© Sunrise Science 2018
Eggium! (a fictitious element)
A lesson on Isotopes & Atomic Mass
TEACHER NOTES
This is a great activity for helping your students to understand what isotopes are, how mass
number and atomic mass are different, and why atomic mass is a decimal number on the Periodic
Table.
The materials for this activity are simple! The fictitious element is called ‘Eggium’ because you will
be using plastic Easter eggs as your atomic ‘nuclei’. I always do this activity as a class activity so
that I can talk the class through the steps and facilitate a discussion, but if you have enough time
and resources, you could make the following materials for groups of 4 students.
For a whole-class activity you will need:
o |0 plastic Easter eggs (preferably in 4 different colors– 3 greens, 4 pinks, 2 blues, | yellow)
[The colors don’t matter, just the number of each color– and this is just for easier counting for you. If you
don’t have different colors, don’t worry! If you don’t have Easter eggs, simply use Petri dishes or even just
sandwich bags!]
o
o
o
o
Around |00 small pom-poms in one color [these are your neutrons]
Around 70 small pom-poms in another color [these are your protons]
A black Sharpie marker
|0 paper plates or trays [optional]
To make your |0 Eggium isotopes, you will be putting a certain number of ‘neutrons’ and ‘protons’
into each egg and labeling it with the letter from the table below. If you use the exact numbers
below, then the answer key for this activity will be accurate for you!
Egg Color
(examples)
Green
Green
Green
Pink
Pink
Pink
Pink
Blue
Blue
Yellow
Label with
Letter
A
B
C
D
E
F
G
H
I
J
# of
neutrons
8
8
8
9
9
9
9
|0
|0
||
# of
protons
7
7
7
7
7
7
7
7
7
7
The answer is that the atomic mass is a weighted average mass of all of an atom’s isotopes!
[but don’t tell them this yet!]
Tell the students that they are going to be working with a new element that was just discovered– its
name is Eggium and its symbol is Eg.
Hand out the student worksheet to each student and randomly hand out an Eggium isotope to groups of
2 or 3 students [you have |0 atoms, so this depends on the class size]. Tell the students the color pompom that represents protons and the color that represents neutrons. Have them write this on their
worksheet in the top box. Tell them that this activity depends on exact numbers of the subatomic
particles, so while they are allowed to open their eggs, they need to keep the particles (pom-poms) on
their plate/tray/desk and not let them drop on the floor or get mixed up with other atoms!
Instruct the groups to follow the directions for Step |. They will have to open the eggs and count the
pom-poms. Walk around to make sure they are counting the correct particles and writing the letter of
their Eggium isotope on their sheet. Next, review the shorthand version of an atom– there is a sample
on the worksheet– and instruct the groups to write the shorthand for their Eggium atom. Have the
students raise their hand to have you check their shorthand.
Once all groups are ready, instruct the groups to put the subatomic particles back into their nucleus and
pass their Eggium atom clockwise to the next group. The groups will be counting the pom-poms in each
Eggium atom and writing the shorthand under the matching letter on the worksheet.
Depending on your class, you could have the groups answer the Quick Questions and share aloud, or use
this time to ask the questions to the whole class and have individuals share their ideas. Just make sure
that everyone understands that all of the atoms are Eggium atoms because they all have 7 protons!
Review that their neutron numbers are different, but this affects the mass only, not the charge or the
identity of the element!
Do the same for the discussion questions or have students try to answer #|-4 in small groups and then
discuss answers aloud. You may need to review how to find the fraction/percentage of a whole!
© Sunrise Science 2018
Now, you could begin the lesson by asking your students to discuss in pairs and write down why they
think that atomic mass is in a decimal form on the Periodic Table. Usually, you’ll get an array of answers
including the most common misconceptions– that electrons add a little mass, that there are parts of
protons or neutrons in the nucleus, or that chemists have very precise equipment for measuring mass
and they know mass out to many decimal places.
If you use the same numbers of protons and neutrons as I have suggested, your weighted atomic
masses will be |6.| for both calculations. Hopefully someone will notice that this number is closest to the
most abundant isotope (mass number |6). Now it is time to discuss why the first method is simply not
feasible or effective for finding the weighted average mass of isotopes in the real world.
