LAB- MOLECULAR MODELS
Purpose: To construct models of covalent substances in order to show their
shapes and determine their polarity. To use bond type and polarity to predict the
properties of a substance.
Materials: Student molecular model sets, Chemistry Reference Table
Introduction:
A covalent bond consists of a pair of shared electrons. Covalent bonds
are classified as either polar covalent bonds or nonpolar covalent bonds.
A pair of electrons shared equally between identical atoms produces a nonpolar
covalent bond. The unequal sharing of the electron pair between two different
nonmetal atoms produces a polar covalent bond. If the electrons are not
shared, but transferred, from a metal atom to a nonmetal atom, an ionic bond is
formed. Ionic bonds are much stronger than covalent bonds. The polarity
(how unequally the electrons are shared) of a bond depends upon the
electronegativity difference between the atoms involved in the chemical bond.
Bonds between atoms with larger electronegativity differences are more polar
than bonds between the atoms with smaller electronegativity differences. Recall
that electronegativity is a numerical value from 0 to 4.0 that indicates the
amount of an atom’s attraction for shared electrons in a chemical bond.
Electronegativity values are listed in Reference Table S.
Molecules may also be classified as polar or nonpolar. The bond type
doesn’t determine whether a molecule is polar or nonpolar. The molecule’s
shape determines its polarity. Molecules with symmetry are nonpolar
molecules because the shape of the molecule allows the electrons to be evenly
distributed among the atoms in the molecule. Asymmetrical molecules are
polar molecule because one end of the molecule is slightly negative while the
other end is slightly positive due to uneven or unequal sharing of the electrons
among the atoms in the molecule.
Bonding has a huge impact on the chemical and physical properties of
a substance. In general, ionic substances tend to have very strong bonds, high
melting points and boiling points, and are solids at room temperature. Ionic
solids will conduct electricity when melted or dissolved in water. Covalent
substances tend to have weaker bonds, lower melting points & boiling points,
and may exist as solids, liquids, or gas at room temperature. Covalent
substances will not conduct electricity; in fact, they are sometimes used as
insulators. Polarity plays an important role in the properties of a substance also.
Generally, as polarity increases (electronegativity difference increases), bond
strength increases, melting points and boiling points increase, and the substance
is more likely to be a solid at room temperature.
Procedure:
In this experiment molecular models will be built to represent a number of
compounds. The colored wooden balls represent atoms and have holes bored in
them according to the number of bonds which they can form at the generally
accepted bond angles. Use the color code on the next page to key the models
with the elements.
Covalent bonds may be single, double, or triple. A single bond consists of
one pair of shared electrons while a double bond consists of two pairs of shared
electrons and a triple bond consists of three pairs of shared electrons. Use your
Periodic Table to look up each atom’s number of valence electrons. All atoms
want to obtain a stable octet (8 valence electrons) except hydrogen and helium,
which only need 2 valence electrons. If an atom has 7 valence electrons, it
needs one more electron and generally will form a single bond. If an atom has 6
valence electrons, it needs 2 more electrons and generally will form a double
bond. If an atom has 5 valence electrons, it needs 3 more electrons and
generally will form a triple bond.
A wooden stick is used to represent a single bond, two springs are used
for a double bond and three springs are used for a triple bond. The models
should be assembled by the insertion of the springs/sticks with a partial
clockwise turn. In order to avoid damaging the springs/sticks, the models should
be dismantled with the same clockwise turn.
Assemble 2 or 3 models. Fill in the data table. Have your work checked
by the teacher. Carefully take the models apart and proceed similarly with a
second set. At the end of the period, NEATLY reassemble the kit.
MOLECULAR MODEL COLOR KEY
BLACK-----CARBON
RED-----OXYGEN
PURPLE-----IODINE
BLUE-----NITROGREN
GREEN-----CHLORINE
YELLOW-----HYDROGEN
ORANGE-----BROMINE
ELECTRONEGATIVITY AND BOND TYPE
Electronegativity Difference
Zero
Greater than zero, but less the 1.7
Greater than 1.7
Bond Type
Nonpolar Covalent
Polar Covalent
Ionic
Results
Directions: Draw the electron dot diagram for each model in the box provided
and answer the related questions. Show both the model and your answers to
your teacher for verification.
