Transcript Isotopes - Firefly

Hydrogen-1
• Contains 1 proton and
1 electron – as all
isotopes of H must
contain.
• No neutrons
• The most common
type of H atom.
Hydrogen-2, deuterium
• Contains 1 proton and
1 neutron in the
nucleus.
Hydrogen-3, tritium
• Contains 1 proton and
2 neutrons in the
nucleus.
Isotopes
 Atoms with the same number of protons,
but different numbers of neutrons.
 Atoms of the same element (same atomic
number) with different mass numbers
Isotopes of chlorine
35Cl
37Cl
17
17
chlorine - 35 LecturePLUS
chlorine
- 37
Timberlake
4
Isotopes
Neutron
+
Electrons
Nucleus
+
+
+
+
+
Nucleus
Proton
Proton
Nucleus
Carbon-12
Neutrons 6
Protons
6
Electrons 6
Isotopes have same numbers
of protons and electrons but
have different NEUTRONS.
+
+
+
+
Neutron
Electrons
+
+
Carbon-14
Neutrons 8
Protons
6
Electrons 6
Nucleus
Learning Check AT 2
Naturally occurring carbon consists of three
isotopes, 12C, 13C, and 14C. State the number of
protons, neutrons, and electrons in each of
these carbon atoms.
12C
13C
14C
6
6
6
#P _______
_______
_______
#N _______
_______
_______
_______
_______
#E _______
LecturePLUS Timberlake
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Solution AT 2
12C
6
13C
14C
6
6
#P __6___
_ 6___
___6___
#N __6___
_ _7___
___8___
#E __6___
_ 6___
___6___
LecturePLUS Timberlake
7
Learning Check AT 3
An atom of zinc has a mass number of 65.
A. Number of protons in the zinc atom
1) 30
2) 35
3) 65
B. Number of neutrons in the zinc atom
1) 30
2) 35
3) 65
C. What is the mass number of a zinc isotope
with 37 neutrons?
1) 37
2)
65
LecturePLUS Timberlake
3) 67
8
Solution AT 3
An atom of zinc has a mass number of 65.
A. Number of protons in the zinc atom
1) 30
B. Number of neutrons in the zinc atom
2) 35
C. What is the mass number of a zinc isotope
with 37 neutrons?
3) 67
LecturePLUS Timberlake
9
Learning Check AT 5
An atom has 14 protons and 20 neutrons.
A. Its atomic number is
1) 14
2) 16
3) 34
B. Its mass number is
1) 14
2) 16
3) 34
C. The element is
1) Si
2) Ca
3) Se
D. Another isotope of this element is
1)
34X
16
2)
34X
14
LecturePLUS Timberlake
3)
36X
14
10
Solution AT 5
An atom has 14 protons and 20 neutrons.
A. It has atomic number
1) 14
B. It has a mass number of
3) 34
C. The element is
1) Si
D. Another isotope of this element would be
3) 36X
14
LecturePLUS Timberlake
11
6Li
7Li
3 p+
3 n0
3 p+
4 n0
2e– 1e–
2e– 1e–
Neutron
Neutron
Electrons
Electrons
+
Nucleus
+
+
Nucleus
+
Nucleus
Lithium-6
Neutrons 3
Protons
3
Electrons 3
Proton
+
+
Nucleus
Lithium-7
Neutrons 4
Protons
3
Electrons 3
Proton
Calculating Atomic Mass
 Percent(%) abundance of isotopes
 Mass of each isotope of that element
 Weighted average =
mass isotope1(%) + mass isotope2(%) + …
100
100
LecturePLUS Timberlake
15
Atomic Mass of Magnesium
Isotopes
Mass of Isotope
Abundance
24Mg
=
24.0 amu
78.70%
25Mg
=
25.0 amu
10.13%
26Mg
=
26.0 amu
11.17%
Atomic mass (average mass) Mg = 24.3 amu
Mg
24.3
LecturePLUS Timberlake
16
Learning Check AT7
Gallium is a metallic element found in
small lasers used in compact disc players.
