Cumberland High School · Chemistry
Name ________________________________
Period ________ Date ___ /___ /___
1 · Atomic Particles
PRACTICE 1: AVERAGE ATOMIC MASS (CP)
Elements come in a variety of isotopes, meaning they are made up of the same atoms with the same atomic
number but different atomic masses. These atoms differ in the number of neutrons.
The average atomic mass is the weighted average of all the isotopes of an element.
% abundance isotope #1
100
Average Atomic Mass (AAM) = (mass isotope #1)(
% abundance isotope # 2
) + (mass isotope #2)(
100
% abundance
Relative abundance =
Example:
100
A sample of cesium is 75% 133Cs, 20% 132Cs, and 5% 134Cs. What is the
average atomic mass?
Answer:
75
133 amu 100 =
99.75 amu
20
132 amu 100 =
26.4 amu
5
134 amu 100 =
6.7 amu
.
.
.
TOTAL = 132.85 amu = Average Atomic Mass
DIRECTIONS: Determine the average atomic mass of the following mixtures of isotopes
1.
80% 127I, 17% 126I, 3% 128I
2.
50% 197Au, 50% 198Au
3.
15% 55Fe, 85% 56Fe
)+
4. Natural uranium mined from the earth contains the isotopes uranium-238, uranium-235 and uranium-234. The
majority (99.2745%) of all the atoms in natural uranium are uranium-238. Most of the remaining
atoms (0.72%) are uranium-235, and a slight trace (0.0055%) are uranium-234. What is the atomic mass of
uranium?
5. An element has three naturally occurring isotopes with the following masses and abundances:
ISOTOPIC MASS (amu)
PERCENTABUNDANCE (%)
27.977
92.21
28.976
4.70
29.974
3.09
Calculate the atomic mass of this element. What is the identity of this element?
6. Sulfur has four known isotopes. Sulfur-32 has a mass of 31.972 amu and relative abundance of 0.95002.
Sulfur-33 has a mass of 32.971 amu and a percent abundance of 0.76%. Sulfur-34 has a mass of 33.967 amu
and a relative abundance of 0.0422. Finally, sulfur-36 has a mass of 35.967 amu and is found in 0.014%
abundance in nature. What is the atomic mass of sulfur?
Cumberland High School · Chemistry
Name ________________________________
Period ________ Date ___ /___ /___
1.3 · Atomic Particles
PRACTICE 2: AVERAGE ATOMIC MASS (CP)
1. The element X has three naturally occurring isotopes. The isotopic mass (amu) and percent abundances (%)
of the isotopes are given below. What is the relative abundance for the 16X isotope?
Isotope
15
X
X
17
X
16
Isotopic Mass
(amu)
15.33
18.11
17.26
% Abundance
(%)
28.60
13.30
2. Find the percent abundances of the two known isotopes of silver if the atomic mass of silver is 107.868 amu,
isotope #1 has a mass of 106.905 amu and isotope #2 has a mass of 108.905 amu.
3. A new element with an average atomic mass of 2.792amu has been discovered and is known to have two
naturally occurring isotopes. Isotope 1 has an atomic mass of 2.814amu and isotope 2 has an atomic mass of
2.710amu. Calculate the relative abundances of each isotope.
4. Chlorine has two stable isotopes. Chlorine-35 has an atomic mass of 34.968852amu and chlorine-37 has an
atomic mass of 36.965903amu. Calculate the percent abundances of each of these isotopes.
5. Antimony has two stable isotopes, 121Sb and 123Sb, with masses of 120.9038 amu and 122.9042 amu,
respectively. Calculate the percent abundances of each of these isotopes.