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Chapter 4 Atoms and Elements

4.5Isotopes and Atomic Mass

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Isotopes

• are atoms of the same element that have different mass numbers

• have the same number of protons, but different numbers of neutrons

Isotopes

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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

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• 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

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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

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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

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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

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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

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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

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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.

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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

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Isotopes of Magnesium

In naturally occurring magnesium, there are three isotopes.

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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

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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

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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.

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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.

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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.

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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

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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

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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.