Lab 2: Molecular Structure and VSEPR
Objectives:
- To systematically determine the shapes of molecules using VSEPR theory.
- To learn determination of molecular polarity based on shape and bond polarity
- To apply lessons of polarity & molecular geometry to the structure of DNA
Notes:
This lab will be completed over 2 weeks as we work through Chapter 3 and Chapter 4 in our
textbook. This is in part an active learning exercise that is intended to build upon
fundamental concepts to develop an understanding of ionic and covalent bonds. By the end
of this activity you should be able to determine the ionic compound that will form from two
elements; draw lewis dot structures representing covalent molecules, and determine the
polarity of covalent molecules.
Pre-lab: (To be submitted at the beginning of lab)
 Week 1: Complete questions 1 & 2 in Part 1 of the lab worksheet. (ONLY these
questions, the rest will be completed in groups in class). Please complete this in the
Data & Observations Worksheet and not on a separate piece of paper.
 Week 2: Draw the lewis dot structures of the molecules that you will build in Part 2 of
this lab. Please complete this in the Data & Observations Worksheet and not on a
separate piece of paper.
Lab Procedures:
Part 1: Bonding Active Learning Exercise
1. Work through the Bonding worksheet in groups of 4.
Part 2: Small Molecule Modeling
1. Attempt to draw the lewis dot structure for the given molecule (pre-lab assignment).
2. Build a model of the given molecule using the provided modeling kits. Use a stick with
no ball on the end to represent a lone pair. Be sure to pick a ball with holes placed at the
appropriate angles to match the bond angles. Build the molecule so that all “parts” (the lone
pairs and atoms attached to the central atom) are as far away as possible from each other.
Draw a picture of this model in column 2. (If the molecule has no lone pairs, leave column 2
blank.)
3. Now, take out any lone pairs (so that your model only has atoms and bonds). This
shows the “molecular geometry” of the molecule. Draw a picture of this model in column 3.
(There should be no blanks in column 3.)
4. In column 4, write the name of the geometry of the molecule (linear, bent, trigonal planar,
pyramidal or tetrahedral). Refer to section 4.8 in your textbook.
5. In column 5, write the approximate angle between each bond/lone pair in the molecule.
6. In column 6, write whether the molecule is polar or nonpolar.
Edited by J. Pikul 2015
VESPR
1
Part 3: Modeling Acetaminophen
1. Using the given lewis dot structure for acetaminophen, build a 3D model using your
modeling kid. If necessary use 2 modeling kits, but make sure pieces are returned to the
correct container.
2. Determine the geometry at each central atom
3. Identify any polar bonds
4. Determine if the molecule is polar
Edited by J. Pikul 2015
VESPR
2
Lab 2 Data & Observations Worksheet
Part 1: Bonding
Adapted from Garoutte’s General, Organic, & Biological Chemistry: A Guilded Inquiry
1. What is the definition of an ionic bond?
ChemActivity 10
Covalent and Ionic Bonds
(Why do atoms bond together?)
2. State the octet rule:
Model 1: Two types of chemical bonding*
Ions held together by opposite charges are said to be ionically bonded.
Ionic compounds contain ions—typically a metal ion along with nonmetals.
A. The definition
of a covalent
bond is are
the said
sharing
electronsbonded.
between two atoms, and
Atoms sharing
valence electrons
to beofcovalently
covalent bondsCovalent
typicallymolecules
only contain
non-metals.
typically contain only nonmetals.
B. The
lewis electron-dot
electron dot structures
Model
2: Lewis
structures for
forhydrogen
hydrogenand
andthe
thesecond
secondrow
rowelement are given
elements
below.
H
Li
Be
B
C
N
O
F
Ne
Critical Thinking Questions:
3. What is the electron dot structure for sulfur, chlorine, and sodium?
1. Which two elements in Model 2 are metals?
2. Are these two elements likely to be in a covalent molecule? Explain.
4. Which of the elements given in B will form a covalent bond with sulfur?
3. Consider Model 2. How is the number of dots related to the number of valence
electrons?
5. Which of the elements given in B will form an ionic bond with sulfur?
4. By extension, write the electron-dot (Lewis) structures for sulfur, chlorine, and sodium.
6. The ions formed in compounds from group 1 almost always form M+ ions (they have
a +1 charge). Considering question 3 and the given electron dot structures in B
explain this result.
5. The ions formed in compounds from group 1 atoms (the alkali metals) are almost always
M+ ions (that is, they have a +1 charge). Considering Model 2, explain this result.
7. The ions formed in compounds from the halogen group almost always form X - ions
(they have a -1 charge). Considering question 3 and the given electron dot structures
in B explain this result.
6. The ions formed in compounds from group 2 atoms (the alkaline earth metals) are
almost always M2+ ions. Explain this result.
Edited by J. Pikul 2015
VESPR
3
-2– and F-.
8.8. Draw
electron
dot structures
(symbols)
forthe
each
, Cl–ions
, andOBr
.
Write the
electron
dot (Lewis)
structures
for each of
ionsofF–the
7. The ions formed in compounds from group 17 atoms (the halogens) are almost always
X– ions. Explain this result.
