57 / 123
EXAMPLE
Problem 3 (Continued)
2
SOLVE FOR THE UNKNOWN
6
X: mass contribution
=
(mass)(percent abundance)
Calculate
6
X’s contribution.
mass contribution
=
(6.015 amu)(0.0759)
=
0.4565 amu
Substitute mass
=
6.015 amu
and abundance
=
0.0759.
7
X: mass contribution
=
(mass)(percent abundance)
Calculate
7
X’s contribution.
mass contribution
=
(7.016 amu)(0.9241)
=
6.483 amu
Substitute mass
=
7.016 amu
and abundance
=
0.9241.
atomic mass of X
=
(0.4565 amu
+
6.483 amu)
=
6.939 amu
Total the mass contributions to
find the atomic mass.
The
element
with a mass nearest 6.939 amu is
lithium (Li).
Identify the element using the
periodic table.
3
EVALUATE THE ANSWER
The result agrees with the atomic mass given in the periodic table. The isotope masses have
four significant figures, so the atomic mass needs to have four significant figures.
PRACTICE
Problems
17.
Boron (B) has two naturally occurring isotopes: boron-10 (abundance
=
19.8%, mass
=
10.013 amu)
and boron-11 (abundance
=
80.2%, mass
=
11.009 amu). Calculate the atomic mass of boron.
18. CHALLENGE
Nitrogen has two naturally occurring isotopes, N-14 and N-15. Its atomic mass is
14.007. Which isotope is more abundant? Explain your answer.
Go online to follow your personalized learning path to review, practice,
and reinforce your understanding.
Summary
• The atomic number of an atom
is given by its number of
protons. The mass number of
an atom is the sum of its
neutrons and protons.
• Atoms of the same element
with different numbers of
neutrons are called isotopes.
• The atomic mass of an element
is a weighted average of the
masses of all of its naturally
occurring isotopes.
Demonstrate Understanding
19.
Recall
Which subatomic particle identifies an atom
as belonging to a particular element?
20.
Describe
the difference between atomic number
and mass number.
21.
Explain
how isotopes are related to why atomic
masses are not whole numbers.
22.
Calculate
Copper has two isotopes Cu-63
(abundance
=
69.2%, mass
=
62.930 amu) and
Cu-65 (abundance
=
30.8%, mass
=
64.928 amu).
Calculate the atomic mass of copper.
23.
Explain
how patterns in the periodic table can
provide evidence as to how the structures of
elements differ.
Check Your Progress
ADDITIONAL PRACTICE
Lesson 3 • How Atoms Differ
97