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1.4 Isotopes, Radioisotopes,
and Atomic Mass
• B3.1 explain the relationship between the
atomic number and the mass number of
an element, and the difference between
isotopes and radioisotopes of an element
• B3.2 explain the relationship between
isotopic abundance of an element’s
isotopes and the relative atomic mass of
the element
Atomic Mass
• If a proton has a mass of 1, and a neutron has a
mass of 1, how come the elements on the periodic
table have masses that are decimals?
• What unit does the periodic table use to measure
mass?
Mass of an Atom
• In the head of a pin there are approximately
8.0 x 1019 atoms of iron.
• Because atoms are so small, standard units of
measurement are not practical.
• Scientists use a smaller unit to describe the
mass of atoms, the Atomic Mass Unit (u or
amu).
Relative Atomic Mass
• Mass12C atom = 1.992 × 10-23 g
• 1 amu = 1/12 the mass of a 12C atom
• 1.992 × 10-23 g / 12 = 1.66 x 10-24 g = 1 amu
Neutron
+
• 1 p = 1.007276 amu
1 n = 1.008665 amu
1 e- = 0.0005486 amu
Electrons
Nucleus
+
+
+
+
+
Proton
Nucleus
Carbon-12
Neutrons 6
Protons
6
Electrons 6
Isotopes
• The number of protons inside the nucleus at the centre
of an atom decides what element it is.
• Different atoms with the same number of protons and a
different number of neutrons are known as isotopes.
• For example, there are three naturally occurring isotopes
of carbon: carbon-12, carbon-13 and carbon-14. Most
(98.9%) of the natural carbon is carbon-12 and the
remaining 1.1% is made up of stable carbon-13 and
radioactive carbon-14.
Isotopes
Neutron
+
Electrons
Nucleus
+
+
+
+
+
Nucleus
Proton
Proton
Nucleus
Carbon-12
Neutrons 6
Protons
6
Electrons 6
+
+
+
+
Neutron
Electrons
+
+
Carbon-14
Neutrons 8
Protons
6
Electrons 6
Nucleus
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6Li
Naming Isotopes
7Li
3 p+
3 n0
2e–
3 p+
4 n0
1e–
2e–
1e–
Neutron
Neutron
Electrons
Electrons
+
Nucleus
+
+
Nucleus
+
Proton
Nucleus
Lithium-6
Neutrons 3
Protons
3
Electrons 3
• Put the mass number after the name of
the element
• carbon- 12
• carbon -14
• uranium-235
+
+
Proton
Nucleus
Lithium-7
Neutrons 4
Protons
3
Electrons 3
Isotopic Abundance
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Isotopic Abundance
• Why is the average mass not the same as the
mass on the periodic table?
• The abundance of each isotope has to be taken
into consideration.
• A regular average calculation treats each isotope
the same
Isotopic Abundance
• A weighted average takes into account the
abundance of each isotope
Average Atomic Mass
• Weighted average of all isotopes
• The mass indicated on the Periodic Table
• Usually rounded to 2 decimal places
Avg.
(mass)(%) + (mass)(%) …
Atomic =
100
Mass
Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem
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13/02/2017
Average Atomic Mass
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.
• EX: Calculate the avg. atomic mass of oxygen if its
abundance in nature is 99.76%
0.20% 18O.
16O,
0.04%
17O,
and
Avg.
(16)(99.76) + (17)(0.04) + (18)(0.20) 16 amu
Atomic =
=
100
Mass
Avg.
(35)(8) + (37)(2)
Atomic =
=
10
Mass
Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem
35 amu
Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem
17
100
Weighing atoms
gas sample
enters here
.
ions accelerate
towards charged
slit
90
35.4594
Cl-35
80
magnetic field
deflects lightest ions
most
Cl
Mass spectrum of chlorine. Elemental chlorine (Cl 2) contains
only two isotopes: 34.97 amu (75.53%) and 36.97 (24.47%)
70
AAM = (34.97 amu)(0.7553) + (36.97 amu)(0.2447)
60
Abundance
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
AAM =
(26.412841 amu)
AAM =
+
(9.046559 amu)
35.4594 amu
50
40
30
• 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
Cl-37
20
10
0
36
35
34
37
Mass
Copyright © 1997-2005 by Fred Senese
Mass Spectrometer
Mass Spectrum for Mercury
(The photographic record has been converted to a scale of relative number of atoms)
The percent natural abundances
for mercury isotopes are:
magnetic field
heaviest
ions
lightest
ions
30
Relative number of atoms
stream
of ions of
different
masses
Hg-196
Hg-198
Hg-199
Hg-200
Hg-201
Hg-202
Hg-204
25
198
0.146%
10.02%
16.84%
23.13%
13.22%
29.80%
6.85%
196
200
199
202
201
204
Mass spectrum of mercury vapor
20
15
10
5
electron
beam
gas
196
197
198
199
200
201
202
203
204
Mass number
Dorin, Demmin, Gabel, Chemistry The Study of Matter 3rd Edition, page 138
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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
Given the average atomic mass of an element is 118.21
amu and it has three isotopes (“A”, “B”, and “C”):
Isotope “A” has a mass of 117.93 amu and is 87.14%
abundant. Isotope “B” has a mass of 120.12 amu and is
12.36% abundant.
Find the mass of isotope “C”.
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
29
Cu
A.A.M . 63.548 amu for Copper
AAM  63.5 amu
63.548
80
Hg
200.59
The percent natural abundances
for mercury isotopes are:
A
B
C
D
E
F
G
Hg-196
Hg-198
Hg-199
Hg-200
Hg-201
Hg-202
Hg-204
Using a periodic table and what you know about atomic
number, mass, isotopes, and electrons, fill in the chart:
Element
0.146%
10.02%
16.84%
23.13%
13.22%
29.80%
6.85%
Symbol
Atomic
Number
Atomic
Mass
# of
protons
# of
neutron
# of
electron
8
8
8
39
Potassium
charge
+1
Br
45
-1
(% "A")(mass "A") + (% "B")(mass "B") + (% "C")(mass "C") + (% "D")(mass "D") + (% "E")(mass "E") + (% F)(mass F) + (% G)(mass G) = AAM
(0.00146)(196) + (0.1002)(198) + (0.1684)(199) + (0.2313)(200) + (0.1322)(201) + (0.2980)(202) + (0.0685)(204) = x
30
35
30
0.28616 + 19.8396 + 33.5116 + 46.2600 + 26.5722 + 60.1960 + 13.974 = x
Atomic Number = Number of Protons
x = 200.63956 amu
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.
Isotope
Percent
Abundance
Mass
Mg-24
78.99
23.9850
18.94575
Mg-25
10.00
24.9585
2.49585
Mg-26
11.01
25.9826
2.86068
24.304 amu
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