NAME:
PERIOD:
COVALENT COMPOUNDS MODELS LAB
(67 points)
PURPOSE:
The purpose of this lab is to give the student experience building 3-D models of some simple covalent
molecules. Students will then predict the shapes of the molecules and use their prior knowledge of bond
polarity to predict the polarity of the molecule.
INTRODUCTION
Covalent Bonds
A group of atoms held together by covalent bonds is called a molecule. The properties of a molecule,
including its role in nature, depends primarily on its molecular structure, or shape. Molecular shape
contributes toward determining a compound’s boiling point, freezing point, viscosity, solubility, types of
reactions it can participate in, and a host of other physical and chemical properties.
The covalent bonds holding the atoms of a molecule together can be classified as three different types.
Covalent bonds are classified by comparing the differences in electronegativities of the two bonded atoms.
Electronegativity is a scale used to determine an atom’s attraction for an electron in the bonding process.
Bond Polarity
Differences in electronegativities are used to predict whether the bond is nonpolar covalent, polar covalent,
or ionic. The range of electronegativity differences and bond classification is:
0.0 – 0.4
0.4 – 2.1
2.1 – 4.0
nonpolar covalent
polar covalent
ionic
In a polar covalent bond, the electrons are more attracted to the atom with the greater electronegativity.
This results in a partial negative charge on that atom. The atom with the smaller electronegativity value
acquires a partial positive charge.
Molecular Polarity
Molecules composed of covalently bonded atoms may also exhibit polar or nonpolar properties. For the
molecule to be polar, it must, of course have polar bonds. But the key factor in determining the polarity of
a molecule is its shape, or geometry. If the polar bonds are arranged symmetrically around the central atom
they will offset each other and the resulting molecule is nonpolar. However, if the arrangement of the polar
bonds is not symmetrical around the central atom, the electrons will be pulled to one end of the molecule
and the resulting molecule is polar.
Lewis Structures
The structures used to show bonding in covalent molecules are called Lewis structures. When bonding,
atoms tend to achieve a noble gas configuration. By sharing electrons, individual atoms can complete the
outer energy level.
OVER
Predicting Shapes
The model used to determine the molecular shape is referred to as the Valence Shell Electron pair
Repulsion (VSEPR) model. The model is based on an arrangement that minimizes the repulsion of shared
and unshared pairs of electrons around the central atom. By examining the central atom and identifying the
number of atoms bonded to it and the number of unshared electron pairs surrounding it, one can determine
the geometry, shape, of a molecule.
Molecular Shapes
TOTAL
PAIRS
SHARED
PAIRS
UNSHARED
PAIRS
SHAPE/
GEOMETRY
EXAMPLE
2
2
0
linear
N2 or CO2
3
3
0
trigonal planar
BF3
4
4
0
tetrahedral
CH4
4
3
1
trigonal pyramidal
NH3
4
2
2
bent
H2O
5
5
0
trigonal bipyramidal
NbBr5
6
6
0
octahedral
SF6
Models
To study covalent molecules, chemists find the use of models helpful. Colored wooden, or plastic, balls are
used to represent atoms. These balls have holes drilled into them according to the number of covalent
bonds they will form. The holes are bored at angles that approximate the accepted bond angles.
EQUIPMENT
Molecular model building set.
COLOR KEY (amount)
Black (6)
White (10)
Red (4)
Green (4)
Blue (2)
Yellow (2)
Light brown (2)
Gray (2)
Medium gray links (20)
Long gray links (6)
NUMBER OF HOLES
4
1
2
1
4
4
5
6
ELEMENT
Carbon
Hydrogen
Oxygen
Halogen
Nitrogen
Sulfur
Trigonal bipyramidal
Octahedral
SAFETY PRECAUTION
Do not place model pieces in your mouth – they present a choking hazard.
PROCEDURE
1. Obtain a molecular model set.
2. For each of the molecules in the data table:
a.
Build a model of the molecule.
b.
Draw the Lewis structure for each molecule.
c.
Determine the number of bonding pairs and lone pairs around each central atom.
d.
Draw a 3-D sketch of the molecule.
e.
Determine the shape of the molecule at each central atom.
f.
Determine whether the bonds between the central atom and the other atoms are nonpolar, polar, or
ionic.
g.
Determine whether the molecule is polar or nonpolar.
OVER
DATA TABLE (44 points)
Formula
Cl2
HBr
NH3
Electron
Dot Diagram
Bond
Pairs
Lone
Pairs
N/A
N/A
N/A
N/A
3
1
N/A
N/A
SO3
N2
SbI5
CCl4
3D Drawing
Name of
Molecular Shape
Bonds
Ionic, Polar
or Nonpolar
Molecule
Polar or
Nonpolar
Formula
Electron
Dot Diagram
Bond
Pairs
Lone
Pairs
3D Drawing
CH2Cl2
CH4
TeBr6
C2H4
CH3OH
CO2
H2O
OVER
Name of
Molecular Shape
Bonds
Polar or
Nonpolar
Molecule
Polar or
Nonpolar
POST-LAB DISCUSSION: (23 points)
Read the entire lab and the relevant parts of your textbook (Chapter 9; pp. 259 – 267). Then answer the
questions that follow.
(1 point) 1. Define a covalent bond.
(1 point) 2. Define a dipole.
(2 points) 3. List the two factors that determine whether a molecule is polar.
(5 points) 4. List the seven different molecular geometries (shapes) that were studied in this lab.
(4 points) 5. Calculate the electronegativity difference and predict the type of bond for the following
examples:
a. Na―Cl
b. C―H
c. S―O
d. N—N
(5 points) 6. List five molecules in this lab which are nonpolar because all bonds are nonpolar.
(3 points) 7. List three molecules in this lab that have polar covalent bonds but are nonpolar molecules
because of symmetry.
(2 points) 8. Which two shapes appear to produce polar molecules?