Ultimately, your students should see that scientists use a sampling method to determine the approximate
percentages of the different isotopes of an atom that exist on Earth. They use these percentages
(natural abundances) to find a relative or weighted average atomic mass and that is what is written on
the Periodic Table1 For the most part (there are exceptions), the weighted atomic mass is closest to the
most abundant isotope of an element. For example, boron’s atomic mass is |0.8|; its isotope boron-||
has a natural abundance of 80% and its isotope boron-|0 has a natural abundance of 20%. The atomic
mass |0.8| rounds up to ||, the mass number of the most abundant isotope!
At the end of this lesson, I like to really drive home the point of why atomic mass is a decimal by asking
the students: “So when I ask you on a quiz why atomic mass is a decimal number on the Periodic Table,
some of you are still going to say it’s because electrons add a little bit of mass– NO! This is wrong!” I
write this on the board and dramatically put a big line through it. “Some of you are still going to say
that it’s because there are parts of protons or neutrons in the nucleus- NO! This is wrong! There are
only whole protons and whole neutrons!” I write it on the board and cross it off. “Some of you are still
going to say that chemists have very precise equipment for measuring mass and they know mass out to
many decimal places— NO! This is wrong! Mass number is only ever a whole number because protons and
neutrons each have a mass of | amu.” Again, write it on the board and cross it off. Then, I write the
correct reason and put a bunch of stars and happy faces around that one!
Hopefully they remember the reason!!
Print pages || and |3 for each student so that they can practice with the problems on isotopes and
calculating weighted atomic mass. Have the students complete these worksheets as formative
assessment or homework! The answer keys are included.
Thank you very much for your purchase! If this product has met your needs,
please consider leaving feedback at TeachersPayTeachers.com or feel free to
email me at SunriseScienceTPT@gmail.com with any questions or concerns!
© Sunrise Science 2018
© Sunrise Science 2018
Once you have reviewed the terms isotope and natural abundance, your students are ready to calculate
the average atomic mass of this sampling of Eggium atoms. I suggest asking the class how they might
find the average mass and maybe they will offer these two ways. Have the groups try the calculations
on their own before reviewing aloud or showing them the calculations. Usually some students get
confused on the second calculation and need to see an example of the first term as a model.
Eggium! (a fictitious element)
A lesson on Isotopes & Atomic Mass
Objectives:
Students will be able to explain what an isotope of an atom is
Students will be able to carry out calculations of relative atomic mass
Students will be able to explain why the atomic mass is a decimal on the periodic table
Eggium Model Subatomic Particle Representations:
pink
green
Protons = _______________
Neutrons = _______________
(example colors!)
Directions:
1) When your group receives your first Eggium isotope, answer the questions below.
C
a) What is the letter written on your Eggium atom? ________
7
b) What is the atomic number of this atom? ________
c) What is the mass number of this atom?
(example info)
15
________
2) Write the shorthand for your Eggium atom in the box to the right. The symbol for Eggium is Eg.
For example:
12
6
Mass number
C
15
Chemical symbol
Eg
7
Atomic number
3) Have your shorthand checked by the teacher. If given the okay,
copy the shorthand in the correct box below (look for the letter).
As the Eggium atoms get passed around the room, write the shorthand of each one in the box below
with the same letter as it written on the egg:
A
B
C
D
E
F
G
H
I
J
15
15
15
16
7
7
7
16
16
16
17
17
18
7
7
7
7
7
Eg Eg Eg Eg Eg Eg Eg Eg Eg Eg
7
7
Wait for Directions
© Sunrise Science 2018
KEY
Name ________________________________________________
Class _______ Date _______
2)
If these atomic models were actual atoms, what are two things that are different between these
atoms?
Some of these atoms have different numbers of neutrons and therefore, different masses
(because neutrons have mass).
Discussion and Calculations:
1) What is the definition of an isotope and what are some examples?
Isotopes are the same type of atom with different numbers of neutrons. Some examples are:
Carbon-14 and Uranium-235. We can write the isotope as the element name followed by a dash
and the isotope’s mass number (total number of protons plus neutrons).
2)
Do you think that all isotopes of an element occur on the Earth with the same frequency?
No- isotopes of an element definitely do not occur on the Earth with the same frequency. For
example, 92% of lithium atoms may be lithium-6 and only 8% lithium-7.
3)
If I have 20 animals on my farm and 12 of them are cows, then 12/20 (or 3/5 as a reduced fraction,
0.6 as a decimal, or 60% as a percentage) of my animals are cows. Calculate the fraction or
percentage of each isotope represented in the sampling of Eggium.
