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1
Chapter 4 Atoms and Elements
4.5Isotopes and Atomic Mass
Basic Chemistry Copyright © 2011 Pearson Education, Inc.
2
Isotopes
• are atoms of the same element that have different mass numbers
• have the same number of protons, but different numbers of neutrons
Isotopes
Basic Chemistry Copyright © 2011 Pearson Education, Inc.
3
A nuclear symbol
• represents a particular atom of an element
• gives the mass number in the upper left corner and the atomic number in the lower left corner
Example: An atom of magnesium with an atomic number of 12 and a mass number of 24 has the following atomic symbol:
Nuclear Symbol
Basic Chemistry Copyright © 2011 Pearson Education, Inc.
4
• The nuclear symbol indicates the number of protons (p+), neutrons, (n), and electrons (e-) in a particular atom. 16 31 65
O P Zn 8 15 30
8 p+ 15 p+ 30 p+
8 n 16 n 35 n 8 e- 15 e- 30 e-
Information from Nuclear Symbols
Basic Chemistry Copyright © 2011 Pearson Education, Inc.
5
Naturally occurring carbon consists of three isotopes, 12C, 13C, and 14C. State the number of protons, neutrons, and electrons in each of the following.
12C 13C 14C 6 6 6
protons ______ ______ ______
neutrons ______ ______ ______
electrons ______ ______ ______
Learning Check
Basic Chemistry Copyright © 2011 Pearson Education, Inc.
6
Naturally occurring carbon consists of three isotopes, 12C, 13C, and 14C. State the number of protons, neutrons, and electrons in each of the following.
12C 13C 14C 6 6 6
protons 6 p+ 6 p+ 6 p+
neutrons 6 n 7 n 8 n
electrons 6 e- 6 e- 6 e-
Solution
Basic Chemistry Copyright © 2011 Pearson Education, Inc.
7
Write the nuclear symbols for atoms with the following subatomic particles:
A. 8 p+, 8 n, 8 e- ___________
B. 17 p+, 20 n, 17e- ___________
C. 47 p+, 60 n, 47 e- ___________
Learning Check
Basic Chemistry Copyright © 2011 Pearson Education, Inc.
8
Write the nuclear symbols for atoms with the following subatomic particles:
16OA. 8 p+, 8 n, 8 e- 8
B. 17p+, 20 n, 17e- 37Cl 17
C. 47p+, 60 n, 47 e- 107Ag 47
Solution
Basic Chemistry Copyright © 2011 Pearson Education, Inc.
9
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. 15X 15X 8 7
B. 12X 14X 6 6
C. 15X 16X 7 8
Basic Chemistry Copyright © 2011 Pearson Education, Inc.
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Solution
1. B. 12X 14X
6 6
Both nuclear symbols represent isotopes of carbon with six protons each, but one has 6 neutrons and the other has 8.
2. C. 15X 16X
7 8
The atom of nitrogen (7) and the atom of oxygen (8) each have 8 neutrons.
Basic Chemistry Copyright © 2011 Pearson Education, Inc.
11
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 12C
• is not the same as the mass number
11Na
22.99
Basic Chemistry Copyright © 2011 Pearson Education, Inc.
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Isotopes of Magnesium
In naturally occurring magnesium, there are three isotopes.
Basic Chemistry Copyright © 2011 Pearson Education, Inc.
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Examples of Isotopes and Their Atomic Masses
Most elements have two or more isotopes that contribute to the atomic mass of that element.
Basic Chemistry Copyright © 2011 Pearson Education, Inc.
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Calculating Atomic Mass
The calculation for atomic mass requires the
• percent(%) abundance of each isotope
• atomic mass of each isotope of that element
• sum of the weighted averages
mass isotope(1) x (%) + mass isotope(2) x (%) + …
100 100
Basic Chemistry Copyright © 2011 Pearson Education, Inc.
15
Atomic Mass of Magnesium
The atomic mass of Mg• is due to all the Mg
isotopes• is a weighted average• is not a whole number
Basic Chemistry Copyright © 2011 Pearson Education, Inc.
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Calculating Atomic Mass Isotope Mass Abundance
24Mg = 23.99 amu x 78.70/100 = 18.88 amu25Mg = 24.99 amu x 10.13/100 = 2.531 amu26Mg = 25.98 amu x 11.17/100 = 2.902 amu
Atomic mass (average mass) Mg = 24.31 amu
Mg24.31
Basic Chemistry Copyright © 2011 Pearson Education, Inc.
17
Using the periodic table, specify the atomic mass of
each element:
A. calcium __________
B. aluminum __________
C. lead __________
D. barium __________
E. iron __________
Learning Check
Basic Chemistry Copyright © 2011 Pearson Education, Inc.
18
Using the periodic table, specify the atomic massof each element:
A. calcium 40.08 amu
B. aluminum 26.98 amu
C. lead 207.2 amu
D. barium 137.3 amu
E. iron 55.85 amu
Solution
Basic Chemistry Copyright © 2011 Pearson Education, Inc.
19
Atomic Mass for Cl
• The atomic mass of chlorine is the weighted average of two isotopes 35Cl and 37Cl.
Basic Chemistry Copyright © 2011 Pearson Education, Inc.
Chlorine, with two naturally occurring isotopes, has an atomic mass of 35.45.
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35Cl has atomic mass 34.97 (75.76%) and 37C has
atomic mass 36.97 (24.24%).
• Use the atomic mass and percent of each isotope to calculate the contribution of each isotope to the weighted average. 34.97 x 75.76 = 26.49 amu 10035.97 x 24.24 = 8.962 amu
100 35.45 amu
• The sum is the weighted average or atomic mass of Cl. 35.45 amu
Calculating Atomic Mass for Cl
Basic Chemistry Copyright © 2011 Pearson Education, Inc.
21
Gallium is an element found in lasers used in compact disc players. In a sample of gallium, there is 60.11% of 69Ga (atomic mass 68.93) atoms and 39.89% of 71Ga (atomic mass 70.92) atoms.
What is the atomic mass of gallium?
Learning Check
Basic Chemistry Copyright © 2011 Pearson Education, Inc.
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69Ga
68.93 amu x 60.11 = 41.43 amu (from 69Ga)
10071Ga
70.92 amu x 39.89 = 28.29 amu (from 71Ga)
100
Atomic mass Ga = 69.72 amu
Solution
31
Ga
69.72
Basic Chemistry Copyright © 2011 Pearson Education, Inc.