Isotopes and Average Atomic Mass

Unit 3—Atomic Theory and the Periodic Table
Name
Pre-AP Chemistry: Worksheet #3.3
Class Period
Isotopes and Average Atomic Mass
1.
Name two ways that isotopes of an element differ.
2.
What data must you know about the isotopes of an element to calculate the atomic mass of the element?
3.
The four isotopes of lead are shown below, each with its percent by mass abundance and the composition of its
nucleus. Using these data, calculate the approximate atomic mass of lead.
82p
82p
82p
82p
122n
124n
125n
126n
1.37%
26.26%
20.82%
51.55%
4.
Lithium has two naturally occurring isotopes. Lithium-6 has an atomic mass of 6.015 amu; lithium-7 has an
atomic mass of 7.016 amu. The atomic mass of lithium is 6.941 amu. What is the percentage of naturally
occurring lithium-7? (Make Li-6’s percent abundance x and Li-7’s percent abundance 1-x)
5.
What is the atomic mass of an element, and how does it differ from the mass number?
6.
Imagine you are standing on top of a boron-11 nucleus. Describe the numbers and kinds of subatomic particles
you would see looking down into the nucleus, and those you would see looking out from the nucleus.
Unit 3—Atomic Theory and the Periodic Table
7.
The element boron, B, has an atomic mass of 10.81 amu according to the periodic table. However, no single
atom of boron has a mass of exactly 10.81 amu. How can you explain this difference?
The peri
odic table reports the average atomic mass, which is a weighted
average of all isotopes of boron.
8.
A certain element has two isotopes. One isotope, which has a percent abundance of 72.15% has a mass of
84.9118 amu. The other isotope has a mass of 86.9092 amu.
a.
Calculate the average atomic mass of this element to three decimal places
84.9118 ×
72.15
100
86.9092 ×
27.85
100
= 61.264 amu
61.264 amu + 24.2amu = 85.468 amu
b.
ANS.
Identify the element.
Rubidium
9. Three isotopes of uranium occur in nature. If the relative atomic masses and abundances of each of these
isotopes are as follows, calculate the average atomic mass of uranium to three decimal places.
Isotope
Atomic Mass
Percent Abundance
uranium-234
234.0409amu
0.0055%
uranium-235
235.0439 amu
0.7200%
uranium-238
238.0508 amu
99.2745%
234.0409 amu ×
238.0508 ×
0.0055
100
99.2745
=
100
= 0.0129
236.324 amu
0.0129 amu + 1.692 amu +236.324 amu = 238.029 amu
ANS.

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