There are 10 atoms in the sample of Eggium.
3 of them are Eggium-15, so 3/10 (or 0.33 or 33%)
4 of them are Eggium-16, so 4/10 (or 2/5 or 0.6 or 60%)
2 of them are Eggium-17, so 2/10 (or 1/5 or 20%)
1 of them is Eggium-18, so 1/10 (or 10%)
4)
What you just calculated was each isotope’s natural abundance. What do you think natural abundance
means?
Natural abundance is the percentage of one isotope of a type of atom that is naturally occurring
on Earth. This percentage is an estimate based on a sampling of that element’s known isotopes.
© Sunrise Science 2018
Quick Questions to Answer:
1) How can we be sure that these ‘nuclei’ are all representing ‘Eggium’ atoms?
We know that all of the Eggium atoms are the same type of atom because they all have the same
number of protons, 7.
4)
There are two ways of calculating the average mass of the isotopes of the element Eggium. Show
your work for each of the calculations in the boxes below. The unit for the mass of atoms (because
it’s soooo small) is called an atomic mass unit (amu).
Calculation #|: Add up the masses and divide by the number of masses
|5 + |5 + |5 + |6 + |6 + |6 + |6 + |7 + |7 + |8 = |6|
|6| divided by 10 = |6.|
|6.| amu (atomic mass units)
The relative atomic mass of Eggium using this calculation is ____________
Calculation #2: Find the sum of: (the % natural abundance of each type of isotope times its mass).
(3/|0 x |5) + (4/|0 x |6) + (2/|0 x |7) + (|/|0 x |8)
= 4.5 + 6.4 + 3.4 + |.8 = |6.|
|6.| amu (atomic mass units)
The relative atomic mass of Eggium using this calculation is ____________
5)
In the real world, the second method of calculating the relative atomic mass of an element’s isotopes
is both more effective and more feasible. Why do you think this is?
It would be impossible to actually count every single atom of an element, which is what you would need to do
for the first method. Since we can’t count every single atom, we can approximate the numbers of each isotope
by taking a sampling of an element’s isotopes and determining a percentage of each isotope that exists in the
sample. We can estimate the amounts of each isotope on Earth using the natural abundance.
6)
Now the most important question of this lesson: Why is atomic mass in a decimal form on the Periodic
Table?
Atomic mass is in a decimal form on the Periodic Table because it is a weighted mass of all isotopes of that
element. In other words, the atomic mass is an average mass of all of the element’s isotopes that exist on
Earth, but it also takes into account the percentage of each that exist in a sample. For our Eggium atoms,
the isotope Eggium-|6 occurs most frequently, so the relative atomic mass is closest to |6.
© Sunrise Science 2018
Discussion and Calculations, continued:
Eggium! (a fictitious element)
A lesson on Isotopes & Atomic Mass
Objectives:
Students will be able to explain what an isotope of an atom is
Students will be able to carry out calculations of relative atomic mass
Students will be able to explain why the atomic mass is a decimal on the periodic table
Eggium Model Subatomic Particle Representations:
Protons = _______________ Neutrons = _______________
Directions:
1) When your group receives your first Eggium isotope, answer the questions below.
a) What is the letter written on your Eggium atom? ________
b) What is the atomic number of this atom? ________
c) What is the mass number of this atom?
________
2) Write the shorthand for your Eggium atom in the box to the right. The symbol for Eggium is Eg.
For example:
12
6
Mass number
C
Chemical symbol
Atomic number
3) Have your shorthand checked by the teacher. If given the okay,
copy the shorthand in the correct box below (look for the letter).
As the Eggium atoms get passed around the room, write the shorthand of each one in the box below
with the same letter as it written on the egg:
A
B
C
D
E
F
G
H
I
J
Wait for Directions
© Sunrise Science 2018
Name ________________________________________________ Class _______ Date _______
2)
If these atomic models were actual atoms, what are two things that are different between these
atoms?
Discussion and Calculations:
1) What is the definition of an isotope and what are some examples?
2)
Do you think that all isotopes of an element occur on the Earth with the same frequency?
3)
If I have 20 animals on my farm and 12 of them are cows, then 12/20 (or 3/5 as a reduced fraction,
0.6 as a decimal, or 60% as a percentage) of my animals are cows. Calculate the fraction or
percentage of each isotope represented in the sampling of Eggium.
