Transcript 3_6 Isotopes and Atomic Mass

Chapter 3 Atoms and Elements
3.6
Isotopes and Atomic Mass
24Mg
25Mg
26Mg
12
12
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Copyright © 2009 by Pearson Education, Inc.
1
Isotopes
Isotopes
• are atoms of the same element that have different
mass numbers.
• have the same number of protons, but different
numbers of neutrons.
2
Atomic Symbol
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
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Information from Atomic Symbols
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-).
4
Information from Atomic Symbols
Examples of number of subatomic particles for atoms
Atomic symbol
16
8
31
O
8 p+
8n
8 e-
P
15
15 p+
16 n
15 e-
65
Zn
30
30 p+
35 n
30 e-
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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:
12C
6
13C
14C
6
6
protons
______
______
______
neutrons
______
______
______
electrons ______
______
______
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Solution
12C
6
13C
14C
6
6
protons
6 p+
6 p+
6 p+
neutrons
6n
7n
8n
electrons 6 e-
6 e-
6 e-
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Learning Check
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-
___________
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Solution
A. 8 p+, 8 n, 8 e-
16O
8
B. 17p+, 20 n, 17e-
37Cl
17
C. 47p+, 60 n, 47 e-
107Ag
47
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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
8
B.
C.
12X
15X
7
14X
6
6
15X
16X
7
8
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Solution
B.
12X
6
14X
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
7
16X
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 naturally
occurring sulfur contains
several isotopes with the
following abundances
Isotope % abundance
32S
95.02
33S
0.75
34S
4.21
36S
0.02
32S, 33S, 34S, 36S
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16
16
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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
Na
22.99
each element compared to 12C.
• is not the same as the mass number.
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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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Learning Check
Using the periodic table, specify the atomic mass of
each element.
A. calcium
__________
B. aluminum
__________
C. lead
__________
D. barium
__________
E. iron
__________
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Solution
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
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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 for Cl
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
100
36.97 x 24.24
=
8.961 amu
100
• Sum is atomic mass of Cl
35.45 amu
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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 =
Atomic mass (average mass) Mg
2.902 amu
= 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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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
100
= 41.42 amu (from 69Ga)
71Ga
70.925 amu x 39.90 = 28.30 amu (from 71Ga)
100
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Atomic mass Ga
= 69.72 amu
Ga
69.72
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