Covalent Bonds
Ionic Bonds form when two atoms (usually a metal and a nonmetal) exchange electrons, become
oppositely charged and then bond, like two, oppositely charged magnets. Substances formed
through ionic bonds are called ionic compounds. This is one way atoms can form bonds. The other
way is by SHARING electrons instead of giving them away entirely.
The motivation for bonding in covalent bonds is the same as it is in ionic bonds. Atoms are still trying
to get to a Noble Gas configuration with a full outer shell of 8 (or 2). They do this by merging their
electron orbitals. The electrons involved in the bond will spend some time hovering around the
nucleus of one atom and the rest of the time around the nucleus of the other.
This type of bonding creates a weaker bond than the bonds created between ions. Covalently bonded
substances are called molecules and because their bonds are weaker, their chemical and physical
properties are different than those of ionic compounds.
Using the textbook Pages L32-L33
1. Property: __________________________________________________________________________
Explanation of how the characteristics of covalent bonds make this property possible:
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2. Property: __________________________________________________________________________
Explanation of how the characteristics of ionic bonds make this property possible:
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From your textbook pages L31-L32, copy the electron dot diagrams from the following figures:
3. Draw Figure 22 showing the bonding of two fluorine atoms into a fluorine gas molecule.
What is the molecular formula for this molecule? ________________________________
4. From Figure 23
a. Draw the covalent bonding of Hydrogen to oxygen to form a water molecule.
What is the molecular formula for this molecule? _____________________________
b. Draw the Covalent bonding of Hydrogen to Nitrogen to form Ammonia.
What is the molecular formula for this molecule? ___________________________
5. From Figure 24
a. Draw the covalent bonding of Oxygen to Oxygen to form Oxygen Gas
What is the molecular formula for this molecule? __________________________
b. Draw the covalent bonding of Oxygen to Carbon to produce Carbon Dioxide Gas.
What is the molecular formula for this molecule? __________________________
c. Draw the covalent bonding of Nitrogen to Nitrogen to produce Nitrogen Gas.
What is the molecular formula for this molecule? __________________________
Polarity in Covalently Bonded Molecules.
1. What do you think of when you hear the word polar? _______________________________________
In science, anytime the word ‘polar’ is used it is describing the characteristic of an object possessing a
positive and negative side. A magnet is polar because one side of the magnet has a positive charge
and the other side has a negative charge. The earth has poles and is polar because it is like a giant
magnet and has a positive and a negative side. The sun has poles (which are currently switching and
causing a tremendous amount of strange weather here on earth…) because it is also like a magnet
with a positive and a negative side.
2. Draw a polar magnet. Show that it has a positive side and a negative side
3. What does it mean to be polar? ________________________________________________________
Polarity is a HUGE concept to understand because…like Density…it explains so many behaviors. Why
does water dissolve stuff?...polarity. Why does water have surface tension? …polarity? Why does
friction occur?...polarity. etc. etc.
All Ionic Compounds are also polar objects because…yup…they have a positive and a negative side.
4. Draw a Salt Molecule (NaCl). Which side of the compound has a positive charge and which side has a
negative charge? Label the sides.
So what about covalently bonded molecules, can they be polar? The answer is yes and no. A
covalently bonded molecule can become polar if electrons are not shared equally. Sometimes the
nucleus of one atom is so much bigger and so much more attractive than the other nucleus that the
electrons will not divide their time between the two nuclei evenly. Unequal sharing of electrons
creates polarity.
Let’s look at water as a really important example of a polar molecule formed through covalent
bonding.
5. Draw a water molecule.
Notice that the atomic mass of oxygen is 16 times bigger than the mass of Hydrogen. The electrons are
HIGHLY attracted to the Oxygen nucleus and so they spend MOST Of their time there.
6. Which side of a water molecule will become negatively charged? ____________________________
7. Which side of a water molecule will become positively charged? _____________________________
8. Draw a water molecule and label which side is positively charged and which side is negatively
charged. If you need help, there is a drawing in Figure 26 Page L34 in the textbook.