4)
What you just calculated was each isotope’s natural abundance. What do you think natural abundance
means?
© Sunrise Science 2018
Quick Questions to Answer:
1) How can we be sure that these ‘nuclei’ are all representing ‘Eggium’ atoms?
4)
There are two ways of calculating the average mass of the isotopes of the element Eggium. Show
your work for each of the calculations in the boxes below. The unit for the mass of atoms (because
it’s soooo small) is called an atomic mass unit (amu).
Calculation #|: Add up the masses and divide by the number of masses
The relative atomic mass of Eggium using this calculation is ____________ amu (atomic mass units)
Calculation #2: Find the sum of: (the % natural abundance of each type of isotope times its mass).
The relative atomic mass of Eggium using this calculation is ____________ amu (atomic mass units)
5)
In the real world, the second method of calculating the relative atomic mass of an element’s isotopes
is both more effective and more feasible. Why do you think this is?
6)
Now the most important question of this lesson: Why is atomic mass in a decimal form on the Periodic
Table?
© Sunrise Science 2018
Discussion and Calculations, continued:
Isotopes
Recall the Shorthand:
Mass number
Atoms of the same element with
Define Isotope: ________________________________
different numbers of neutrons, and therefore different mass.
________________________________________________
Example: These atoms are all hydrogen:
|
2
H
| p+
0 n0
3
H
| p+
| n0
20
(p+ + n0)
|0
Atomic number
Ne
(p+) can be invisible since
it never changes for an
element
H
| p+
2 n0
Additional neutrons add mass to the mass
number, but don’t change the fact that this is
representing hydrogen (| p+)
Complete the missing information: (hint—use your Periodic Table!)
1
Most Common
4
Another Isotope
6
He
Most Common
39
___
40
K
|9
___
20 n0
___
Most Common
7
___
Another Isotope
|6
|4
___
He
Another Isotope
4
14
___
Li
8
Another Isotope
|6
N
7 p+
___
7 n0
___
p+
6
O
8 p+
___
6 n0
Most Common
K
Another Isotope
O
8 p+
___
8 n0
|9
___
2|
___ n0
p+
5
Most Common
2___ p+
4 n0
__
2 p+
2___ n0
3
2
N
7 p+
___
9 n0
___
Most Common
Another Isotope
35
___
37
Li
Cl
Cl
3 p+
3 p+
___
___
|7 p+
|7 p+
___
___
4 n0
5 n0
|8 n0
___
___
20 n0
___
___
Are the pairs of elements listed below ISOTOPES or two DIFFERENT ELEMENTS? Mark an X in the box:
Ele ment |
Ele ment 2
Isotopes?
Element X has 6 p+ and 7 n0
Element Z has 7 p+ and 7 n0
Element X has 27 p+ and 32 n0
Element Z has 27 p+ and 33 n0
Element X has 17 p+ and 18 n0
Element Z has 18 p+ and 17 n0
Element X has 56 p+ and 81 n0
Element Z has 56 p+ and 82 n0
X
Element X has an atomic # of
20 and an atomic mass of 40
Element Z has 20 p+ and 21 n0
X
Different
Elements?
X
X
X
© Sunrise Science 2018
KEY
Name ________________________________________________
Class _______ Date _______
Isotopes
Recall the Shorthand:
Mass number
Define Isotope: ________________________________
________________________________________________
Example: These atoms are all hydrogen:
|
2
H
| p+
0 n0
3
H
| p+
| n0
20
(p+ + n0)
|0
Atomic number
Ne
(p+) can be invisible since
it never changes for an
element
H
| p+
2 n0
Additional neutrons add mass to the mass
number, but don’t change the fact that this is
representing hydrogen (| p+)
Complete the missing information: (hint—use your Periodic Table!)