1. Hydrogen Gas (H2)
a) Electronegativity Difference _______________
b) Bond Type_____________________________
c) Symmetrical or Asymmetrical? _____________
d) Kind of Molecule ________________________
e) Teacher’s Initials _______________________
2. Chlorine Gas (Cl2)
a) Electronegativity Difference _______________
b) Bond Type_____________________________
c) Symmetrical or Asymmetrical? _____________
d) Kind of Molecule ________________________
e) Teacher’s Initials _______________________
3. Oxygen Gas (O2)
a) Electronegativity Difference _______________
b) Bond Type_____________________________
c) Symmetrical or Asymmetrical? _____________
d) Kind of Molecule ________________________
e) Teacher’s Initials _______________________
4. Nitrogen Gas (N2)
a) Electronegativity Difference _______________
b) Bond Type_____________________________
c) Symmetrical or Asymmetrical? _____________
d) Kind of Molecule ________________________
e) Teacher’s Initials _______________________
5. Hydrochloric Acid (HCl)
a)
Electronegativity Difference _______________
b) Bond Type_____________________________
c) Symmetrical or Asymmetrical? _____________
d) Kind of Molecule ________________________
e) Teacher’s Initials _______________________
6. Water (H2O)
a) Electronegativity Difference _______________
b) Bond Type_____________________________
c) Symmetrical or Asymmetrical? _____________
d) Kind of Molecule ________________________
e) Teacher’s Initials _______________________
7. Carbon Dioxide (CO2)
a) Electronegativity Difference _______________
b) Bond Type_____________________________
c) Symmetrical or Asymmetrical? _____________
d) Kind of Molecule ________________________
e) Teacher’s Initials _______________________
8. Ammonia (NH3)
a) Electronegativity Difference _______________
b) Bond Type_____________________________
c) Symmetrical or Asymmetrical? _____________
d) Kind of Molecule ________________________
e) Teacher’s Initials _______________________
9. Methane (CH4)
a) Electronegativity Difference _______________
b) Bond Type_____________________________
c) Symmetrical or Asymmetrical? _____________
d) Kind of Molecule ________________________
e) Teacher’s Initials _______________________
MOLECULAR MODEL CONCLUSION QUESTIONS
1. Which type of chemical bond generally forms between a:
a) metal atom and a nonmetal atom? ________________________
b) two different nonmetallic atoms? __________________________
c) two of the same nonmetal atoms? _________________________
2. In terms of valence electrons, differentiate between the following types of
chemical bonds:
a) Polar Covalent Bonds _______________________________________
b) Ionic Bonds ______________________________________________
c) Nonpolar Covalent Bonds ____________________________________
3. In terms of electronegatitivity difference, differentiate between the
following types of chemical bonds:
a) Nonpolar Covalent Bonds ___________________________________
b) Polar Covalent Bonds ______________________________________
c) Ionic Bonds _______________________________________________
4. In a complete sentence, explain how to determine if a substance is a polar
molecule or a nonpolar molecule. Use the words polar molecule and
nonpolar molecule in your answer.
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5. What type of chemical bond is found in water? Explain your answer.
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6. Is water a polar molecule or a nonpolar molecule? Explain your answer.
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7. Given the following substances:
F2
Al2O3
CCl4
H2S
a) Which is a polar molecule? [Hint: Please draw the structures in the
space below to help aid you in answering this question correctly.]
Answer: ____________________
b) Which has the highest melting point? _________
8. Given the following substances:
N2
NaCl
MgBr2
H2S
a) Which molecule is a nonpolar molecule? [Hint: Please draw the
structures in the space below to help aid you in answering this
question correctly.]
Answer: _______________________
b) Which has the highest melting point? _________
9. In the boxes provided, draw Lewis Structures for:
a) CF4
b) Na2S
10. Answer the following questions regarding the compounds in question #9.
a) What type of chemical bond is found in CF4? _________________
b) What type of chemical bond is found in Na2S? ________________
c) What type of molecule is CF4? ____________________________
d) Compare the two compounds in terms of differences in boiling
points.
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11. What is the number of pairs of electrons that are shared between the
nitrogen atoms in a molecule of N2? _________
12. Which atom attains a stable valence electron configuration by bonding with
another atom?
(1) neon
(2) helium
(3) radon
(4) hydrogen
Please explain your reasoning in the space below as to why you chose the
atom you chose.
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