Lab # 12 - Electron Glue
Homework: Reading Assignment:
Here are some possible models to explain how electrons provide the glue to stick
atoms together. Each sphere in the drawing represents an atom. The gray shaded
areas represent where the negatively charged electrons might be found in each
model.
Model 1: IONIC
Properties:
Made of metal and non-metal atoms
Dissolves in water
Conducts electricity when dissolved but not when solid
Description of drawing: Spheres without gray areas represent
metal atoms. Spheres with gray areas are non-metal atoms. Metal
atoms have "given up" their valence electrons to non-metal atoms.
Model 2: METALLIC
Properties:
Made entirely of metal atoms
Do not dissolve in water
Conduct electricity
Bendable solids
Description of drawing: Valence electrons are free to move
throughout the substance like a "sea" of electrons.
Model 3: COVALENT
Molecular Covalent, like H2.
Properties:
Made of nonmetal atoms
Some dissolve in water, some do not
Do not conduct electricity
Tend to be liquids or gases or softer solids, but network
covalent compounds, like a diamond, are hard solids.
Description of drawing: Valence electrons are shared between
some atoms. This creates small stable units within the substance, or a
large network of bonds.
Network Covalent, like a diamond C(s).
Very hard!
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Model 1 is a description of what chemists call ionic bonding. Ionic bonding occurs strictly between metal and
nonmetal atoms. In ionic bonding some of the valence electrons of a metal atom are transferred to a nonmetal
atom so that each atom ends up with a noble gas configuration. Usually one, two, or three electrons are
transferred from one atom to another. This transfer of an electron causes the metal atom to have a net positive
charge (+) and the nonmetal atom to have a net negative charge (−). The
Cl
individual atoms in ionic solids are referred to as ions because of their
charges. These opposite charges are attracted to one another. On the right is
a drawing of a chunk of salt, NaCl, a very common ionic substance.
Na+
Notice how the sodium and chloride ions alternate throughout the
structure. The positive and negative ions alternating in three
dimensions make the solid quite strong because of their strong
attractions to one another. The sodium ion is written Na+ and the
chloride ion is written Cl .
−
−
When ionic solids are placed in contact with water, they dissolve. They remain ions, with charges, but now the
individual ions are surrounded by water molecules and distributed throughout the water. Once dissolved, ionic
compounds will conduct electricity .
Model 3 represents bonding that is referred to by chemists as covalent bonding. The valence electrons are
shared between atoms, such that the electrons are attracted to two nuclei. In a
molecular covalent compound atoms are connected into small units, called
molecules, by covalent bonds. However, the molecules themselves are not connected
by covalent bonds to one another. Thus, molecular covalent substances consist of a
large group of individual molecules. The picture shows an artist's idea of a small
collection of water molecules. Note that whenever we are talking about molecules in
chemistry, we are referring to covalently bonded groups of atoms. Molecular
water
covalent substances tend to be liquids, gases or soft solids. This is because the
molecules
individual molecules have more freedom to move within the substance.
Network covalent bonding is a little different. This means that there is a
covalent connection between each atom and all of the adjacent atoms that
surround it. This extended network covalent bonding creates a very strong
substance that is not easily broken apart. Diamond, silicon, glass are
examples of substances that have network covalent bonding. Diamond is
made entirely of carbon atoms. If we looked at a chunk of diamond on a
particulate level, we'd see that each carbon atom is bonded in three
dimensions to the carbon atoms surrounding it. Silicon dioxide, sand or glass
(SiO2), is another network covalent substance. You may have chosen to put it
in the molecular covalent category because it is made of two nonmetal atoms.
Nevertheless, it is a very hard solid, unlike the molecular covalent
SiO2
compounds. Its hardness comes from an extended network of covalent bonds
throughout the material. The illustration shows an artist's rendering of a piece of glass from an atomic level. The
shared electrons have been replaced by sticks showing the connections between all the atoms.
Model 2 is referred to as metallic bonding. In the case of metallic bonding
each atom contributes electrons to the solid. Thus, each atom becomes
essentially positive in charge, having lost some electrons. The electrons
then form a sort of sea of electrons around the atoms. The atoms in turn are
attracted to the negatively charged "sea". On the right is an artist's
rendering of a block of iron, Fe, atoms. The "sea of electrons" is not visible.
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Fe
2
Lab Activity Part 1. You Light Up My Life
Purpose: This lesson allows you to collect evidence regarding some of the properties of metallic, ionic
compounds and covalent compounds by looking for patterns.
Predict: Make some predictions regarding the following substances. Predict: 1) whether the substance will
conduct electricity, 2) whether the substance will dissolve in water. Put your predictions in the left hand portion
of the table.
