Advanced Physical Science Isotope Problems #1 Name ____________________________
Example
The isotopes of neon are Ne-20, Ne-21, and Ne-22.
Ne-20
Ne-21
Ne-22
-------------------------------------------------------------------------------------------------------The average atomic mass of an element is the weighted average of the masses of its
naturally occurring isotopes. The relative atomic masses are based on 12C = 12.000
Example
The isotopes of chlorine are Cl-35 and Cl-37
75.8 % of all Cl atoms have a mass of 35 amu (Cl-35)
24.2 % of all Cl atoms have a mass of 37 amu (Cl-37)
Therefore, the average atomic mass of Cl is (.758) (35 amu) + (.242) (37 amu) = 35.5 amu
Average atomic mass = (mass X relative abundance)isotope 1 + (mass X relative abundance)isotope 2
+ (mass X relative abundance)isotope 3+ (mass X relative abundance)isotope 4
The actual equation used in calculation would be:
𝑚𝑎𝑣𝑔 = 𝑚𝑖𝑠𝑜𝑡𝑜𝑝𝑒1 (%)𝑖𝑠𝑜𝑡𝑜𝑝𝑒1 + 𝑚𝑖𝑠𝑜𝑡𝑜𝑝𝑒2 (%)𝑖𝑠𝑜𝑡𝑜𝑝𝑒2 + 𝑚𝑖𝑠𝑜𝑡𝑜𝑝𝑒3 (%)𝑖𝑠𝑜𝑡𝑜𝑝𝑒3
+ 𝑚𝑖𝑠𝑜𝑡𝑜𝑝𝑒4 (%)𝑖𝑠𝑜𝑡𝑜𝑝𝑒4
---------------------------------------------------------------------------------------------------------The average atomic mass of an element is often close to the mass of the most
abundant isotope.
Example
The average atomic mass of H is 1.00794 amu and it is calculated from H-1, H-2, and H-3.
Therefore, H-1 is the most abundant isotope for hydrogen since 1 amu is the closest value to
1.00794 amu.
---------------------------------------------------------------------------------------------------------How isotopes are represented.
238
92𝑈
or
U-238
or
Uranium-238
These are three common ways isotopes are
listed.
The mass number is 238.
The atomic number is 92.
The symbol for Uranium is U.
For Problems 1- 9 circle the correct answer.
1. Which pairs of atoms are isotopes of the same element?
a)
16
17
8O and 8O
b)
122
122
50Sn and 52Te
39
39
c) 17Cl and 19K
d)
119
121
50Sn and 50Sn
Identify the most abundant isotope for each element.
2. Fe-54
3. O-16
Fe-56
O-17
4. Mg-24
Fe-57
O-18
Mg-25
Mg-26
5. K-39
K-40
K-41
6. S-32
S-33
S-34
7. Ar-36
8. Hg-196
9. Kr-78
Ar-38
S-36
Ar-40
Hg-198
Kr-80
Fe-58
Hg-199
Kr-82
Hg-202
Kr-83
Kr-84
Hg-204
Kr-86
-------------------------------------------------------------------------------------------------------_______10. All the isotopes of a given atom have:
a) The same mass number and the same atomic number
b) The same mass number but different atomic numbers
c) Different mass numbers but the same atomic number
d) Different mass numbers and different atomic numbers
_______11. What is the total number of protons in the nucleus of an atom of potassium-42?
a) 15
b) 19
c) 39
d) 42
_______12. An atom of carbon-12 and an atom of carbon-14 differ in:
a) atomic number b) mass number c) nuclear charge d) number of electrons
_______13. Hydrogen has three isotopes with mass numbers of 1, 2, and 3 and has an average
atomic mass of 1.00794 amu. This information indicates that:
a) Equal numbers of each isotope are present.
b) More isotopes have an atomic mass of 2 or 3 than of 1.
c) More isotopes have an atomic mass of 1 than of 2 or 3.
d) Isotopes have only an atomic mass of 1.
________14. The atomic mass of an element is calculated using the:
a) Atomic number and the ratios of its naturally occurring isotopes.
b) Atomic number and the half-lives of each of its isotopes.
c) Masses and the ratios of its naturally occurring isotopes.
d) Masses and the half-lives of each of its isotopes
For Problems 15- 16 draw the isotopes of the following elements as shown on page
one.
15. The isotopes of oxygen are O-16, O-17 & O-18
O-16
O-17
16. The isotopes of boron are B-10 and B-11.
B-10
B-11
O-18
17. Element Z has two natural isotopes. The isotope with mass 10.012 amu has a percentage
abundance of 19.91%. The isotope with mass 11.009 amu has a percentage abundance of
80.09%. Calculate the average atomic mass of this element using the following format.
Step
Step
Step
Step
1)
2)
3)
4)
Identify the unknown quantity you are solving for. (Let X=)
Show the equation used.
Show the equation with known number values inserted.
Show the answer with the correct units.
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e) Based on the calculated average atomic mass, which element is most likely Element Z?
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Base your answers to questions 18 through 19 on the data table below, which shows three
isotopes of neon.
Isotope
Ne
Ne
22
Ne
20
21
Atomic mass
Percent Natural
(atomic mass units)
Abundance
19.99
90.9%
20.99
0.3%
21.99
8.8%
18. In terms of atomic particles, state one difference between these three isotopes of neon.
Refer to the diagram on page one if needed.
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19. Based on the atomic masses and the natural abundances shown in the data table show a
correct numerical setup for calculating the average atomic mass of neon using the same format
as question 17.
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