On page L34 Figure 26, there is a picture of a NON-POLAR molecule (Carbon dioxide).
9. Draw the Carbon dioxide molecule from the figure and label where the molecule has positive and
negative charges?
10. Why is the molecule NOT considered polar? Be clear.
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Soap and Solutions
I am sure you have noticed that oily substances do not mix in water? Why is this? Once again…the
answer is polarity. Polar and non polar substances will refuse to have anything to do with each other.
They will NOT interact. Water is a polar substance, while oil is a non-polar substance. The
intermolecular forces between molecules of a non polar substance are different than the attractive
forces that keep polar substances together. The non polar substance is not able to distract the polar.
1. So how do we typically deal with this problem? How do we get oil off of our dishes or clothes?
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I hope you said you use soap, otherwise…I am worried.
Soaps and Detergents are odd molecules because they are both polar and non-polar at the same time.
They have a polar head and a non-polar tail. The polar side of soap is attracted to water and the non
polar side of soap is attracted to the oil molecule. The soap molecule acts like a lasso holding the two
molecules together and allowing the oil to travel with the water as it flows down the drain!
2. Draw a soap molecule lassoing water and oil and tying them together.
A really practical knowledge that comes from understanding polarity is knowing why some things can
be washed away with water and why other things can’t. A common chemistry phrase that I use all the
time is “Like dissolves Like” which means that a polar substance will dissolve a polar substance and a
non polar substance will dissolve a non polar substance.
3. What does the phrase “like dissolves like” mean?
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4. Example: Nail polish cannot be removed with water, but can be removed with acetone. What can
you infer about nail polish and acetone from this fact?
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5. Example: When working with automobiles, the grease and oils from the car cannot be washed away
with water, but can be washed away with gasoline. What can you infer about oils and gasoline from this
fact?
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6. Example: When painting with oil paints or stains, the brushes cannot be cleaned with water but must
be cleaned with paint thinner. What can you infer about oil based paint and paint thinner from this
fact?
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7. Example: Sugar and salt can dissolve in water but will never dissolve in oil. What can you infer about
sugar, salt and water from this fact?
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8. Example: Your cells are made of a cell membrane that is basically fat molecules. You do not dissolve
in water when you take a bath. What can you infer about your cell membranes and water from this
fact?
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A solution is a mixture of two substances where one substance (usually a fluid) called a solvent,
dissolves the other substance (usually a solid) called the solute. In the case of salt dissolving in water,
water is the solvent and salt is the solute. A substance dissolves when the molecules of the solvent
are able to distract the molecules of the solute from their intermolecular attraction for each other.
9. What is a solute? ___________________________________________________________________
10. What is a solvent? __________________________________________________________________
If you will recall, one of the properties of ionic compounds is that they tend to form crystal structures
as the positive side of one molecule is attracted to the negative side of the other.
11. What can you infer about minerals by the fact that they also form crystal structures? (wow! That
goes back in time a ways…)
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Recall that salt is a mineral that forms crystal structures because the Na atom is positively charged
while the Cl atom is negatively charged. Every salt molecule is attracted to every other salt molecule
in every direction because of the polar nature of the compound. When two atoms or molecules of a
substance are attracted to each other, they are bonded by INTERMOLECULAR BONDS. Some
intermolecular bonds are stronger than others, which is why some substances have a low boiling point
and others have a high melting point.
12. Draw the molecules of a salt crystal and show where the intermolecular bonds occur between each
salt molecule.
13. Can you infer what would occur if polar water molecules came in contact with polar salt molecules?
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14. Draw a picture of a salt water solution. Be sure to indicate where the positive and negative charges
are and where new intermolecular bonds are being formed.
15. What is an intermolecular bond? ______________________________________________________
16. What determines the melting or boiling point of a substance?
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17. When will a substance change from a liquid to a gas?
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18. When will a substance dissolve?
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