C-3
1
Valence Electrons and Molecules
How many electrons are in the outermost level?
Element
Atomic Number
Number of electrons in
the outermost energy
level
Lithium
Potassium
Fluorine
Bromine
Neon
Krypton
Number of empty spaces
in the outermost level
a. What do lithium and potassium have in common?
b. What do fluorine and chlorine have in common?
c. What do neon and argon have in common?
2
What are valence electrons?
a. How many electrons are in an s orbital of a given energy level?
b. How many electrons are in the three p orbitals of a given energy level?
c. How many electrons are in an s orbital plus three p orbitals for a given energy level?
d. If valence electrons include the electrons in the s and p orbitals for an energy level, what is the maximum number of valence electrons an atom can have?
e. Use the table below to group the atoms from the first table according to their number of valence electrons. In the first column, write a number of valence electrons. In the second column, write the
names of the elements that have that number of valence electrons.
Number of valence electrons
Elements
3
4
f. How is the placement of an element on the periodic table related to the number of valence electrons
the element has?
Modeling a chemical bond
a. In order to fill its outermost energy level, do you thing sodium will lose its only valence electron, or
gain seven electrons?
b. Explain your answer.
c. When sodium loses one electron, what does the “new” outermost energy level look like?
d. In order to fill its outermost energy level, do you think chlorine will tend to lose all seven of its valence electrons, or gain one electron?
e. Explain your answer.
f. Describe the outermost energy level of chlorine with one electron added.
g. Why do you think these two atoms bond together to form a molecule? In your answer, describe what
you think happens to the electrons when sodium and chlorine form a chemical bond.
Determining oxidation numbers
a. Remove the valence electrons from sodium. What is the total charge on the atom now?
b. The total charge on sodium after it ionizes is equal to sodium’s oxidation number. What is sodium’s
oxidation number?
c. Move the electron you took from sodium into the chlorine. What happens to chlorine’s total charge
when it gains the electron from the sodium atom?
d. In its ionized state, chlorine has a full outermost energy level. What is chlorine’s oxidation number?
e. When sodium and chlorine form a chemical bond, what is the overall charge of the molecule?
5
6
f. Sodium and chlorine combine in a 1:1 ratio. This means that for every one sodium atom, there is a
chlorine atom. Why do you think these atoms combine in this way?
Making molecules
Use the Atom Building Game to build each of the two kinds of atoms in these molecules. As you build
the atoms, think about these questions:
Why do you think some atoms combine in different ratios like 1:1, 2:1, or 1:4?
What is the role of valence electrons in forming molecules?
Does the placement of an atom on the periodic table tell you anything about what atoms it combines with to form a molecule?
Use your answer to these questions to help you figure out three rules for predicting molecules.
Molecular Formula
Ratio of first atom to second atom
(first atom:second atom)
NaCl
1:1
CaO
1:1
H2O
2:1
CH4
1:4
Rule 1:
Rule 2:
Rule 3:
Average atomic mass?
Filling in Table 4 will help you understand average atomic mass. The numbers listed are some possible
average atomic masses for silver. In the space next to these values in Table 4, write down what you
think these values mean regarding how much of each isotope of silver might be in a sample of silver
from nature. For example, there are two isotopes of silver: silver-107 and silver-109. If the average
atomic mass of silver was 107, would there be more silver-107 than silver-109? Would there be any silver-109 on Earth? The answer to the first question is “Yes, there would be more silver-107,” and the
answer to the second question is “No, there would be no silver-109.” An average atomic mass of 107
implies that there are no silver isotopes with a mass of 109. However, an average atomic mass of 107.5
does imply that there is a small amount of silver isotopes with mass numbers larger than 107.
Average atomic mass of silver
What this number means about the abundance of each isotope of
silver in the universe
107
107.5
108
108.5
109
a. The actual average atomic mass of silver is 107.87 grams per mole. What does this value tell you
about the abundance of silver-107 versus silver-109?
b. If you have a sample of silver from nature and you were able to select one atom from this sample,
what are the chances you would select a silver-107 atom?
c. Bromine has two isotopes: bromine-79 and bromine-81. the abundance of bromine-79 is 50.69%.
The abundance of bromine-81 is 49.31%. Based on this information, what is the average atomic
mass of bromine? (Show your work)
1.
2.
3.
4.
5.
Questions
Fill in the number of valence electrons for the listed elements in the table below.
Element
Valence electrons
Sodium
Calcium
Tin
Nitrogen
Radon
Arsenic
Carbon
Helium
One of the rules that is used for figuring out which atoms will combine tor form molecules is called the “octet rule.” You may have figured out this rule on your own during the Investigation. Write a definition
for what you think the octet rule is. Use the term valence electrons in your answer.
Which atoms on the periodic table are the least likely to ionize?
Why?
In the space, make diagrams of each of the atoms listed in the table. In your diagrams, be sure to include the
nucleus, energy levels, protons, neutrons, and electrons. Once you have completed your diagrams, fill in
the table for each molecule that you can build with the atoms. An example of how to fill in the table is
provided in the first row for two pretend elements, “X” and “Y.”
First element (symbol)
Element X (X)
Hydrogen
Fluorine
Nitrogen
Oxygen
Sulfur
Magnesium
Second element (symbol)
Element Y (Y)
Ratio of first element to
second element (1st:2nd)
1:4
6. List the oxidation numbers for the elements in the table above.
Molecular formula of
the molecule
XY4