1
Most Common
4
Another Isotope
6
He
2 p+
___ n0
3
Another Isotope
___
40
K
___
___ n0
___
Another Isotope
|6
___
He
Most Common
Most Common
Most Common
___ p+
___ n0
p+
5
2
___ p+
8 n0
4
___ p+
6 n0
Another Isotope
K
___
|6
p+
___ p+
___ n0
Another Isotope
8
O
Most Common
___
___ n0
Li
O
6
N
N
___ p+
___ n0
Most Common
Another Isotope
___
37
Li
Cl
Cl
___ p+
___ p+
___ p+
___ p+
___ n0
___ n0
___ n0
___ n0
Are the pairs of elements listed below ISOTOPES or two DIFFERENT ELEMENTS? Mark an X in the box:
Ele ment |
Ele ment 2
Element X has 6 p+ and 7 n0
Element Z has 7 p+ and 7 n0
Element X has 27 p+ and 32 n0
Element Z has 27 p+ and 33 n0
Element X has 17 p+ and 18 n0
Element Z has 18 p+ and 17 n0
Element X has 56 p+ and 81 n0
Element Z has 56 p+ and 82 n0
Element X has an atomic # of
20 and an atomic mass of 40
Element Z has 20 p+ and 21 n0
Isotopes?
Different
Elements?
© Sunrise Science 2018
Name ________________________________________________ Class _______ Date _______
Atomic Mass
Use the % natural abundances of each isotope of the elements below to calculate the approximate weighted atomic mass.
Compare your answer to the actual weighted atomic mass for that element from the Periodic Table.
You will need to recall that a percentage is easily converted into a decimal by moving the decimal point two times to the
left. For example, 88% = 0.88 or 7% = 0.07.
Use the example to help you set up the calculations for the other problems.
Calculation
Approximate
Weighted Atomic
Mass
Atomic Mass
from the
Periodic Table
#
Isotopes
% Natural
Abundance
|
Chlorine-35
Chlorine-37
76%
24%
(0.76 x 35) +
(0.24 x 37)
35.48
35.45
Boron-|0
Boron-||
Rubidium-85
Rubidium-87
Lithium-6
Lithium-7
20%
80%
72.|7%
27.83%
7.59%
92.4|%
(0.20 x |0) +
(0.80 x ||)
|0.80
|0.8|
(0.72|7 x 85) +
(0.2783 x 87)
85.56
85.47
(0.0759 x 6) +
(0.924| x 7)
6.92
6.94
5
Silicon-28
Silicon-29
Silicon-30
92.2%
4.7%
3.|%
(0.922 x 28) +
(0.047 x 29) +
(0.03| x 30)
28.||
28.085
6
Neon-20
Neon-2|
Neon-22
90.48%
0.27%
9.25%
(0.9048 x 20) +
(0.0027 x 2|) +
(0.0925 x 22)
20.|9
20.|8
7
Strontium-84
Strontium-86
Strontium-87
Strontium-88
0.56%
9.86%
7.00%
82.58%
(0.0056 x 84) +
(0.0986 x 86) +
(0.07 x 87) +
(0.8258 x 88)
87.7|
87.62
2
3
4
Why might your calculated answers be slightly different from the actual weighted atomic mass from the Periodic Table?
The calculated answers on this worksheet might be slightly different for a few reasons. For one, there are very trace amounts
of other isotopes that weigh the average just a little bit higher or lower. Second, we have not accounted for significant digits so
our answers may not be as precise. Thirdly, if these natural abundance percentages are different from the source used to
calculate the weighed atomic masses on the Periodic Table, then the numbers would come out slightly different.
© Sunrise Science 2018
KEY
Name ________________________________________________
Class _______ Date _______
Atomic Mass
Use the % natural abundances of each isotope of the elements below to calculate the approximate weighted atomic mass.
Compare your answer to the actual weighted atomic mass for that element from the Periodic Table.
You will need to recall that a percentage is easily converted into a decimal by moving the decimal point two times to the
left. For example, 88% = 0.88
Use the example to help you set up the calculations for the other problems.
#
Isotopes
% Natural
Abundance
|
Chlorine-35
Chlorine-37
76%
24%
Boron-|0
Boron-||
Rubidium-85
Rubidium-87
Lithium-6
Lithium-7
20%
80%
72.|7%
27.83%
7.59%
92.4|%
5
Silicon-28
Silicon-29
Silicon-30
92.2%
4.7%
3.|%
6
Neon-20
Neon-2|
Neon-22
90.48%
0.27%
9.25%
7
Strontium-84
Strontium-86
Strontium-87
Strontium-88
0.56%
9.86%
7.00%
82.58%
2
3
4
Calculation
Approximate
Weighted Atomic
Mass
Atomic Mass
from the
Periodic Table
(0.76 x 35) +
(0.24 x 37)
35.48
35.45
Why is an element’s atomic mass in a decimal form on the Periodic Table?
© Sunrise Science 2018
Name ________________________________________________ Class _______ Date _______
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