Isotopes and Average Atomic Mass and Ions
Name________________________________
Period_________Date___________________
Elements come in a variety of isotopes, meaning they are made up of atoms with the same atomic number,
but different atomic masses. These atoms differ in the number of neutrons. The average atomic mass if
the weighted average of all the isotopes of an element.
Example: A sample of cesium is 75% 133
55 Cs , 20.%
What is the average atomic mass?
Answer:
132
55 Cs ,
and 5%
134
55 Cs .
.75 x 133 = 99.75
.20 x 132 = 26.4
.05 x 134 = 6.7 +
Total = 132.85 amu = average atomic mass
Determine the average atomic mass of the following mixtures of isotopes.
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1. 80.%
127
53 I ,
17%
126
53 I ,
3%
128
53 I
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198
2. 50.% 197
79 Au , 50.% 79 Au
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55
3. 15% 26
Fe , 85% 56
26 Fe
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4. 99% 11 H , 0.8% 21 H , 0.2% 31H
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5. 95% 147 N , 3% 157 N , 2% 167 N
6. 98%
12
6C ,
2%
14
6C
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7. Uranium has 3 isotopes with the following relative abundances: Uranium-234 (0.0058%),
U-235 (0.71%), and U-238 (99.23%). Calculate the average atomic mass of uranium:
8. Fill in the blanks in the following table using the periodic table when necessary. If not otherwise noted,
assume the elements to be neutral atoms.
Symbol
A
Protons
Electrons
Neutrons Mass Number
42 Ca +2
20
B
59
141
C
60
D
24
E
Atomic Number
44
45
34 P-3
15
F
22
28
9. Write the hyphen notation for each of the six isotopes listed above:
A. _____________________
B. _____________________
C. ____________________
D. _____________________
E. _____________________
F. ____________________
10. Indicate the charge (+1, -1, etc) on the ions that are formed in the following situations. Record the
resultant ion as a cation or anion.
Charge
Cation/Anion
a. Lithium loses 1 electron
_____________
_______________
b. Oxygen gains 2 electrons
_____________
_______________
c. Magnesium loses 2 electrons
_____________
_______________
d. Nitrogen gains 3 electrons
_____________
_______________