Average Atomic Mass and Nuclear Chemistry Lab
Background Info
The atomic mass you find on the periodic table is actually an average atomic mass. To be more specific,
it is a weighted average of all of the isotopes of that element. Isotopes are atoms of the same element
that have different atomic masses. Remember, the number of protons an element has determines the
identity of the element. The number of protons in one particular element will never change. Also,
electrons have almost no mass. So the difference in mass between isotopes must be due to having a
different number of neutrons. Carbon-12 (meaning the atomic mass is 12) has 6 protons and 6
neutrons. Carbon has 3 isotopes.
Carbon - 12
Carbon - 13
Carbon - 14
Atomic Mass
12.000
13.003
14.003
Number of
Protons
6
6
6
Number of
Neutrons
6
7
8
Percentage of Natural abundance
98.90
1.10
trace
Carbon-14 is radioactive and there is very little to be found in the environment. So when calculating
average atomic mass we do not include it.
To calculate average atomic mass you multiply each isotope by its percentage abundance and add the
totals together. This is called a weighted average. Remember to convert the percent to a decimal
before multiplying. For example
12.000 x 0.9890 =
13.003 x .0110 =
11.868
0.143
12.011
So the average atomic mass of C is 12.011, and this is what we find when we look at the periodic table.
Your Task
Build all isotopes of the atoms listed below with the beans provided.
White Beans =
Dark Beans =
Pinto Beans =
Neutrons
Protons
Electrons
After building the isotopes, calculate the average atomic mass of each element. Include the completed
chart for each element in the data section of your lab report. Show your calculations in detail in the
calculations section.
Oxygen - 16
Oxygen - 17
Oxygen - 18
Neon - 20
Neon - 21
Neon - 22
Atomic Mass
15.995
16.999
17.999
Atomic Mass
19.992
20.994
21.991
Number of
Protons
Number of
Neutrons
Number of
Protons
Number of
Neutrons
Percentage of Natural abundance
99.762
0.038
0.200
Percentage of Natural abundance
90.48
0.27
9.25
You do not have to build the following isotopes with the beans. Still include the completed chart in your
data section and your calculations in the calculations section.
Uranium - 234
Uranium - 235
Uranium - 238
Atomic Mass
234.041
235.044
238.051
Number of
Protons
Number of
Neutrons
Percentage of Natural abundance
0.005
0.720
99.275
Answer the following questions in your conclusion
1.
2.
3.
4.
What is average atomic mass?
What is an isotope and why do isotopes have different atomic masses?
How do we calculate the mass number you find on the periodic table?
How many protons, neutrons and electrons does chlorine – 36 have?
Second Investigation
As part of your introduction, define alpha particle, beta particle, and gamma radiation. Use the beans to
work out the following nuclear reactions.
1. Si – 30 emits an alpha particle. Write a balanced equation
2. B – 11 emits a beta particle. Write a balanced equation. Draw a picture of this in your data
section.
Answer the following in your conclusion.
1.
2.
3.
4.
What is radioactive decay?
Why do atoms emit particles?
What is a decay series? Give an example.
What is one way we use radioactive decay to help society?
LAB ENDS HERE
Weekend Homework assignment
Read section 22-1 p701 and complete the section review, skip #1
Read section 22-4 p717 and complete the section review
In preparation for a group project read about the experiments and results of Dalton, Thompson, E
Rutherford, and Bohr (use the index in your book)