TEKS 6D: Use isotopic composition to calculate average atomic mass of an element.
Atomic mass
The atomic mass of an element is a weighted
average mass of the atoms in a naturally occurring
sample of the element.
•A weighted average mass reflects both the mass
and the relative abundance of the isotopes as they
occur in nature.
2. Contrast How does a weighted average differ from an arithmetic mean?
TEKS 6D: Use isotopic composition to calculate average atomic mass of an element.
How do you calculate the average atomic
mass of an element?
To determine the atomic mass of an element you
must know:
•the number of stable isotopes of the element
•the mass of each isotope
•the natural percent abundance of each isotope
TEKS 6D: Use isotopic composition to calculate average atomic mass of an element.
Calculating atomic mass
To calculate the atomic mass of an element:
• Multiply the mass of each isotope by its natural
abundance; express the percentage as a decimal.
• Examples: 1.5% = 0.015 & 23.5% = 0.235
• Add the products.
• The resulting sum is the weighted average mass of
the atoms of the element as they occur in nature.
TEKS 6D: Use isotopic composition to calculate average atomic mass of an element.
Sample problem: Calculating atomic mass
Element X has two naturally occurring isotopes.
• The isotope with a mass of 10.012 amu (10X) has a
relative abundance of 19.91 percent.
• The isotope with a mass of 11.009 amu (11X) has a
relative abundance of 80.09 percent.
Calculate the atomic mass of element X.
Isotopes’ Average Atomic Mass
1. Calculate average atomic mass of the following mixtures of isotopes. PSYW
Example: A sample of cesium is 75% cesium-133, 20% cesium-132, and 5% cesium-134. What is
the average atomic mass?
Answer: 0.75 x 133 = 99.75
0.20 x 132 = 26.4
0.05 x 134 = 6.7
Total = 132.85 amu = average atomic mass
a. 15% iron-55, 85% iron-56
b. 98% carbon-12, 2% carbon-14
c. 95% nitrogen-14, 3% nitrogen-15, 2% nitrogen-16
d. 80% iodine-127, 17% iodine-126, 3% iodine-128
e. 50% gold-197, 50% gold-198
f.
99.98% hydrogen-1, 0.015% hydrogen-2
Name:____________________________ Period:________
Isotope Practice
1. Here are three isotopes of an element:
12C
6
13C
6
14C
6
a. The element is: __________________
b. The number 6 refers to the _________________________
c. The numbers 12, 13, and 14 refer to the ________________________
d. How many protons and neutrons are in the first isotope? _________________
e. How many protons and neutrons are in the second isotope? _________________
f. How many protons and neutrons are in the third isotope? _________________
2. Complete the following chart:
Isotope name atomic #
mass #
# of
protons
19
17
# of
neutrons
# of
electrons
18
19
8
Charge
Symbol
notation
-2
uranium-235
92
uranium-238
0
5
10
boron-11
+3
2
DIRECTIONS: For the following problems, show your work! Be thorough.
3. Naturally occurring europium (Eu) consists of two isotopes was a mass of 151 and 153. Europium151 has an abundance of 48.03% and Europium-153 has an abundance of 51.97%. What is the
average atomic mass of europium?
4. Strontium consists of four isotopes with masses of 84 (abundance 0.50%), 86 (abundance of 9.9%),
87 (abundance of 7.0%), and 88 (abundance of 82.6%). Calculate the average atomic mass of
strontium.
46
47
48
5. Titanium has five common isotopes: Ti (8.0%), Ti (7.8%), Ti (73.4%),
What is the average atomic mass of titanium?
49Ti
(5.5%), 50Ti (5.3%).
6. Calculate the average atomic mass of copper if copper-63 is 69.17% abundant and copper-65 is
30.83% abundant.
7. Boron exists in two isotopes, boron-10 and boron-11. Based on the average atomic mass on the
periodic table, which isotope should be more abundant?
8. Lithium-6 is 4% abundant and lithium-7 is 96% abundant. What is the average mass of lithium?
9. Iodine is 80% 127I, 17% 126I, and 3% 128I. Calculate the average atomic mass of iodine.
10. The natural abundance for boron isotopes is 19.9% 10B and 80.1% 11B . Calculate boron’s average
atomic mass.
11. Hydrogen is 99% 1H, 0.8% 2H, and 0.2% 3H. Calculate its average atomic mass.
85
87
12. Rubidium is a soft, silvery-white metal that has two common isotopes, Rb and Rb. If the
85
87
abundance of Rb is 80.2% and the abundance of Rb is 19.8%, what is the average atomic mass of
rubidium?
13. What is the average atomic mass of hafnium if, out of every 100 atoms, 5 have a mass of 176, 19 have
a mass of 177, 27 have a mass of 178, 14 have a mass of 179, and 35 have a mass of 180.0?