Exploring Isotopes and Average Atomic Mass
Name______________________________
Go to the PhET Isotopes simulation. Click “run now” (Note: If this option doesn’t work, click download)
1. To begin, select the Make Isotopes tab and expand the Symbol and Abundance in Nature menus.
2. Add neutrons to the hydrogen isotope. How does adding neutrons affect the isotope symbol?
3. As neutrons are added, comment on the stability of the isotope.
4. How does the stability of an isotope relate to its abundance in nature?
5. Play around with the sim until you are comfortable with the relationship between neutrons, mass number,
symbol, and abundance in nature. You can select a new element by using the periodic table at the top right.
6. Complete the following table. This information will be important later.
Isotope
Symbol
Hydrogen -1 (Protium)
Abundance in
Nature
99.9885%
Mass Number
1
Atomic Mass
(amu)
1.00783
Hydrogen -2 (Deuterium)
0.0115%
2
2.01410
Hydrogen -3 (Tritium)
Very small
3
3.01605
7. Next, select the Mix Isotopes tab and make sure your screen looks like this before proceeding.
8. Drag hydrogen isotopes (purple and green atoms) into the black box. Notice how the percent composition
and average atomic mass data changes. As you add more atoms to the box, record at least three different
observations. You may add as many of each isotope (color) as you like.



Number of atoms and neutrons increases
Percent composition changes
Average atomic mass changes
9. Reset All. In order to discover the relationship between percent composition and average atomic mass,
complete the following table by adding purple and green atoms to the black box. In order to add larger
amounts, Click “More” and use the slider bar or numerically enter data.
# atoms Hydrogen-1
Purple
# atoms Hydrogen-2
Green
1
1
5
% Hydrogen-1
Purple
% Hydrogen-2
Green
Average Atomic Mass
(amu)
50
50
1.51
5
50
50
1.51
5
10
33.3
66.7
1.68
10
5
66.7
33.3
1.34
1
10
9.1
90.9
1.92
10
1
90.9
9.1
1.10
20
1
95.2
4.8
1.06
50
1
98.0
2.0
1.03
10. The average atomic mass for hydrogen is listed as 1.007 amu on the periodic table. Predict the
combination of purple and green atoms required to achieve this mass.
Much more than 50 purple (hydrogen-1) : 1 green (hydrogen-2)
Estimate : 1000 hydrogen-1: 1 hydrogen-2
11. Check your prediction by clicking on “Nature’s mix of isotopes”.
12. Look at the table you completed in Step 6 as well as the information on this page. What conclusions can
you draw between abundance in nature, percent composition, and average atomic mass?
The more abundant (common) the isotope in nature, the greater it’s percent composition. Also, the closer
the amu to the mass of the most common isotope
13. Carbon has an average atomic mass of 12.011 amu (as given on the periodic table). Which isotope of
carbon do you think is most abundant: carbon-12 or carbon-13? Explain your answer. Check your response by
using the sim to select carbon and clicking on “Nature’s mix of isotopes”.
The amu is very close to 12, therefore carbon-12 must be much more abundant than carbon-13
Verification : 98.93% carbon-12 versus 1.07% carbon-13
14. Boron has an average atomic mass of 10.81 amu (as given on the periodic table). Which isotope of boron
do you think is most abundant: boron-10 or boron-11? Explain your answer. Check your response by using
the sim to select boron and clicking on “Nature’s mix of isotopes”.
The amu is close to 11, therefore boron-11 must be much more abundant than boron-10
Verification : 80.1 % boron-11 versus 19.9% boron-10
15. Bromine’s two major isotopes are bromine-79 and bromine-81. Based on the average atomic mass given
for bromine on the periodic table (79.904 amu), what can you conclude about the percent
abundance/composition of the two isotopes in nature?
Bromine-79 should be only slightly more abundant than bromine-81 as the average mass is slightly less
than 80 (the half-way point between 79 and 81)
16. Define the term average atomic mass using your own words.
Average atomic mass is the average mass of atoms of an element, based on the abundance of its isotopes
in nature.