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Chapter 3 Atoms and Elements

3.6Isotopes and Atomic Mass

24Mg 25Mg 26Mg 12 12 12Copyright © 2005 by Pearson Education, Inc.

Publishing as Benjamin Cummings

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

mass number 23 Na

atomic number 11

Nuclear Symbol

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From the nuclear symbol, we can determine 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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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. 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 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

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.

C. 15X 16X

7 8

An atom of nitrogen (7) and an atom of oxygen (8) each have 8 neutrons.

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Isotopes of Magnesium

In naturally occurring magnesium, there are three isotopes.

24Mg 25Mg 26Mg 12 12 12

Isotopes of Mg

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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 © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings

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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.

Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings

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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.

Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings

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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