Modelis3:
of number
covalentofbonds
for bonds
elements
biologyin biology:
C. Below
a Typical
table of number
the typical
covalent
for common
elements in
common
Element
Number of Bonds
H
1
O
2
N
3
C
4
8. Write electron dot (Lewis) structures for each of the ions F–, Cl–, and Br–.
Thinking
Questions:
9.Critical
Referring
to C &
B answer the following questions:
9. How
in the dot
for O in Model
2 are paired?
a.many
Howelectrons
many electrons
in structure
the dot structure
for Oxygen
(O) are____
paired?
Model 3: Typical number of covalent bonds for elements common in biology
10. How many unpaired electrons are in the dot structure for O? ____
Element
Number of Bonds
H
1
11. How many electrons in the dot structure for N are paired? ____
12. How many unpaired electrons are in the dot structure for N? ____
O electrons2are in the dot structure for O?
b. How many unpaired
13. How many electrons in the
N dot structure
3 for C are paired? ____
C
14. How many unpaired electrons
are in the4 dot structure for C? ____
15. How
is the number of covalent bonds that an atom makes related to its electron-dot
Critical
Thinking
c. HowQuestions:
is the number of covalent bonds that an atom makes related to its
structure?
9. How many electrons
in the
dot structure for O in Model 2 are paired? ____
electron-dot
structure?
10. How many unpaired electrons are in the dot structure for O? ____
16. Which nonmetal in Model 2 is unlikely to be in a covalent molecule? Explain.
11. How many electrons in the dot structure for N are paired? ____
12. How many unpaired electrons are in the dot structure for N? ____
13. How many electrons in the dot structure for C are paired? ____
Model 4: Covalent bonding (sharing valence electrons) between H and F
14. How many unpaired electrons are in the dot structure for C? ____
10. Which nonmental in B is unlikely to be in a covalent molecule? Explain.
15. How
an atom makes
F is the+ number
H of covalent bonds that
or relatedF to its
F H
H electron-dot
structure?
lone pair
bonding
pair
usuallyExplain.
use
16. Which
nonmetal in Model 2 is unlikely
to be
in a covalent molecule?
(covalent bond)
this notation
spacefilling model
D. Example of covalent bonding (sharing valence electrons) between Hydrogen and
Model 5: Covalent bonding between H and O
Fluorine:
Model 4: Covalent bonding (sharing valence electrons) between H and F
O
F
+
H
lone pair
+ 2 H
F H
O H
HF H
or
- 41 usually use
this notation
bonding pair
(covalent bond)
or
O
H
H
CA10
spacefilling model
Model 5: Covalent bonding between H and O
11. How many valence electrons does Fluorine experience in the covalent molecule HF?
O H
O
+ 2 H
or
O H
H
H
- 41 -
Edited by J. Pikul 2015
CA10
VESPR
4
12. How many valence electrons does Hydrogen experience in the covalent molecule
HF?
13. Draw the lewis dot structure of F2.
14. How many lone pairs does each Fluorine atom have in the molecule F 2 (above)?
15. Draw the lewis dot structure of water, H2O, using the following steps:
a. Identify the atoms that are bonded in a molecule of water and draw their
electron dot symbol.
b. Arrange the electron dot symbols so the element that will form the most bonds
is the central atom, and the elements that form the least bonds surround it.
c. Connect the unpaired electrons between elements. How many bonds will the a
molecule of water have?
d. Draw the lewis dot structure of water, H2O, using lines to indicate covalent
bonds and including all lone pairs of electrons.
Edited by J. Pikul 2015
VESPR
5
Part 2: Small Molecule Modeling
Column 1
Lewis Dot
Structure
Column 2
Sketch (with
lone pairs)
Column 3
Sketch (no lone
pairs)
Column 4
Molecular
Geometry
Column
5
Bond
Angles
Column 6
Polar or
Nonpolar
180
Nonpolar
BeH2
H Be H
Linear
BH3
H2O
NH3
CH4
Edited by J. Pikul 2015
VESPR
6
CO2
NH4 +
(cation)
Part 3: Modeling Acetaminophen
Observations: (geometry, polar bonds, overall polarity)
Edited by J. Pikul 2015
VESPR
7
Lab 2: Post Lab Report
 Results:
o Tables & worksheet neatly filled out with data from lab
 Error Analysis: There is no error analysis for this lab.
 Post Lab Questions: Note that single sentence answers will not suffice, except
questions 5 a and b. State the answer to the question followed by a brief description
of the evidence supporting that answer.
1. Describe the underlying principle of VSEPR theory in your owns words.
2. Explain why two compounds with the same generic formula, NH3 and BH3,
have different molecular geometries.
3. Why are the structures of CH4 and NH4+ so similar?
Edited by J. Pikul 2015
VESPR
8
4. Write a detailed procedure in your own words for converting a chemical
formula into a Lewis structure and how to determine the geometry and polarity.
5. Answer the following questions about acetaminophen:
a. Does the molecule have an overall geometry?
b. What are the different types of geometry represented by the carbon
atoms in acetaminophen?
c. Is the molecule polar? Please support your answer with evidence from
your observations.
Edited by J. Pikul 2015
VESPR
9