Simulations at http://phet.colorado.edu/
Name: ________________________
Isotopes and Atomic Mass PhET Lab
Introduction: Breath in…Breath out. Again! When you inhale air, you are not just
inhaling a mixture of oxygen, nitrogen, and trace gasses, but a mixture of different
oxygen atoms and different nitrogen atoms. It turns out that all oxygen atoms have the
same number of protons, but some may have different numbers of neutrons. These
different-but-still-oxygen atoms are called isotopes. Some atoms have just two isotopes;
some have dozens!
Go to the website https://phet.colorado.edu/en/simulation/isotopes-and-atomic-mass
Procedure: PhETPlay with the Sims  Chemistry  Isotopes and Atomic Mass
PART A – Exploring Isotopes

Click on the “Isotope” button

Be sure to activate
and
1) Select the “Atomic Mass (amu)” button in the atomic balance. For the hydrogen atom, what is the atomic
mass in atomic mass units (amu)? ______________
2) The chemical symbol for hydrogen is H. One by one, click the other shaded chemical symbols in the Periodic
Table and observe the change in the red number in the lower left-hand corner of the chemical symbol shown.
Using your guided notes, write down the two pieces of information the red number convey?
a.
b.
3) Return to the hydrogen atom by pressing the reset button
a. Does the red number change? (Yes/No)
b. Does the black number change? (Yes/No)
and add a neutron to the atom.
4) Using your guided notes, what two meanings can be given to the black number?
a.
b.
5) The hydrogen isotope with only one proton,11 𝐻 , is also represented by the name hydrogen – 1. What is the
atomic mass of hydrogen -1, in amu? _________________
6)
The second isotope, 21 𝐻 is named hydrogen – 2. What is the atomic mass of hydrogen – 2, in a.m.u.?
_______________
7) Which isotope is more abundant in nature, 11 𝐻 or 21 𝐻? _________
8) Write the isotope symbol of a hydrogen atom with two neutrons: _________
a. Build the atom above by dragging a neutron to the nucleus. Is this atomic nucleus stable? (Yes/No)
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Name: ________________________
Note: Unstable nuclei are radioactive; that is, they undergo nuclear change by emitting subatomic particles and radiant
energy.
b. How is the natural abundance of this isotope in nature described? ________________
c. What is the name that is given to this third isotope? __________________
9) Add a third neutron to the hydrogen atom.
a. What is the chemical symbol of this isotope?_______
b. Is this isotope stable? (Yes/No)
c. Do you expect this isotope to be radioactive? (Yes/No)
d. What is the reported natural abundance of this isotope in nature? ___________
10) Add a fourth and fifth neutron to the hydrogen atom
a. What happens to the stability of an atom as more and more neutrons are added?
b.
How does the stability of an isotope relate to its abundance in nature? Why might this be
the case??
__________________________________________________________________________________________________
__________________________________________________________________________________________________
__________________________________________________________________________________________________
11) Click on the helium (He) chemical symbol in the Periodic Table. Remove the two neutrons by dragging them
to the neutron bin.
a. What can you state about the stability and abundance of 22 𝐻e?
b. Write the chemical symbol of the isotope containing two protons and one neutron: _________
c. Build the atom, in part b, by dragging a neutron and placing it in the nucleus of the atom.
a) Does the addition of a neutron help stabilize the helium nucleus ? (Yes/No)
b) Do you expect this isotope to be radioactive? (Yes/No)
c) What is the reported natural abundance of this isotope in nature? ___________
d) What is the name of this isotope? ___________________________
12) Draw the nucleus of the most abundant isotope of each of the following atoms in the boxes below. Be sure to count and
label the protons and neutrons. Also show the full atomic symbol. Hydrogen has been done for you.
Hydrogen: H
1
1
H
Carbon: C
Oxygen: O
MASS #
ATOMIC#
X
Neon: Ne
Proton
12. Complete the chart below. In some cases, you will need to work backwards to fill out missing information.
Simulations at http://phet.colorado.edu/
Isotope
Name
Atomic
Number
# of
Protons
# of
Neutrons
Name: ________________________
Mass
Number
Hydrogen-2
Stable?
