Transcript 5. Isotopes and Atomic Mass

General, Organic, and
Biological Chemistry
Fourth Edition
Karen Timberlake
Chapter 3
Atoms and Elements
3.5
Isotopes and
Atomic Mass
© 2013 Pearson Education, Inc.
Lectures
Isotopes
Isotopes
 are atoms of the same element that have different
mass numbers.
 have the same number of protons but different
numbers of neutrons.
 can be distinguished by atomic symbols.
© 2013 Pearson Education, Inc.
Chapter 3, Section 5
2
Isotopes and Mass and Atomic
Symbols
Since each isotope of an element has a different
number of neutrons, each isotope’s mass number will
be different. We write these as atomic symbols:
 Mass numbers are in the upper left corner.
 Atomic numbers are in the lower left corner.
Example: An atom of sodium with atomic number 11
and a mass number 23 has the following atomic
symbol:
23
mass number
11
atomic number
Na
© 2013 Pearson Education, Inc.
Chapter 3, Section 5
3
Atomic Symbols
For an atom, the atomic symbol gives the number of
 protons (p+),
 neutrons (n), and
 electrons (e–).
16
8
O
8 p+
8n
8 e–
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31
15
P
15 p+
16 n
15 e–
Chapter 3, Section 5
65
30
Zn
30 p+
35 n
30 e–
4
Learning Check
Naturally occurring carbon consists of three isotopes:
C-12, C-13, and C-14. State the number of protons,
neutrons, and electrons in each of the following.
______
13
6
______
14
6
______
Neutrons ______
______
______
Electrons ______
______
______
12
6
Protons
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C
C
Chapter 3, Section 5
C
5
Solution
12
6
C
C
14
6
C
6 p+
6 p+
Neutrons 6 n
7n
8n
Electrons 6 e–
6 e–
6 e–
Protons
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6 p+
13
6
Chapter 3, Section 5
6
Learning Check
Write the nuclear symbols for atoms with the
following subatomic particles:
1. 9 p+, 10 n, 9 e–
___________
2. 17p+, 20 n, 17 e–
___________
3. 47p+, 60 n, 47 e–
___________
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Chapter 3, Section 5
7
Solution
Write the nuclear symbols for atoms with the
following subatomic particles:
19
1. 9 p+, 10 n, 9 e–
2. 17p+, 20 n, 17 e–
3. 47p+, 60 n, 47 e–
© 2013 Pearson Education, Inc.
9
F
37
17
Cl
107
47
Ag
Chapter 3, Section 5
8
Learning Check
1. Which of the following pairs are isotopes of the
same element?
2. In which of the following pairs do both atoms have
8 neutrons?
A.
12
6
B.
12
6
C.
15
7
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X
15
7
Z
14
6
Z
16
8
X
Z
Z
Chapter 3, Section 5
9
Solution
1. Which of the following pairs are isotopes of the
same element?
B.
Same number of protons,
12
14
6Z
6Z
different number of
neutrons.
2. In which of the following pairs do both atoms have
8 neutrons?
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Chapter 3, Section 5
10
Isotopes of Magnesium
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Chapter 3, Section 5
11
Isotopes of Sulfur
A sample of naturally occurring sulfur contains
several isotopes with the following abundances:
Isotope
32
16
S
% Abundance
95.02
33
16
S
0.75
34
16
S
4.21
36
16
S
0.02
© 2013 Pearson Education, Inc.
Chapter 3, Section 5
12
Atomic Mass
The atomic mass of an element
 is listed below the symbol of each
element on the periodic table.
 gives the mass of an “average” atom of
each element compared to C-12.
 is not the same as the mass number.
 is calculated using a weighted average.
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Chapter 3, Section 5
Na
22.99
13
Some Elements and Their Atomic
Mass
Most elements have two or more isotopes that
contribute to the atomic mass of that element.
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Chapter 3, Section 5
14
Atomic Mass for Cl
The atomic mass of chlorine is
 based on all naturally
occurring Cl isotopes.
 not a whole number.
 the weighted average
of the Cl-35 and Cl-37
isotopes.
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Chapter 3, Section 5
15
Calculating the Atomic Mass for Cl
To calculate the atomic mass of an element, we
need to know the percent abundance of each
isotope and its mass.
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Chapter 3, Section 5
16
Atomic Mass for Mg
The atomic mass of Mg is
 based on all naturally
occurring Mg isotopes.
 not a whole number.
 the weighted average
of the Mg-24, Mg-25, and
Mg-26 isotopes.
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Chapter 3, Section 5
17
Calculating Atomic Mass for Mg
To calculate the atomic mass of Mg, we use a weighted
average because the atomic mass is calculated from
the percent abundance of each isotope and its mass.
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Chapter 3, Section 5
18
Learning Check
Using the periodic table, give the atomic mass for each
of the following elements.
1. calcium
__________
2. aluminum
__________
3. lead
__________
4. barium
__________
5. iron
__________
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Chapter 3, Section 5
19
Solution
Using the periodic table, give the atomic mass for each
of the following elements.
1. calcium
40.08 amu
2. aluminum
26.98 amu
3. lead
207.2 amu
4. barium
137.3 amu
5. iron
55.85 amu
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Chapter 3, Section 5
20
Learning Check
The atomic mass of chlorine is 35.45 amu. Based on
this information, which of the following isotopes of Cl is
present in the largest amount?
A. Cl-35
B. Cl-37
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Chapter 3, Section 5
21
Solution
The atomic mass of chlorine is 35.45 amu. Based on
this information, which of the following isotopes of Cl is
present in the largest amount?
Solution: A. Cl-35
Since 35.45 amu is very close to the mass number of
35, the isotope Cl-35 is the most prevalent isotope in a
naturally occurring sample of chlorine atoms.
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Chapter 3, Section 5
22