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Chlorine exists naturally as a mixture of about 76% chlorine-35 and 24% chlorine-37.
It has an atomic mass of 35.453 amu. Because atomic mass is a weighted average, the
chlorine-35 atoms, which exist in greater abundance than the chlorine-37 atoms, have a
greater effect in determining the atomic mass. The atomic mass of chlorine is calculated
by multiplying each isotope’s percent abundance by its atomic mass and then adding
the products. The process is similar to calculating an average grade. You can calculate
the atomic mass of any element if you know the number of naturally occurring isotopes,
their masses, and their percent abundances.
Isotope abundances
Analyzing an element’s
mass can indicate the most abundant isotope for
that element. For example, fluorine (F) has an
atomic mass that is extremely close to 19 amu. If
fluorine had several fairly abundant isotopes, its
atomic mass would not likely be so close to a
whole number. Thus, you might conclude that
all naturally occurring fluorine is probably in
the form of fluorine-19
(
19
9
F
)
. Indeed, 100% of
naturally occurring fluorine is in the form of
fluorine-19. While this type of reasoning
generally works well, it is not foolproof. Consider
bromine (Br). It has an atomic mass of 79.904 amu.
With a mass so close to 80 amu, it seems likely
that the most common bromine isotope would be
bromine-80. However, bromine’s two isotopes are
bromine-79 (78.918 amu, 50.69%) and bromine-81
(80.917 amu, 49.31%). There is no bromine-80 isotope.
Figure 19
shows one of the major production sites of bromine, located in the Dead Sea area.
Figure 19
Bromine is extracted from sea water and salt lakes.
The Dead Sea area of Israel is one of the major bromine
production sites in the world. Applications of bromine include
microbe and algae control in swimming pools. It is also used
in medicines, oils, paints, pesticides, and flame retardants.
Get It?
Infer
how atomic mass changes from element to element as you move horizontally
across the periodic table.
EXAMPLE
Problem 3
CALCULATE ATOMIC MASS
Given the data in the table, calculate the atomic mass of unknown
Element X. Then, identify the unknown element, which is used medically to treat some mental
disorders.
1
ANALYZE THE PROBLEM
Calculate the atomic mass and use the periodic table to confirm.
Known
Unknown
6
X: mass
=
6.015 amu
atomic mass of X
=
? amu
abundance
=
7.59%
=
0.0759
7
X: mass
=
7.016 amu
element X
=
?
abundance
=
92.41%
=
0.9241
Isotope Abundance for Element X
Isotope Mass (amu)
Percent
Abundance
6
X
6.015
7.59%
7
X
7.016
92.41%
96
Module 3 • The Structure of the Atom