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Isotopes

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Title: Isotopes

1
Isotopes
Pisgah High School M. Jones
Rev. 022609
2
Isotopes
• have the same number of protons and electrons
but different numbers of neutrons.

Therefore, isotopes of the same element have
different masses.
3
Isotopes
have different mass numbers but the same atomic
number.
• Atomic number - the number of protons in the
nucleus of an atom
• Mass number - the number of protons added to
the number of neutrons

4
Symbols for Isotopes
A is the symbol for mass number
A
E
Z
Z is the symbol for atomic number
5
Symbols for Isotopes
235
U
• 92

6
Symbols for Isotopes
U
235
92
7
Symbols for Isotopes
Find U in the periodic table. Z 92
U-235
How do you know the atomic number?
8
Determining the Atomic Number and Mass Number
• The atomic number is the number of protons.
• The mass number is the sum of the protons
neutrons.
• Consider an atom of aluminum with 13 protons and
15 neutrons. What is Z and A?

9
Determining the Atomic Number and Mass Number
Atomic number Z 13 Mass number A 28
A p n0 13 15 28
10
Finding the number of Protons, Neutrons, and
Electrons
• The atomic number is the number of protons in the
nucleus.
• The number of electrons in a neutral atom equals
the number of protons.

11
Finding the number of Protons, Neutrons, and
Electrons
The number of neutrons is the difference between
the mass number and the atomic number.
of neutrons A - Z
12
Finding the number of Protons, Neutrons, and
Electrons
• Look at the Periodic Table and find the element
by using the symbol

U-235
Z 92 protons 92 electrons 92
A 235 protons neutrons 235
13
Finding the number of Protons, Neutrons, and
Electrons
U-235
235 prot. neut. - 92 protons 143
neutrons
Z 92 protons 92 electrons 92
A 235 protons neutrons 235
14
Finding the number of Protons, Neutrons, and
Electrons
Q. Find the number of neutrons in the Ba-137
isotope.
• In the Ba-137 isotope
• Z 56 and A 137
• 137 56 81 neutrons

15
Finding the number of Protons, Neutrons, and
Electrons
Fill in the following table.
16
Finding the number of Protons, Neutrons, and
Electrons
Element Symbol Z A p n e
Zinc 66
In 68
85 38
82 210
Rn 136
35 47
17
Finding the number of Protons, Neutrons, and
Electrons
Element Symbol Z A p n e
Zinc Zn 30 66 30 36 30
In 68
85 38
82 210
Rn 136
35 47
18
Finding the number of Protons, Neutrons, and
Electrons
Element Symbol Z A p n e
Zinc Zn 30 66 30 36 30
Indium In 49 117 49 68 49
85 38
82 210
Rn 136
35 47
19
Finding the number of Protons, Neutrons, and
Electrons
Element Symbol Z A p n e
Zinc Zn 30 66 30 36 30
Indium In 49 117 49 68 49
Strontium Sr 38 85 38 47 38
82 210
Rn 136
35 47
20
Finding the number of Protons, Neutrons, and
Electrons
Element Symbol Z A p n e
Zinc Zn 30 66 30 36 30
Indium In 49 117 49 68 49
Strontium Sr 38 85 38 47 38
Lead Pb 82 210 82 128 82
Rn 136
35 47
21
Finding the number of Protons, Neutrons, and
Electrons
Element Symbol Z A p n e
Zinc Zn 30 66 30 36 30
Indium In 49 117 49 68 49
Strontium Sr 38 85 38 47 38
Lead Pb 82 210 82 128 82
Radon Rn 86 222 86 136 86
35 47
22
Finding the number of Protons, Neutrons, and
Electrons
Element Symbol Z A p n e
Zinc Zn 30 66 30 36 30
Indium In 49 117 49 68 49
Strontium Sr 38 85 38 47 38
Lead Pb 82 210 82 128 82
Radon Rn 86 222 86 136 86
Bromine Br 35 82 35 47 35
23
How are isotopes of the same element alike and
different?
• Alike
• Number of protons and electrons
• Atomic number
• Chemical properties
• Different
• Number of neutrons
• Mass Number
• Atomic weight of the isotopes

24
Which of the following is the same for the three
isotopes of magnesium?
1. The atomic number of 12
2. The number of protons and electrons
3. The number of neutrons
4. The atomic weight of 24.986 AMU
5. The reaction with hydrochloric acid
6. The speed of gaseous Mg atoms

25
Which of the following is the same for the three
isotopes of magnesium?
1. The atomic number of 12

Same
All three isotopes of magnesium have the same
atomic number.
26
Which of the following is the same for the three
isotopes of magnesium?
• 2. The number of protons and electrons

Same
All isotopes of the same element have the same
number of protons in the nucleus, and electrons
outside the nucleus.
27
Which of the following is the same for the three
isotopes of magnesium?
• 3. The number of neutrons

Not the same
The number of neutrons varies with the isotope.
Different isotopes have different numbers of
neutrons.
28
Which of the following is the same for the three
isotopes of magnesium?
• 4. Atomic weight of 24.986 AMU

Not the same
Mg-24 ? 23.985 AMU Mg-25 ? 24.986 AMU Mg-26 ?
25.983 AMU
29
Which of the following is the same for the three
isotopes of magnesium?
• 5. The reaction with HCl

Same
All isotopes of the same element react the same
chemically.
The number and arrangement of electrons is the
same for each isotope.
30
Which of the following is the same for the three
isotopes of magnesium?
• 6. The speed of gaseous Mg atoms

Not the same
The speeds of atoms depend on mass. Heavier
atoms move more slowly, and lighter atoms move
faster.
31
How did knowing that the speeds of gaseous
isotopes are different, allow the United States
to win World War II?
32
Who were the two guys responsible for winning
World War II?
Fat Man, and
Little Boy
Atomic bombs dropped on Hiroshima and Nagasaki
33
Manhattan Project
Oak Ridge, TN
Gaseous Diffusion
Most naturally occurring uranium is U-238
Less than 1 of naturally occurring uranium is
U-235
To sustain a nuclear chain reaction, uranium must
be at least 4 U-235.
34
To make either bomb-grade uranium, or fuel for a
nuclear reactor you need enriched uranium.
Enriched uranium has a higher percentage of
U-235. 12 for a reactor, 90 for a bomb.
35
The gaseous diffusion plant at Oak Ridge was used
to make enriched uranium.
The uranium oxide is reacted with fluorine to
make UF6 which is a gas.
The UF6 is introduced into a container with a
porous barrier.
36
The molecules of UF6 containing U-235 diffuse
through the porous barrier slightly faster.
Produces a slightly higher percentage of
U-235 in the UF6.
Then the process is repeated many times.
37
The K-25 building at Oak Ridge
38
Atomic Weights
39
Calculating Average Atomic Weight
Atomic mass (atomic weight) is the weighted
average of the masses of all the isotopes of the
element.
40
Calculating Average Atomic Weight
A weighted average puts more weight or importance
on one value that another.
Example Compute the average age of everyone in
this room.
41
Calculating Average Atomic Weight
Suppose there were two isotopes of the element Q.
Q-30 95 Q-32 5
Determine the average atomic mass.
42
Calculating Average Atomic Weight
Is the average atomic weight 31?
( 30 32 ) / 2 31
Assume 100 atoms of Q
Much closer to 30
95 x 30 amu 2850 5 x 32 amu 160
/ 100 30.1 amu
43
Calculating Average Atomic Weight
You can compute the atomic weight by knowing
the mass (M) and the relative abundance (RA)
of each naturally
occurring isotope.
44
Calculating Average Atomic Weight
The mass is the actual mass of the isotope in
amu.
The relative abundance is the percentage of that
isotope divided by 100.
45
Calculating Average Atomic Weight
Example Suppose that 48.6 of the atoms of zinc
in the world were Zn-64.
Then the relative abundance of zinc-64 would be
0.486.
46
Calculating Average Atomic Weight
Average atomic weight
(M1 RA1)
(M2 RA2)
Mass of each isotope times its relative abundance
(M3 RA3)
47
Calculating Average Atomic Weight
Calculate the average atomic weight for Cu-63 and
Cu-65.
Actual mass of isotope in AMU Percent abundance
Cu-63 62.9298 69.09
Cu-65 64.9278 30.91
48
Calculating Average Atomic Weight
Calculate the average atomic weight for Cu-63 and
Cu-65.
AW (M1 RA1) (M2 RA2)
AW (62.9298 .6909)
(64.9278 .3091)
AW 63.55 AMU
49
Calculate the Average Atomic Weight of Chlorine
Actual mass of isotope in AMU Percent abundance
Cl-35 34.969 75.77
Cl-37 36.966 24.23
Stop Do the calculations, then continue.
50
Calculate the Average Atomic Weight of Chlorine
Actual mass of isotope in AMU Percent abundance
Cl-35 34.969 75.77
Cl-37 36.966 24.23
AW (34.969 0.7577) (36.966 0.2423) AW
35.45 AMU
51
Which isotope of Boron is more abundant?
B-10 ??? 10.01 amu B-11 ??? 11.01
amu
A qualitative answer does not require
calculations, just thought.
52
Which isotope of Boron is more abundant?
B-10 ??? 10.01 amu B-11 ??? 11.01
amu
B-11 is more abundant because 11.01 is closer to
10.81, the actual average mass of Boron.
53
Which isotope of Boron is more abundant?
B-10 ??? 10.01 amu B-11 ??? 11.01
amu
Now stop and compute the actual percentages.
54
Which isotope of Boron is more abundant?
B-10 ??? 10.01 amu B-11 ??? 11.01
amu
Did you get 20.00 B-10 and 80.00 B-11?
55
Some isotope trivia
56
Only one element has unique names for its
isotopes
Deuterium and tritium are used in nuclear
reactors and fusion research.
57
• Radioisotopes can emit alpha, beta or gamma
• Radioisotopes are important in a number of
research fields.

58
with streams of protons or neutrons.
• Cobalt-59 occurs naturally. Cobalt-60 occurs in
reactors after neutron exposure.

59
Uses for Isotopes
• Various radioisotopes are used to kill cancer
cells. (Co-60, Bi-212)
• Radioisotopes are used in imaging living and
nonliving systems. Tc-99
• Radioisotopes are used as tracers in chemical
reactions.

60
Isotopes