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Chapter 3
Atoms and Elements
3.5Isotopes and Atomic Mass
1
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc.
Isotopes
Isotopes• are atoms of the same element• have different mass numbers• have same number of protons but different numbers of
neutrons
2
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc.
Atomic Symbols and Subatomic Particles
3
Examples of number of subatomic particles for atoms:
Atomic symbol
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-
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc.
Atomic Symbols for Isotopes of Magnesium
4
Mass Numbers
€
1224 Mg
€
1225 Mg
€
1226 Mg
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc.
Atomic Symbols for Isotopes of Magnesium
5
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc.
Learning Check
Naturally occurring carbon consists of three isotopes: 12C, 13C, and 14C. State the number of protons, neutrons, and electrons in each of the following:
6
Isotope 12C 13C 14C
protons
neutrons
electrons
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc.
Solution
Naturally occurring carbon consists of three isotopes: 12C, 13C, and 14C. State the number of protons, neutrons, and electrons in each of the following:
7
Isotope 12C 13C 14C
protons 6 6 6
neutrons 6 7 8
electrons 6 6 6
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc.
Learning Check
Write the atomic symbols for atoms with the following subatomic particles:
A. 8 protons 8 neutrons 8 electrons
B. 17 protons 20 neutrons 17 electrons
C. 47 protons 60 neutrons 47 electrons
8
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc.
Solution
Write the atomic symbols for atoms with the following subatomic particles:
A. 8 protons 8 neutrons 8 electrons
B. 17 protons 20 neutrons 17 electrons
C. 47 protons 60 neutrons 47 electrons
9
€
816O
€
1737Cl
€
47107 Ag
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc.
Learning Check
1. Which of the pairs are isotopes of the same element?
2. In which of the pairs do both atoms have 8 neutrons?
A.
B.
C.
10
€
815 X
€
715 X
€
612 X
€
614 X
€
715 X
€
816 X
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc.
Learning Check
1. Which of the pairs are isotopes of the same element?
B.
2. In which of the pairs do both atoms have 8 neutrons?
C.
11
€
612 X
€
614 X
€
715 X
€
816 X
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc.
Atomic Mass
Atomic mass is the• weighted average of all
naturally occurring isotopes of that element
• number on the periodic table below the chemical symbol with two decimal places
12
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc.
Calculating Atomic Mass
To calculate atomic mass,• use an experimental percent abundance of each isotope
of the element • multiply the percent abundance by the atomic mass of
that isotope• sum the total mass of each isotope
13
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc.
Calculating Atomic Mass
To calculate atomic mass of Cl, use experimental data for both isotopes of Cl:
Isotope atomic mass x % abundance
35Cl 34.97 amu x 75.76 = 26.49 amu 100
37Cl 36.97 amu x 24.24 = 8.962 amu 100Atomic mass of Cl = 35.45 amu
14
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc.
The Atomic Mass of Some Elements15
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc.
Learning Check
Lithium consists of two naturally occurring isotopes, 6Li and 7Li. Use the periodic table to predict which isotope is the most prevalent one.
Periodic table atomic mass of lithium is 6.941 amu.
16
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc.
Solution
Lithium consists of two naturally occurring isotopes, 6Li and 7Li. Use the periodic table to predict which isotope is the most prevalent one.
Periodic table atomic mass of lithium is 6.941 amu.6Li and 7Li
Since the total atomic mass of 7Li is closer to 7, the mass number of 7Li, the most prevalent isotope is 7Li.
17
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc.
Learning Check
Gallium is an element found in lasers used in compact disc players. In a sample of gallium, there is 60.10% of 69Ga (atomic mass 68.926) atoms and 39.90% of 71Ga (atomic mass 70.925) atoms.
What is the atomic mass of gallium?
18
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc.
Solution
69Ga 68.926 amu x 60.10 = 41.42 amu (from 69Ga) 10071Ga70.925 amu x 39.90 = 28.30 amu (from 71Ga) 100 Atomic mass Ga = 69.72 amu
19
31
Ga
69.72
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc.