Transcript Atomic Mass

Chapter 4 Atoms and
Elements
4.5
Isotopes and Atomic Mass
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Isotopes
Isotopes
• are atoms of the same element that have
different mass numbers
• have the same number of protons, but different
numbers of neutrons
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Nuclear Symbol
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:
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Information from Nuclear
Symbols
• The nuclear symbol indicates the number of
protons (p+), neutrons, (n), and electrons (e-) in
a particular atom.
16
31
8
15
8 p+
8n
8 e-
15 p+
16 n
15 e-
O
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P
65
30
Zn
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
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
6 p+
6 p+
neutrons 6 n
7n
8n
electrons 6 e-
6 e-
6 e-
protons
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6 p+
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Learning Check
Write the nuclear symbols for atoms with the
following subatomic particles:
7
A. 8 p+, 8 n, 8 e-
___________
B. 17 p+, 20 n, 17e-
___________
C. 47 p+, 60 n, 47 e-
___________
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Solution
Write the nuclear symbols for atoms with the
following subatomic particles:
16O
A. 8 p+, 8 n, 8 eB. 17p+, 20 n, 17e-
8
37Cl
17
C. 47p+, 60 n, 47 e-
107Ag
47
8
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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
8
B.
12X
6
C.
9
15X
7
14X
6
15X
16X
7
8
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Solution
1.
B.
12X
6
14X
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
7
16X
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
11
Na
22.99
• gives the mass of an “average” atom
of each element compared to 12C
• is not the same as the mass number
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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.
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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 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
Mg
24.31
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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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Atomic Mass for Cl
• The atomic mass of
chlorine is the
weighted average of
two isotopes 35Cl and
37Cl.
Chlorine, with two naturally occurring
isotopes, has an atomic mass of 35.45.
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Calculating Atomic Mass for
Cl
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
100
35.97 x 24.24 =
8.962 amu
100
35.45 amu
• The sum is the weighted average or atomic
mass of Cl. 35.45 amu
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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 60.11% of 69Ga (atomic mass 68.93)
atoms and 39.89% of 71Ga (atomic mass 70.92)
atoms.
What is the atomic mass of gallium?
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Solution
69Ga
68.93 amu x
60.11
100
= 41.43 amu (from 69Ga)
71Ga
70.92 amu x 39.89 = 28.29 amu (from 71Ga)
100
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Atomic mass Ga
= 69.72 amu
Ga
69.72
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