO4. S is the central atom with four
S-O bonds and the H’s are each attached to different oxygen atoms.
a. Assign formal charges to all atoms in the structure above.
Part II. Structures containing atoms with less than an octet
10) Draw four resonance structures for BF3 all of them having a skeletal structure with B as the
central atom. In three of the structures B obeys the octet rule, and in one of the structures it
has a sextet (6 electrons) instead of an octet.
a. Based on the formal charges which of these structures would you predict to be the most
stable?
b. Assign the hybridization to all the atoms in the structure you selected in part a.
11) Nitrogen oxide (NO) is a reasonably stable species despite the fact that it is a radical (it has
an unpaired electron). Draw two resonance structures for NO that come as close to obeying
the octet rule as is possible.
a. Based on the formal charges, which is the preferred resonance structure?
b. Assign the hybridization to all the atoms in the structure you selected in part a.
Part III. Structures containing atoms that exceed an octet
12) Draw a Lewis structure with an expanded octet for H2SO4. S is the central atom, and it is
bonded to four O’s. The H’s are each attached to different oxygen atoms. Hint: All the
oxygen atoms have their normal valence in this structure, and the S exceeds an octet.
a. Assign formal charges to all atoms in the structure above.
13) Draw a valid Lewis structure for PBr5 with P as the central atom.
a. Use VSEPR theory to determine the molecular geometry of the PBr5 molecule.
b. Does this molecule contain polar bonds?
c. Does this molecule have a significant dipole moment?
d. Is PBr5 a polar or non-polar substance?
e. Assign formal charges to all atoms in PBr5.
14) Draw a valid Lewis structure for I3- .The accepted skeletal structure does not have a ring.
a.
Use VSEPR theory to determine the arrangement of electron pairs (aka the electron pair
geometry) of I3-.
b. Use VSEPR theory to determine the molecular geometry of I3-.
c. Is I3- a polar or non-polar species?
15) Draw a valid Lewis structure for IF5 with I as the central atom.
a. Use VSEPR theory to determine the arrangement of electron pairs (aka the electron pair
geometry) of the IF5 molecule.
b. Use VSEPR theory to determine the molecular geometry of the IF5 molecule.
c. Does this molecule contain polar bonds?
d. Does this molecule have a significant dipole moment?
e. Is IF5 a polar or non-polar substance?
16) Draw a valid Lewis structure for SeF6 with Se as the central atom.
a. Use VSEPR theory to determine the molecular geometry of the SeF6 molecule.
b. Does this molecule contain polar bonds?
c. Does this molecule have a significant dipole moment?
d. Is SeF6 a polar or non-polar substance?
e. SeF5Br has Se as the central atom and is isoelectronic with SeF6. Does this molecule contain
polar bonds?
f. Does SeF5Br have a significant dipole moment?
g. Is SeF5Br a polar or non-polar substance?
17) Draw a valid Lewis structure for XeF4 with Xe as the central atom.
a. Use VSEPR theory to determine the molecular geometry of the XeF4 molecule.
b. Does this molecule contain polar bonds?
c. Does this molecule have a significant dipole moment?
d. Is XeF4 a polar or non-polar substance?
18) Draw valid Lewis structures for H3PO4 (phosphoric acid) and the hypothetical acid H3NO4
(unnamed because it does not exist!). The skeletal structures have the P and the N as the
central atoms, respectively. And H atoms are each attached to an O atom. Use the stability
guidelines to rationalize why H3PO4 exists, but H3NO4 does not.
19) Molecular oxygen is paramagnetic substance. Experimental evidence suggests that it is
actually a di-radical (it has two unpaired electrons)! Draw two Lewis structures for O2: one
which obeys the octet rule and is not a di-radical and the other which violates the octet rule
and is a di-radical. Considering the stability guidelines, which structure would you predict to
be more stable?
Part IV. Skeletal structures
21) Draw Lewis structures, including any resonance structures for each of the three possible
skeletal structures of carbon dioxide, CO2: three atoms in sequence with C as the central
atom, three atoms in sequence with O as the central atom, and a three-membered ring. Which
skeletal structure would you predict to be the most stable? Why?
22) Draw Lewis structures, including any resonance structures for each of the three possible
skeletal structures of N2O: three atoms in sequence with N as the central atom, three atoms
in sequence with O as the central atom, and a three-membered ring. Which skeletal structure
would you predict to be the most stable? Why?
23) Draw Lewis structures, including any resonance structures for each of the three possible
skeletal structures of Ozone, O3: three O atoms in sequence with, and a three-membered
ring. Which skeletal structure would you predict to be the most stable? Why?