Name: _____________________________ Group Members _______________________________________
Period: _____
Date: ______________
_______________________________________
_______________________________________
Molecular Geometries of Covalent Molecules:
Lewis Structure and the VSEPR Model
OBJECTIVE
The student will be able to become familiar with Lewis Structures, the principles of the VSEPR model
(theory), and the three-dimensional structures of covalent molecules.
MATERIALS
molecular model kit provided by the instructor
VSEPR chart (page 260 Class Textbook)
large & small marshmallows
regular and multi-colored toothpicks
BACKGROUND INFORMATION
Whenever atoms or ions are attracted to one another, we say that there is a chemical bond between them.
There are three types of bonds: ionic, covalent, and metallic. The term ionic bond refers to electrostatic forces
that exist between ions of opposite charge. Ions may be formed by the transfer of one or more electrons from an
atom with a low ionization energy to an atom with a high electron affinity. Thus, ionic substances generally
result from the interaction of metals on the far right side of the periodic table with nonmetals on the far right
side of the periodic table (excluding the noble gases, Group 8A). The term covalent bond results from the
sharing of electrons between two atoms. The more familiar examples of covalent bonding are found among
nonmetallic elements interacting with one another. This experiment illustrates the geometric (three-dimensional)
shapes of molecules and ions resulting from covalent bonding among various numbers of elements, and two of
the consequences of geometric structure – resonance structures and polarity. Metallic bonds are found in metals
such as gold, iron, and magnesium. In the metals, each atom is bonded to several neighboring atoms. The
bonding electrons are relatively free to move throughout the three-dimensional structure of the metal; known as
the “sea” of mobile electron model. Metallic Bonds give rise to such typical metallic properties as high
electrical and thermal conductivity and luster.1
PRE-LAB ASSIGNMENT
Use the BACKGROUND INFORMATION above, the Content Video related to this experiment, and a
textbook to answer the following questions before entering the lab:
1. Define the term “electrostatic forces”.
2. Distinguish among ionic, covalent, and metallic bonds. How do the electrons behave in the
bond? Be clear and thorough.
3. Define the term formal charge. Draw a lewis structure for the phosphate ion and prove its
polyatomic charge by indicating the formal charge of all atoms in the molecule.
Consider the following substances for the next three questions: H2, N2, HCl, HCN, CO2, NO3─
4. Which of these molecules possess polar covalent bonds.
5. Which of these molecules have molecular dipole moments (i.e. which is polar)?
6. List all possible molecular shapes for these molecules.
1
From Laboratory Experiments: Chemistry The Central Science. 10th Ed..Nelson and Kemp. To accompany Brown, Lemay, Bursten.
PROCEDURE
1. Using an appropriate set of models, make molecular models of the compounds listed below and complete Data
Table 1. You are required to get your instructor’s signature for each Lewis Structure.
2. The following key may be used to identify the types of atoms (by color) and the types of bonds (by length):
Atoms
Black – Carbon
Yellow – Hydrogen
Orange – Bromine
Red – Oxygen
Green – Chlorine
Purple – Iodine
Blue – Nitrogen
Bonds
Short wooden stick – triple bond
Silver Spring – used to make a double bond
Log wooden stick – single bond
The molecular model kits will only allow you
to build models for linear to tetrahedral
molecules. For larger molecules (trigonal
bipyramidal, seesaw, octahedral, etc.) use the
marshmallows and regular and multi-colored
toothpicks. The large marshmallows will
represent the atoms. The small
marshmallows will represent external atoms.
The regular toothpicks are bonds while the
multicolored toothpicks represent lone pair
electrons.
3. Because lone pairs are larger than bonding pairs, lone pair-lone pair interactions are greater than lone pairbonding pair interactions, which are in turn larger than bonding pair-bonding pair interactions. Using this notion,
suggest how the following species would distort from regular geometries (i.e. what are the actual bond angles for
these structures and how do they differ from the predicted bond angles)?
(a) OF2
(b) SCl2
(c) PF3
4. Calculate the formal charges of all atoms in NH3, NO21-, NO31-, SO3.
5. Which of the substances in question 4 exhibit resonance?
6. ESSAY. On a separate sheet of paper, consider the following prompt: Describe the difference between molecular
and bond polarities for compounds. A college-ready essay will precisely define both terms, indicate how they are
different, indicate how they are similar, AND have an established and appropriate writer’s voice.
Molecular
Formula
BeCl2
(example)
CH4
NH3
H2O
CO2
CH2O
Lewis Structure
Data Table 1: Molecular Geometries of Covalent Molecules:
Lewis Structure and the VSEPR Model
# of
# of
bond lone
VSEPR Model
Molecular Geometry
pairs pairs
(CA) (CA)
2
0
linear
Dipole
(Y or N)
Bond
Angles
Hybrid.
N
180°
sp
Teacher
Sign.
Molecular
Formula
PCl5
SF4
BrF3
XeF2
IF5
XeF4
Lewis Structure
# of
bond
pairs
(CA)
# of
lone
pairs
(CA)
VSEPR Model
Molecular Geometry
Dipole
(Y or N)
Bond
Angles
Hybrid.
Teacher
Sign.