Physical Science 9
Unit 6: The Power of the Nucleus
Activity #1: Neutrons Make Isotopes
#1 CLASS COPY
To Begin
In previous units we focused on protons (important for atomic identity) and electrons (important
for ions and chemical bonding). Now, it is time to focus our attention to the neutron…another
subatomic particle found in the nucleus along with protons. Neutrons add to the mass of an atom,
but have nothing to do with the atom’s identity. The neutron number may vary from atom to atom,
but if the proton number is constant the atom’s identity remains the same.
Purpose
• Students will discover how neutrons contribute to the atomic mass of atoms. You will learn the
meaning of the term isotope.
• Students will understand that the natural abundance of isotopes, for any given element, determines the
average atomic mass of that element (what we see on the Periodic Table).
• Students will learn that some isotopes are stable and some are unstable.
Process- Part 1: Defining isotopes (both stable and unstable)
1. Using the Google search engine (on Chrome), type “phET Isotopes and Atomic Mass” into the search
window. Select the first website. Choose “download”.
2. Select the (+) on “Symbol” and “Abundance in Nature”. Discuss what abundance in nature means.
3. On the bottom of the scale, select “Atomic Mass (amu)”. Notice the Atomic Mass of Hydrogen is
shown to be 1.00783.
4. Now select the “Mass Number” and use this for the remainder of the activity. The mass number
represents the total number of protons and neutrons (also known as nucleons).
5. Discuss with your partner the meaning of the red and black numbers in the symbol box on the right.
6. The hydrogen isotope with only one proton, 11𝐻 , is also represented by the name hydrogen–1 and is
displayed on the scale.
7. Make the second isotope, 21𝐻 , by adding a neutron from the neutron bucket. Noticed this isotope is
named hydrogen–2. Study the similarities and differences between these two isotopes.
8. As you study these two isotopes, you will notice that they are both stable. What is different about the
two isotopes? What is similar?
9. If they were unstable nuclei, then they would be considered radioactive, and undergo a nuclear
change that results in the emission of radiation.
10. Build a third isotope, hydrogen-3, and notice how it compares to hydrogen-1 and hydrogen-2. You
may notices that the natural abundance of these three isotopes varies significantly. Natural
abundance refers to the frequency of an isotope as naturally found on Earth.
11. Add a 3rd neutron. Discuss your observations with your partner.
12. Complete Table 1. Hydrogen Isotopes on your worksheet. Respond to the prompts below the table.
Note that if an isotope lists a natural abundance of 0%, then this is not a naturally found isotope.
13. Click on the helium (He) chemical symbol in the Periodic Table. Explore and discuss the isotopes of
Helium and see if you notice patterns that are similar to or different from those of Hydrogen isotopes.
14. Complete Table 2. Lithium Isotopes.
15. Complete Table 3. Possible Isotopes of Elements 3-10, by identifying stable and natural radioactive
nuclei and their relative abundances of the first 10 elements. Write the chemical symbol, name and
write the natural abundance of all stable isotopes. If you identify a radioactive nucleus with “very
small” natural abundance, list its chemical symbol and name. You may ignore any isotopes with “0”
abundance. When you finish, highlight elements that have naturally occurring radioactive isotopes.
Process- Part II: Exploring Atomic Mass
1. From the top of the screen, select the Mix Isotopes tab. Be sure to select the (+) for percent
composition and average atomic mass.
2. Drag hydrogen atoms (both purple and green) into the black box. You are dragging in different
isotopes (either hydrogen -1 or hydrogen-2). Notice how the percent composition and average
atomic mass data changes on the right side of the screen. As you add more atoms to the box,
discuss at least three different observations with your partner. You may add as many of each
isotope (color) as you like.
3. After completing Step 2, Reset All. 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 Table 4. Hydrogen Average Atomic Mass 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.
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”.
6. Look at the table you completed in Step 3 as well as the information on this page. Discuss the
connections you can make between abundance in nature, percent composition, and average atomic
mass?
7. 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 simulation to select carbon and clicking on “Nature’s mix of isotopes”.
8. 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 simulation to select boron and clicking on “Nature’s mix of isotopes”.
9. 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?
10. With your partner, define the term average atomic mass , using your own words.
Physical Science 9
Unit 6: The Power of the Nucleus
Activity #1: Neutrons Make Isotopes
Table 1. Hydrogen Isotopes
Isotope
Chemical Atomic
Name
Symbol
#
Hydrogen1
#1
Mass #
# of
Protons
# of
Neutrons
# of
Electrons
Stable?
Natural
Abundance
Hydrogen2
Hydrogen3
Hydrogen4
Complete these sentences:
𝟏
𝟐
𝟏𝑯 and 𝟏𝑯 are both hydrogen atoms because they have the same number of _______________.
They are isotopes of the same element because they each have a different number of ___________.
This also means they each have a _______________ atomic mass from the other.
Make a general statement regarding the stability and abundance of various hydrogen isotopes:
Table 2. Lithium Isotopes
Isotope
Chemical Atomic
Name
Symbol
#
Lithium-5
Lithium-6
Lithium-7
Lithium-8
Lithium-9
Mass #
# of
Protons
# of
Neutrons
# of
Electrons
Stable?
Natural
Abundance
Table 3. Possible Isotopes of Elements 1-10
Chemical Symbols
of Stable Isotopes
Isotope Names and
Relative
Abundance
Chemical Symbol
and Name of
Natural Radioactive
Isotopes
Hydrogen
1
2
1𝐻, 1𝐻
Helium
2
2𝐻𝑒,
Lithium
hydrogen 1
(99.9885%)
hydrogen –
2
(0.0115%)
3
1𝐻
hydrogen -3
(very small)
helium- 4
(100%)
Lithium-6
(7.6%)
none
none
Carbon
Nitrogen
Oxygen
Beryllium
Boron
Fluorine
Neon
6
7
3𝐿𝑖, 3𝐿𝑖
lithium-7
(92.4%)
Chemical Symbols
of Stable Isotopes
Isotope Names and
Relative
Abundance
Chemical Symbol
and Name of
Natural Radioactive
Isotopes
Table 4. Hydrogen Relative Atomic Mass
# of atoms
Hydrogen-1
Purple
# of atoms
Hydrogen-2
Green
1
1
5
5
5
10
10
5
1
10
10
1
20
1
50
1
% Hydrogen-1
Purple
% Hydrogen-2
Green
Average Atomic Mass
(amu)
Make a general statement regarding the stability and abundance of various isotopes of various elements: