Chapter 3 Atoms and Elements 3.5 Isotopes and Atomic Mass 1 Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition

Chapter 3

Atoms and Elements

3.5Isotopes and Atomic Mass

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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

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Atomic Symbols and Subatomic Particles

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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-


Atomic Symbols for Isotopes of Magnesium

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Mass Numbers

€

1224 Mg

€

1225 Mg

€

1226 Mg


Atomic Symbols for Isotopes of Magnesium

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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:

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Isotope 12C 13C 14C

protons

neutrons

electrons


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:

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Isotope 12C 13C 14C

protons 6 6 6

neutrons 6 7 8

electrons 6 6 6


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

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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

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€

816O

€

1737Cl

€

47107 Ag


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.

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€

815 X

€

715 X

€

612 X

€

614 X

€

715 X

€

816 X


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.

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€

612 X

€

614 X

€

715 X

€

816 X


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

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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

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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

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The Atomic Mass of Some Elements15


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.

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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.

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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?

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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

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31

Ga

69.72


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Chapter 3 Atoms and Elements 3.5 Isotopes and Atomic Mass 1 Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition