Name Date Class Isotope and Average Atomic Mass Activity

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Name ________________________________________________________________ Date ___________________________ Class ________________
Isotope and Average Atomic Mass Activity
Directions:
1. Visit the following web address: http://phet.colorado.edu/en/simulation/isotopes-and-atomic-mass
2. Click the green “Run Now” button to launch the simulation.
3. On the right side of the simulation, click the green plus signs next to the words “Symbol” and “Abundance in Nature.”
4. Follow the directions given to work through the simulation. Answer the questions as you progress through the activity.
1. What is an isotope?
2. Click on the element H in the periodic table.
How many protons does the most common isotope of hydrogen have? ______________
How many neutrons? _________________
Write the symbol for this isotope of hydrogen: _______________
What is the abundance of this hydrogen-1 isotope? _________________
3. Add a neutron to the H atom.
How does the symbol change?
What is the abundance of this hydrogen-2 isotope? _________________
4. Using the periodic table in the top right corner, determine which element is described below in statements a-d. Record the
symbol for the isotope using the following notation (an example is shown for chlorine-35):
Mass Number
Atomic Number
Element
35
17
Cl
a.
An atom with 5 protons and 6 neutrons ______________
b.
An atom with a mass number of 19 and 10 neutrons ________________
c.
An atom with an atomic number of 2 and a mass number of 4 __________________
d.
An atom with 8 protons and 9 neutrons _______________
5. Investigate some of the other elements and make different isotopes by adding or subtracting neutrons. Note the symbols,
stabilities, and percent abundances for the different isotopes.
For oxygen, record the three different isotopes you can make that exist in large enough amounts to have a percent
abundance (meaning they occur naturally).
Element
Symbol
Atomic
Number
# of Protons
# of
Neutrons
# of
Electrons
Mass
Number
Atomic Mass
Abundance
in Nature
6. Click on the mix isotopes tab. Put one of each of the oxygen isotopes in the mixing box. Note the average atomic mass. Try
different amounts of the isotopes and see how the average atomic mass changes. Click on “Nature’s Mix of Isotopes.”
Record the symbols and percent abundances for the three isotopes of oxygen in nature:
1. Symbol: ____________________ Percent Abundance: __________________
2. Symbol: ____________________ Percent Abundance: __________________
3. Symbol: ____________________ Percent Abundance: __________________
What is the actual average atomic mass of oxygen? ______________________
7. The average atomic mass of sulfur is 32.065 amu.
Which isotope of sulfur would you predict is the most abundant? (Circle one)
32S
33S
34S
36S
Click on the element sulfur in the periodic table. Click on “Nature’s Mix of Isotopes.”
Record the symbols and percent abundances for the four isotopes of sulfur in nature:
1. Symbol: ____________________ Percent Abundance: __________________
2. Symbol: ____________________ Percent Abundance: __________________
3. Symbol: ____________________ Percent Abundance: __________________
4. Symbol: ____________________ Percent Abundance: __________________
8. The average atomic mass is a weighted average of the masses of naturally occurring isotopes of each element. The equation
for calculating the average atomic mass is to multiply the abundance by the atomic mass for each isotope and add those values
together.
(Abundance of isotope x atomic mass of isotope) + (Abundance of isotope x atomic mass of isotope) + … each isotope
**Note: Convert percent abundances into decimals in this formula (ex: 81.35% = 0.8135)
a.
Chlorine has two naturally occurring isotopes: 76.75% of chlorine is chlorine-35 and 24.24% of chlorine is chlorine37. Using the formula above, calculate the average atomic mass of chlorine.
b.
Silicon has three naturally occurring isotopes: 92.223% of silicon is silicon-28, 4.685% is silicon-29, and 3.092% is
silicon-30. Using the formula above, calculate the average atomic mass of silicon.
c.
There are two isotopes of barium: barium-137 and barium-138. If the average atomic mass is 137.33, which of the two
isotopes do you think is more abundant? Explain your answer.
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