(Y/N)
Ratio of
neutrons to
protons
1:1
Helium-3
Helium-
2
5
Lithium-6
3
3
4
7
8
10
10
18
20
Oxygen-16
Oxygen-17
Neon-23
13. Observe the atoms you determined to be unstable. What can you conclude about the ratio of neutrons to protons and a
nucleus’ stability?
____________________________________________________________________________________________________
____________________________________________________________________________________________________
Part 2 – Exploring Average Atomic Mass
1.
Next, select the Mixtures tab
proceeding.
at the bottom of the screen and make sure your screen looks like this before
Simulations at http://phet.colorado.edu/
Name: ________________________
2. 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.
1.
2.
3.
3. After completing Step 2, press the Reset button
. In order to discover the relationship between percent
composition and average atomic mass, it is helpful to be more systematic when choosing the number of atoms in
the simulation. 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.
# of atoms
Hydrogen-1
Purple
1
# of atoms
Hydrogen-2
Green
1
5
5
5
10
10
5
1
10
10
1
20
1
50
1
% Hydrogen-1
% Hydrogen-2
Average Atomic Mass
Purple
Green
(amu)
4. 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.
_____________________________________________________________________________________________
_____________________________________________________________________________________________
5. Check your prediction by clicking on “Nature’s mix of isotopes”. ________________________________________
NOTE - The average atomic mass of an element with three or more isotopes is the sum of the products of the isotope
relative atomic mass (or the mass number, as an approximation) and the natural abundances (%). As a check for
accuracy, the sum of these natural abundances should be equal to 100%.
Simulations at http://phet.colorado.edu/
Name: ________________________
6. Using “Nature’s mix of isotopes” percentages and the following relationship:
[(𝑀𝑎𝑠𝑠 # 𝑜𝑓 𝐼𝑠𝑜𝑡𝑜𝑝𝑒 1)𝑥 (% 𝑎𝑏𝑢𝑛𝑑𝑎𝑛𝑐𝑒 𝐼𝑠𝑜𝑡𝑜𝑝𝑒 1)] + [( 𝑀𝑎𝑠𝑠 # 𝑜𝑓 𝐼𝑠𝑜𝑡𝑜𝑝𝑒 2) 𝑥 (% 𝑎𝑏𝑢𝑛𝑑𝑎𝑛𝑐𝑒 𝐼𝑠𝑜𝑡𝑜𝑝𝑒 2)]
= 𝑎𝑣𝑒𝑟𝑎𝑔𝑒 𝑎𝑡𝑜𝑚𝑖𝑐 𝑚𝑎𝑠𝑠
100
Confirm the average atomic mass of hydrogen:
NOTE - Hydrogen – 3 is not included in the calculation of relative atomic mass, because its natural abundance is
very small. Other very unstable radioactive isotopes have fleeting existence or are not found in nature and are not
included in relative atomic mass calculations, either.
7. Look at the table you completed in Step 3 as well as the information on this page. What conclusions can you draw
between abundance in nature (%), and average atomic mass? ___________________________________________
_______________________________________________________________________________________________
_______________________________________________________________________________________________
8. 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”.
9. 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 carbon and clicking on “Nature’s mix of isotopes”.
10. 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? _______________________________________________________________________
_______________________________________________________________________________________________
_______________________________________________________________________________________________
_______________________________________________________________________________________________
Simulations at http://phet.colorado.edu/
Name: ________________________
NOTE - A word of caution: If the relative atomic mass is close to the central mass number, you cannot safely
conclude that the middle isotope is most abundant. By chance, large percentages of high and low mass
numbered isotopes might result in a weighed relative atomic mass close to the central isotope mass, leading to
an incorrect prediction.
11. Using a Periodic Table of the Elements, find and record the average atomic mass of magnesium: ___________
Select “My Mix” and on the Periodic Table in the simulation, Select magnesium.
12. Predict which is the most abundant isotope in this mixture: ____________________________
13. Make sure “My Mix” is selected AND WITHOUT looking at “Nature’s mix of isotopes”, mix the three isotopes until
you arrive at a close number for the relative atomic mass. If you need more atoms, up to 99, press “More” and
refine your mix. Record your predicted percentages: After you record your predicted percentages you may select
the “Nature’s Mix” button
Predicted Percentages
Nature’s Mix of Isotopes Percentages
Magnesium – 24
__________________
_________________
Magnesium – 25
_________________
_________________
Magnesium – 26
_________________
_________________
14. Define the term average atomic mass using your own words.