In a sample of gallium, there is 60.2% of
gallium-69 (68.9 amu) atoms and 39.8% of
gallium-71 (70.9 amu) atoms. What is the
atomic mass of gallium?
LecturePLUS Timberlake
17
Solution AT7
Ga-69
68.9 amu x
60.2
=
41.5 amu for
69Ga
28.2 amu for
71Ga
100
Ga-71 (%/100)
70.9 amu x 39.8
=
100
Atomic mass Ga =
69.7 amu
LecturePLUS Timberlake
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• In a sample of carbon, there is 98.93%
of carbon12 (12.000 amu) atoms and
1.07% of carbon13 (13.003 amu) atoms.
What is the atomic mass of carbon?
Finding An Isotopic Mass
A sample of boron consists of 10B (mass
10.0 amu) and 11B (mass 11.0 amu). If
the average atomic mass of B is 10.8
amu, what is the % abundance of each
boron isotope?
LecturePLUS Timberlake
20
Assign X and Y values:
X = % 10B
Y = % 11B
Determine Y in terms of X
X
+
Y
= 100
Y = 100 - X
Solve for X:
X (10.0) + (100 - X )(11.0)
100
100
= 10.8
Multiply through by 100
10.0 X + 1100 - 11.0X = 1080
LecturePLUS Timberlake
21
Collect X terms
10.0 X - 11.0 X
=
1080 - 1100
- 1.0 X = -20
X
=
-20
- 1.0
=
Y = 100 - X
% 11B = 100 - 20% =
20 %
10B
80% 11B
LecturePLUS Timberlake
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Learning Check AT8
Copper has two isotopes 63Cu (62.9 amu)
and 65Cu (64.9 amu). What is the %
abundance of each isotope? (Hint: Check
periodic table for atomic mass)
1) 30%
2) 70%
LecturePLUS Timberlake
3) 100%
23
Solution AT8
2) 70%
Solution
62.9X + 6490 = 64.9X = 6350
-2.0 X = -140
X = 70%
LecturePLUS Timberlake
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Average Atomic Mass
• weighted average of all isotopes
• on the Periodic Table
• round to 2 decimal places
Avg.
(mass)(%) + (mass)(%)
Atomic =
100
Mass
Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem
Average Atomic Mass
• EX: Calculate the avg. atomic mass of oxygen if its
abundance in nature is 99.76% 16O, 0.04% 17O, and
0.20% 18O.
Avg.
(16)(99.76) + (17)(0.04) + (18)(0.20)
16.00
Atomic =
=
amu
100
Mass
Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem
Average Atomic Mass
• EX: Find chlorine’s average atomic mass
if approximately 8 of every 10 atoms are
chlorine-35 and 2 are chlorine-37.
Avg.
(35)(8) + (37)(2)
Atomic =
= 35.40 amu
10
Mass
Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem
17
100
Mass spectrum of chlorine. Elemental chlorine (Cl2) contains
only two isotopes: 34.97 amu (75.53%) and 36.97 (24.47%)
90
80
Cl-35
70
Abundance
AAM = (34.97 amu)(0.7553) + (36.97 amu)(0.2447)
60
AAM =
(26.412841 amu)
AAM =
+
(9.046559 amu)
35.4594 amu
50
40
30
Cl-37
20
10
0
34
36
35
Mass
37
Cl
35.4594
Mass Spectrophotometer
magnetic field
heaviest
ions
stream
of ions of
different
masses
electron
beam
gas
Dorin, Demmin, Gabel, Chemistry The Study of Matter 3rd Edition, page 138
lightest
ions
Weighing atoms
gas sample
enters here
.
ions accelerate
towards charged
slit
magnetic field
deflects lightest ions
most
filament current
ionizes the gas
The first mass spectrograph was
built in 1919 by F. W. Aston, who
received the 1922 Nobel Prize for
this accomplishment
ions separated by mass
expose film
• mass spectrometry is used to experimentally determine isotopic masses
and abundances
• interpreting mass spectra
• average atomic weights
- computed from isotopic masses and abundances
- significant figures of tabulated atomic weights gives some idea
of natural variation in isotopic abundances
Copyright © 1997-2005 by Fred Senese
Mass Spectrometry
198
200
202
Photographic plate
196
-
+
Stream of positive ions
Hill, Petrucci, General Chemistry An Integrated Approach 1999, page 320
199
201
204
Mass spectrum of mercury vapor
Using a periodic table and what you know about atomic
number, mass, isotopes, and electrons, fill in the chart:
Element
Symbol
Atomic
Number
Atomic
Mass
# of
protons
# of
neutron
# of
electron
8
8
8
39
Potassium
+1
Br
45
30
35
-1
30
Atomic Number = Number of Protons
Number of Protons + Number of Neutrons = Atomic Mass
Atom (no charge) : Protons = Electrons
Ion (cation) : Protons > Electrons
charge
Ion (anion) : Electrons > Protons
Using a periodic table and what you know about atomic
number, mass, isotopes, and electrons, fill in the chart:
ANSWER KEY
Element
Symbol
Atomic
Number
Atomic
Mass
# of
protons
# of
neutron
# of
electron
charge
Oxygen
O
8
16
8
8
8
0
Potassium
K
19
39
19
20
18
+1
Bromine
Br
35
80
35
45
36
-1
Zinc
Zn
30
35
30
65
30
0
Atomic Number = Number of Protons
Number of Protons + Number of Neutrons = Atomic Mass
Atom (no charge) : Protons = Electrons
Ion (cation) : Protons > Electrons
Ion (anion) : Electrons > Protons
Atomic Mass
Magnesium has three isotopes.
78.99% magnesium 24 with
a mass of 23.9850 amu,
10.00% magnesium 25 with
a mass of 24.9858 amu, and
the rest magnesium 26 with
a mass of 25.9826 amu.
What is the atomic mass of
magnesium?
If not told otherwise,
the mass of the isotope is
the mass number in amu.
California WEB
Isotope
Percent
Abundance
Mg-24
78.99
23.9850 18.94575
Mg-25
10.00
24.9585
2.49585
Mg-26
11.01
25.9826
2.86068
Mass
24.304 amu
Atomic Mass
Calculate the atomic mass of copper if copper has two isotopes.
69.1% has a mass of 62.93 amu and the rest has a mass of
Percent
64.93 amu.
Isotope
Mass
Abundance
Cu-63
69.1
62.93
43.48463
Cu-65
30.9
64.93
20.06337
63.548
Average atomic mass (AAM)  (% " A" )(mass " A" )  (% " B" )(mass " B" )  ...
A.A.M.  (0.691)(62.93 amu)  (0.309)(64.93 amu)
A.A.M.  43.48463 amu  20.06337 amu
A.A.M.  63.548 amu for Copper
29
Cu
63.548
How can we figure out how long something is dead?
It’s really not
that difficult
https://www.youtube.com/watch?v=phZeE7
Att_s
The Atom
• Fundamental unit of matter
• Made up of components called subatomic
particles
–
–
–
Proton (positive charge)
Neutron (no electrical charge)
Electron (negative charge)
Nucleus
Electron
What Is Carbon?
• Carbon-14 is also referred to as:
–
–
C-14
Radiocarbon
• Types of carbon (isotopes)
Atomic mass
9
14
16
6
6
6
Atomic number
Unstable Isotopes (Atoms)
An atom is generally stable if the number of protons equals the number
of neutrons in the nucleus
Atom (Isotope)
Protons
Neutrons
Stable Carbon
6
6
Stable Nitrogen
7
7
Stable Oxygen
8
8
6
8
Carbon 14
Losing Your Identity: Radioactive Decay
The nucleus of an atom (decays) changes
into a new element
The proton number (atomic number) changes
14
14
6
7
How long does this take?
Carbon-14 and Half-Life?

