3_6 Isotopes and Atomic Mass

8/10/2019 3_6 Isotopes and Atomic Mass

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Chapter 3 Atoms and Elements

3.6

Isotopes and Atomic Mass

24Mg 25Mg 26Mg12 12 12

Copyright © 2009 by Pearson Education, Inc.



2

Isotopes

• are atoms of the same element that have different

mass numbers.

• have the same number of protons, but different

numbers of neutrons.

Isotopes



3

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

4



5

Examples of number of subatomic particles for atoms

Atomic symbol

16 31 65

O P Zn8 15 30

8 p+ 15 p+ 30 p+

8 n 16 n 35 n

8 e- 15 e- 30 e-

Information from Atomic Symbols



6

Naturally occurring carbon consists of three isotopes:12C, 13C, and 14C. State the number of protons,

neutrons, and electrons in each of the following:

12

C13

C14

C6 6 6

protons ______ ______ ______

neutrons ______ ______ ______

electrons ______ ______ ______

Learning Check



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12C 13C 14C6 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 thefollowing subatomic particles:

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

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

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

Learning Check



9

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

8

B. 17p+, 20 n, 17e- 37Cl17

C. 47p+, 60 n, 47 e- 107 Ag47

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.15

X15

X8 7

B. 12X 14X6 6

C. 15X 16X7 8



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Solution

B. 12X 14X6 6

The atomic symbols in “B.” represent isotopes of

carbon with 6 protons each, but one has 6 neutronsand the other has 8.

C. 15X 16X7 8

These isotopes of nitrogen and oxygen have 8

neutrons.



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Isotopes of Magnesium



13

Isotopes of Magnesium



14

Isotopes of Sulfur

A sample of naturally

occurring sulfur contains

several isotopes with the

following abundances

Isotope % abundance32S 95.0233S 0.7534

S 4.2136S 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.

Na

22.99



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Isotopes of Some Elements and

Their Atomic Mass

Most elements have two or more isotopes thatcontribute 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.



18

Using the periodic table, specify the atomic mass of

each element.

A. calcium __________

B. aluminum __________

C. lead __________

D. barium __________

E. iron __________

Learning Check



19

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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Calculating Atomic Mass Mg

Isotope Mass Abundance

24Mg = 23.99 amu x 78.70/100 = 18.88 amu

25Mg = 24.99 amu x 10.13/100 = 2.531 amu

26Mg = 25.98 amu x 11.17/100 = 2.902 amu

Atomic mass (average mass) Mg = 24.31 amu



23

Atomic Mass of Magnesium

The atomic mass of Mg

• is due to all the Mgisotopes.

• is a weighted average.• is not a whole number.



24

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% of71

Ga (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

31

Ga

69.72

3_6 Isotopes and Atomic Mass

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