Predictions
Substances
Conduct? Y/N
Dissolve? Y/N
Test Results
Conduct? Y/N
Dissolve? Y/N
Conduct when
dissolved?
H2O(l), water
Al(s), aluminum
foil
C12H22O11(s),
sugar, sucrose
NaCl(s), salt,
sodium chloride
SiO2(s), sand,
silicon dioxide
C20H42(s), wax,
paraffin
C2H6O(l),
ethanol
Cu(s), copper
CaCl2(s),
calcium chloride
CuSO4(s),
copper sulfate
Test 1: Conductivity Using the conductivity apparatus, test the conductivity of each sample.
Test 2: Dissolving Take a very small portion of each substance and try to dissolve it in 2-5 mL of water in the
well of your well plate. Enter your results in the right hand portion of the table. Before proceeding to the next
substance, test any dissolved substances for conductivity.
Test 3: Conducts When Dissolved. If the substance dissolves in water, test this liquid with the conductivity
apparatus. Enter your results in the final column of the table. When a substance does not dissolve enter N/A in
the last column for Not Applicable. Be sure to rinse the conductivity apparatus in between tests so you do
not contaminate the wells!
Answer questions:
1. Make a list of those substances that conduct electricity but do not dissolve in water. What other things do
these substances have in common? (Think about their properties and their chemical formulas.)
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2. Divide the substances that dissolve in water into two categories: those that conduct electricity once they are
dissolved, and those that don't.
Substances that dissolve in water
Conduct
Don’t conduct
3. What do the substances that conduct electricity once they are dissolved have in common?
4. What do the substances that do not conduct electricity once they are dissolved have in common? (Leave
water out.)
5. Write a generalizing statement about the substances that do not light up the bulb.
6. Write a generalizing statement about the substances that do light up the bulb.
7. Predict whether isopropanol, C3H8O(1), will conduct electricity. State your reasoning.
Making Sense
1. If it is dangerous to take a bath with a blow dryer, what must also be true about the water in the bathtub?
2. Which place would be safer during a lightning storm, the ocean or a bath of pure water? Explain your
reasoning.
3. Predict whether each will conduct electricity, dissolve in water, and/or conduct electricity once dissolved.
State your reasoning in each case.
a. MgCO3(s) magnesium carbonate
b. C4H6O(1) acetone
c. Ti(s) titanium
d. LiNO3(s) lithium nitrate
e. CuZn(s) bronze
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Lab Activity- Part II
Purpose: This lesson helps to explain the physical properties of basic substances by examining the types of
bonds that exist between the atoms of these substances.
Step One: Read the 3 descriptions of possible models for bonding, on the handout.
Chemical formula
Na(s)
NaCl(s) (salt)
Hg(1)
CuCl2(aq)
SiO2(s)
CH3CH20H(1)
NaOH(aq)
NaCl(aq)
MgSO4(s)
Name
Sodium
Sodium chloride
Mercury
Aqueous copper chloride
Silicon dioxide(sand)
Ethanol (drinking alcohol)
Chemical formula
Cu(s)
H2O(1)
C(s)
CH4(g)
Si(s)
Pt(s)
Name
Copper
Water
Carbon (diamond)
Methane (natural gas)
Silicon (computer chips)
Platinum (jewelry)
Aqueous sodium hydroxide (Drano®)
Aqueous sodium chloride (salt water)
Magnesium sulfate (Epsom salt)
Step Two: Classify the 15 substances above according to which of the 3 models best describes their make-up
and properties. Refer to your periodic table as needed.
Ionic
Metallic
Covalent
Are there any substances that don't seem to fit properly in the categories you have placed them into? List them
below and explain.
Circle the network covalent substances in the list of covalent bonds.
Use the models of bonding on the handout to answer the following:
1. Which model would you predict for the following substances? Explain your reasoning.
C6H12O6 (sugar)
KI (potassium iodide)
He (helium)
Au (gold)
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CO2 (carbon dioxide gas)
2. In the ionic solid, Model l, the non-metal atoms have a negative charge and the metal atoms have a positive
charge. What do you think causes these charges?
3. In the metallic solid, Model 2, the "sea" of electrons has a negative charge. What charge would the spherical
atoms have? Explain.
4. Use the models to explain the following evidence:
a. molecular covalent substances are often gases
b. ionic substances are brittle
5. Explain how the location of the "electron bonds" in Model 2 might account for the softness and flexibility of
copper metal.
6. Explain how the location of the electron bonds in a network covalent solid might account for the incredible
hardness of a diamond.
7. Remember from our previous lesson that both sugar and salt dissolve in water, but sugar does not conduct
electricity. We can conclude that they are bonded differently. Use the models to explain how these two
substances might be different once they dissolve.
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