Instructions to CHEM 114 students regarding
Experiment #6: The Structures and Shapes of Molecules
ADEQUATE AND ADVANCE PREPARATION FOR THIS EXPERIMENT are essential to performing
well. It is each student’s responsibility to follow these instructions and prepare for this lab properly.
Please read through the information on this sheet and the relevant material in your text and notes before
proceeding.
1. READ Experiment 6 in the lab manual thoroughly and carefully before the scheduled lab period, and
READ through the instructions on this sheet. All the information you need is there except for dates. The
website for the VSEPR lab is http://gemini.usask.ca/chemtel/ and students will log in with their NSID.
2. Practice session will be available beginning November 7th until the final exam date.

Attendance during the lab the week that Experiment 6 is scheduled (November 13th and 16th ) is
optional. On the day that your lab is scheduled to do Experiment 6, (for example if you are in a
Friday lab, then you may come to your regular lab on Friday, November 13th), you will have an
opportunity to ask any questions to your TAs about how to use the online lab (related to either
how the software works, or how to draw Lewis Structures). This will be the ONLY chance to ask
questions before you are scheduled to do the graded session.

DO NOT START the Graded Session until you’ve had a chance to try the practice sessions.
th
3. Graded session will be available starting November 13 . Only start the graded session
when you are ready to devote 3 hours to complete it, and after you have done the practice sessions and
feel comfortable using the system.
Note: The VSEPR lab is different from Mastering Chemistry. You cannot start the graded session, answer
one level, leave your computer and come back to finish it later. You do not get multiple attempts at an
answer, and there are no part marks.
If you follow instructions in your lab manual and on this sheet, take your time filling out each level, check
your answers carefully, you can achieve 10/10 on this lab! 
4. You must complete Experiment #6 before performing Experiment #7 (November 20th and 23rd ).
You must complete Expt 6 before 12:00 pm the day you are scheduled to do Expt. 7. No extensions, or
excused absences will be given. If you attempt the graded session after you are scheduled to perform
Experiment 7 you will be given a grade of 0/10.
5. Any problems logging in to the online lab should be directed to the lab manager at
a.bartolescott@usask.ca
All students are expected to complete this lab on their own and are expected to comply with the University Of
Saskatchewan’s policies on Academic Honesty. Note that students who do not complete this lab on their own
are expected to perform extremely poorly on related questions on the final exam.
How To Use The VSEPR Model To Determine Molecular Geometry Of A Compound
This document contains the most up-to-date corrections from Section 6.2a-b of the Chem 114 Lab Manual.
Last updated: Nov.2/09
7.2 The Method
a) Determine the Correct Number of Electrons
From its group number in the Periodic Table, identify the number of valence electrons for each atom in the molecule, and
sum them together. If the molecule has a charge (e.g. H3O+) then adjust the total. Remember, a 1+ charge means that one
electron is missing, a 1- charge, that there is one additional electron, etc.
Rules Used for Drawing Lewis Structures in the Online VSEPR Lab

There are several possible formalisms for drawing some Lewis structures. For the purposes of the lab and for
consistency, the rule being used in the VSEPR online lab is to obey the octet rule at all times unless it
absolutely MUST be broken (in a molecule such as PF5, e.g.). This means that if you have the choice between
expanding the octet and reducing formal charge or sticking to an octet with large formal charges, you should stick
with the octet. See the SO42- structure below, for example. The structure on the left would also be considered to
have no resonance structures.
O
O
S
O
+2
O
O
O
O
O
Will be marked correct

S
Will be marked incorrect
Your text gives examples where incomplete octets are plausible (see p389 of your text). These are considered
exceptions to the above octet rule. In the case of neutral molecules where a Group 13 atom is a central atom
you should allow an incomplete octet. For example, the VSEPR Online Lab will treat BF3 as having 3 single
bonds between B and each F atom. The formal charge on B is 0, and there are no contributing resonance
structures. Note: This has been modified from the instructions printed in the 2009-2010 lab manual.
F
F
B
F
Incomplete octet

In some cases you may need to expand the octet of the central atom(s). IF and ONLY IF you have to expand
the octet, then you should continue to expand the octet to reduce formal charges as much as possible. For
example, drawing the appropriate Lewis structure for XeO2F2 leads to lone pair electrons on the Xe atom (octet is
expanded – see diagram on the left). Since the octet around Xe has already been expanded, formal charges on
atoms in this molecule can then be reduced by further expansion of the octet with the addition of multiple bonds.
Note: The addition of multiple bonds does not alter the electron group geometry or molecular geometry of this
molecule but does change the formal charges on several atoms.
O
F
Xe
O
F
O
Expanded octet with
non-zero formal charges
F
Xe
F
O
Will be marked correct
Expanded octet
and reduced formal charges
b) Derive the Best Lewis Dot Structure
Determine which atom(s) will be the central atom(s), and write symbols for terminal atoms around the central atom(s).
Connect up the atoms with “single bonds”, i.e., electron pairs. Complete the octets around each terminal atom using the
remaining electrons, in pairs. Complete the octets around the most electronegative atoms first. If you still have electrons
left, put them on a Period 3 or greater atom (usually, the central atom). Now ask yourself:

Do all atoms have an octet?
o If yes, check formal charges. Reduce formal charges where possible without compromising the octet on
each atom.
o
If no,

Is there an incomplete octet? “Move” some lone pair(s) to form additional bonds, so as to satisfy
the octet rule for as many atoms as possible. Now check formal charges.
 If the molecule contains a Group 13 element, keep in mind that you may need to allow an
incomplete octet.

Is there an expanded octet? If so, then you should continue to expand the octet to reduce formal
charges if necessary.
Determination of Formal Charge: There Are Two Methods Which Are Equivalent
(i) Assign one electron in each bond to each participating atom and assign both electrons in any lone pair to the atom
where they are found. Count this number of assigned electrons to the atom and compare the number with the number of
valence electrons that would be found on that atom in its neutral form. If there is one extra electron, the formal charge on
that atom is -1, etc.
Or
(ii) Use the formula:
Formal charge on
an atom
=
Total # of valence
electrons in the free atom
_
Total number of nonbonding electrons
_
½ (total number of
bonding electrons)
 You may now continue to assign electron group geometry and molecular geometry of a molecule as
described in your lab manual and text.