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Chapter 3 Atoms and Elements

3.6Isotopes and Atomic Mass

24Mg 25Mg 26Mg 12 12 12

Copyright © 2009 by Pearson Education, Inc.

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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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An atomic 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 sodium with atomic number 11 and a mass number 23 has the following atomic symbol:

mass number 23 Na

atomic number 11

Atomic Symbol

The atomic symbol for a specific atom of an element

gives the

• number of protons (p+),

• number of neutrons (n),

• and number of electrons (e-).

Information from Atomic Symbols

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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-

Information from Atomic 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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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 atomic symbols for atoms with the following subatomic particles:

A. 8 p+, 8 n, 8 e- ___________

B. 17p+, 20n, 17e- ___________

C. 47p+, 60 n, 47 e- ___________

Learning Check

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A. 8 p+, 8 n, 8 e- 16O

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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 pairs are isotopes of the same element?

2. In which of the 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

B. 12X 14X

6 6

The atomic symbols in “B.” represent isotopes of carbon with 6 protons each, but one has 6 neutrons and the other has 8.

C. 15X 16X

7 8

These isotopes of nitrogen and oxygen have 8 neutrons.

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Isotopes of Magnesium

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Isotopes of Magnesium

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Isotopes of Sulfur

A sample of naturallyoccurring sulfur containsseveral isotopes with thefollowing abundances

Isotope % abundance 32S 95.02 33S 0.75 34S 4.21 36S 0.02

32S, 33S, 34S, 36S16 16 16 16

Copyright © 2009 by Pearson Education, Inc.

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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.

Na22.99

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Isotopes of 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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Atomic Mass for Cl

The atomic mass of chlorine is

• due to all the Cl isotopes.

• not a whole number.

• the average of two isotopes: 35Cl and 37Cl.

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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

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Using the periodic table, specify the atomic mass of

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

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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 of isotope(1)x (%) + mass of isotope(2) x (%) +

100 100

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35Cl has atomic mass 34.97 amu (75.76%) and 37C

has atomic mass 36.97 amu (24.24%).

• Use 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 10036.97 x 24.24 = 8.961 amu

100

• Sum is atomic mass of Cl 35.45 amu

Calculating Atomic Mass for Cl

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Calculating Atomic Mass Mg

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

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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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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?

Learning Check

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69Ga

68.926 amu x 60.10 = 41.42 amu (from 69Ga)

10071Ga

70.925 amu x 39.90 = 28.30 amu (from 71Ga)

100

Atomic mass Ga = 69.72 amu

Solution

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Ga

69.72