The rate of decay is measured by how
long it takes for half an element to decay
(half-life)
• The half-life of C-14 is 5,730 years
Half-Life Illustration
Time = 0
C-14
11,460 years
2 half-lives
5,730 years
1 half-life
N-14
1/2
N-14
3/4
C-14
1/2
C-14
If C-14 is constantly decaying,
will we run out of C-14 in the atmosphere?
How Carbon-14 Is Produced
Cosmic Rays
(radiation)
Forms C-14
Collision with atmosphere
(N14)
C-14 combines with oxygen to form
carbon dioxide (CO2)
Remember…
Ordinary carbon (C-12) is found in the carbon
dioxide (CO2) in the air, which is taken up by
plants, which in turn are eaten by animals.
A bone, or a leaf or a tree, or even a piece of
wooden furniture, contains carbon.
When the C-14 has been formed, like ordinary
carbon (C-12), it combines with oxygen to give
carbon dioxide (14CO2), and so it also gets
cycled through the cells of plants and animals.
• We can take a sample of air, count how
many C-12 atoms there are for every C14 atom, and calculate the C-14/C-12
ratio.
• Because C-14 is so well mixed up with
C-12, we expect to find that this ratio is
the same if we sample a leaf from a
tree, or a part of the body.
Carbon-14 Life Cycle
Cosmic radiation
14
14
14
7
6
7
Carbon-14 is produced in the atmosphere
Carbon-14 decays into Nitrogen-14
Review

•
•
•
•
There are different types of carbon
There are stable and unstable atoms
(isotopes)
Radioactive decay
Half-life (Carbon-14 = 5,730 years)
C-14 life-cycle (N-14
C-14
N-14)
Next Topics
Equilibrium
How the carbon clocks works
How the Carbon Dating Clock Works
• Once a plant or animal dies the clock starts
• The plant or animal no longer takes in C-14
• The C-14 present in the plant or animal begins
to decay
No more
C-14 intake
C-14 continues
to decay
• In living things, although 14C atoms are
constantly changing back to 14N, they are
still exchanging carbon with their
surroundings, so the mixture remains about
the same as in the atmosphere.
• However, as soon as a plant or animal dies,
the 14C atoms which decay are no longer
replaced, so the amount of 14C in that onceliving thing decreases as time goes on.
• In other words, the 14C/12C ratio gets
smaller.
When Does the Clock Start?
Once a plant or animal dies
the clock starts
Organism dies
No more C-14
intake
C-14 continues to
decay
How the Carbon Clock Works
The C-14 dating method relies on measuring
the amount of C-14 in the fossil
Three Scenarios
1. There is a lot of C-14 remaining in the fossil
2. There is very little C-14 remaining in the
fossil
3. There is no detectable C-14 in the fossil
Two Things We Need to Know
What do we need to know to determine
how many half-lives have expired?
1. How fast it decays (measured in half-lives).
This is known (5,730 years).
2. The starting amount of C-14 in the fossil.
A Critical Detail
•
Determining the Starting
Amount
There are two types
of carbon used in the dating

process: C-12 and C-14
C-12 is a stable isotope (it does not decay)

When an organism is alive it has the same ratio
(C-12 to C-14) that is found in the atmosphere
(1-trillion to 1)
I’m alive
I’m a fossil
Same ratio
Different ratio
Determining the Starting
Amount
No measurable C-14
C-14
C-14
C-12
C-12
C-12
Amount
constant
Amount
constant
Amount
constant
Moment of death
Old
“Infinite” age
Isotopes Research Project
1. Isotopes in nutrition
2. Medical Isotopes used in diagnosis
3. Medical Isotopes used in treatment of disease
4. Isotopic Labelling in Chemical Reactions
5. Isotopes in Pest Control
6. Isotopes used in agriculture
7. Isotopes in climatology and geology
8. Isotopes in nuclear weapons testing
9. Isotopes in smoke alarms
10. Isotopes in biological tracking
In addition you must also
include:
• What is an isotope?
• What is the difference between a stable
and unstable isotope?
• What is mean by the natural abundance of
isotopes?
• Explain how to calculate the atomic mass
of an element containing a mixture of
isotopes.