Transcript Chapter 1

Chapter 4 Lecture
Basic Chemistry
Fourth Edition
Chapter 4
Atoms and Elements
4.5 Isotopes and Atomic Mass
Learning Goal Give the number of
protons, electrons, and neutrons in one or
more of the isotopes of an element;
calculate the atomic mass of an element
using the percent abundance and mass of
its naturally occurring isotopes.
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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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Atomic Symbols
An atomic symbol
• represents a particular isotope of an element
• gives the mass number in the upper left corner
and the atomic number in the lower left corner
mass number
atomic
number
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24
12
Mg
Chemical Symbol
Atomic Symbols, Subatomic Particles
The atomic symbol indicates the number of
protons, neutrons and, electrons in a specific
isotope of an element.
16
8
O
8 protons
8 neutrons
8 electrons
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31
15
P
15 protons
16 neutrons
15 electrons
65
30
Zn
30 protons
35 neutrons
30 electrons
Learning Check
Carbon consists of three naturally occurring
isotopes, 12C, 13C, and 14C. State the number of
protons, neutrons, and electrons in each of these
isotopes.
12
6
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C
13
6
C
14
6
C
Solution
Naturally occurring carbon consists of three
isotopes, 12C, 13C, and 14C. State the number of
protons, neutrons, and electrons in each of these
isotopes.
12
6
C
6 protons
6 neutrons
6 electrons
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13
6
C
6 protons
7 neutrons
6 electrons
14
6
C
6 protons
8 neutrons
6 electrons
Learning Check
Write the atomic symbols for atoms with the
following subatomic particles:
A. 8 protons, 8 neutrons, 8 electrons
____
B. 17 protons, 20 neutrons, 17 electrons ____
C. 47 protons, 60 neutrons, 47 electrons ____
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Solution
Write the atomic symbols for atoms with the
following subatomic particles:
A. 8 protons, 8 neutrons, 8 electrons
B. 17 protons, 20 neutrons, 17 electrons
C. 47 protons, 60 neutrons, 47 electron
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16
8
37
17
107
47
O
Cl
Ag
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. 15 X and 15 X
8
7
B.
12
6
14
6
C.
15
7
X and X
16
8
X and X
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Solution
1. Which of the following pairs are isotopes of
the same element?
12
14
B. 6 X and 6 X
Each has 6 protons and, the element is carbon.
Different mass numbers indicate that each
species has a different number of neutrons.
These are isotopes.
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Solution
2. In which of the following pairs do both atoms
have 8 neutrons?
C. 15 X and 16 X
7
8
The number of neutrons is calculated.
15
7
X mass number – atomic number = number neutrons
16
8
X
15 (mass number) – 7 (atomic number)
= 8 neutrons
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mass number – atomic number = number neutrons
16 (mass number) – 8 (atomic number)
= 8 neutrons
Atomic Mass
The atomic mass of an element is
• listed below the symbol of each element on
the periodic table
• calculated based on the weighted average
of all naturally occurring isotopes
• based on its comparison
11
to the mass of 12C
• not the same as
Na
the mass number
Atomic Mass 22.99
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Isotopes and Atomic Mass
Most elements have two or more isotopes that
contribute to the atomic mass of that element.
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Isotopes of Magnesium
Magnesium has three naturally occurring isotopes.
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Calculating Atomic Mass
The calculation for the atomic mass requires the
• percent(%) abundance of each isotope
• atomic mass of each isotope of that element
To calculate atomic mass of an element with 2 isotopes:
mass isotope(1) × (%)/100
+ mass isotope(2) × (%)/100
Atomic mass of element
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Calculating Atomic Mass
The atomic mass of Mg
• is a weighted average
from all the Mg isotopes
• is not a whole number
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Calculating Atomic Mass
Isotope
Mass
Abundance
= 23.99 amu × 78.70/100
25Mg = 24.99 amu × 10.13/100
26Mg = 25.98 amu × 11.17/100
Atomic mass of Mg
24Mg
12
Mg
24.31
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= 18.88 amu
= 2.531 amu
= 2.902 amu
= 24.31 amu
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 Chlorine
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 of Chlorine
35Cl
has a mass of 34.97 amu (75.76%), and 37C
has a mass of 36.97 amu (24.24%).
Use the atomic mass and percent of each isotope
to calculate the weighted average.
34.97 amu × 75.76/100
= 26.49 amu
35.97 amu × 24.24/100
Atomic mass of Cl
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= 8.962 amu
35.45 amu
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 (68.93 amu) atoms and
39.89% of 71Ga (70.92 amu) atoms.
Given the abundance and mass of these
isotopes, calculate the atomic mass of gallium.
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Solution
Given the abundance and mass of these
isotopes, calculate the atomic mass of gallium.
69Ga
68.93 amu × 60.11/100
= 41.43 amu
71Ga
70.92 amu × 39.89/100
Atomic mass Ga
= 28.29 amu
= 69.72 amu
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Concept Map
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