Post on 05-Jan-2016
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 p = 1.007276 amu
1 n = 1.008665 amu
1 e- = 0.0005486 amu
• 1 amu = 1/12 the mass of a 12C atom
• 1.992 × 10-23 g / 12 = 1.66 x 10-24 g = 1 amu+
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Nucleus
Electrons
Nucleus
Neutron
Proton
Carbon-12Neutrons 6Protons 6Electrons 6
Isotopes• The number of protons inside the nucleus at the centre
of an atom decides what element it is.• Dif ferent atoms with the same number of protons and a
dif ferent number of neutrons are known as isotopes.• For example, there are three nat ur ally occur ring 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 radio active carbon-14.
Isotopes
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Nucleus
Electrons
Nucleus
Neutron
Proton
Carbon-12Neutrons 6Protons 6Electrons 6
Nucleus
Electrons
Carbon-14Neutrons 8Protons 6Electrons 6
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Nucleus
Neutron
Proton
3 p+
3 n02e– 1e– 3 p+
4 n02e– 1e–
6Li 7Li
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Electrons
Nucleus
Neutron
Proton
Lithium-6Neutrons 3Protons 3Electrons 3
Nucleus
Electrons
Nucleus
Neutron
Proton
Lithium-7Neutrons 4Protons 3Electrons 3
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Naming Isotopes
• Put the mass number after the name of the element
• carbon- 12
• carbon -14
• uranium-235
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Isotopic Abundance
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
Isotopes
• Because of the existence of isotopes, the mass of a collection of atoms has an AVERAGE value.
• Average Atomic Mass = weighted average of the masses of the isotopes of an element
Average Atomic Mass
• Weighted average of all isotopes• The mass indicated on the Periodic Table• Usually rounded to 2 decimal places
Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem
Avg.AtomicMass
= (mass)(%) + (mass)(%) …
100
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.
Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem
Avg.AtomicMass
= (16)(99.76) + (17)(0.04) + (18)(0.20)
100= 16 amu
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.
Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem
Avg.AtomicMass
= (35)(8) + (37)(2)
10= 35 amu
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• 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
Weighing atomsgas sampleenters here
filament currentionizes the gas
ions acceleratetowards chargedslit
magnetic fielddeflects lightest ionsmost
ions separated by massexpose film
The first mass spectrograph was built in 1919 by F. W. Aston, who received the 1922 Nobel Prize for this accomplishment
Copyright © 1997-2005 by Fred Senese
100
90
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60
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40
30
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10
034 35 36 37
Ab
un
dan
ce
Mass
Mass spectrum of chlorine. Elemental chlorine (Cl2) contains only two isotopes: 34.97 amu (75.53%) and 36.97 (24.47%)
AAM = (34.97 amu)(0.7553) + (36.97 amu)(0.2447)
AAM = (26.412841 amu) + (9.046559 amu)
AAM = 35.4594 amu
Cl-35
Cl-37
Cl35.4594
17
Mass Spectrophotometer
electron beam
magnetic field
gas
stream of ions of differentmasses lightest
ions
heaviest ions
Dorin, Demmin, Gabel, Chemistry The Study of Matter 3rd Edition, page 138
Mass Spectrum for Mercury
196 197 198 199 200 201 202 203 204
Mass numberMass number
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ativ
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umb
er o
f at
oms
Rel
ativ
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umb
er o
f at
oms
30
25
20
15
10
5
196 199 201 204
198 200 202
Mass spectrum of mercury vaporMass spectrum of mercury vapor
The percent natural abundances The percent natural abundances for mercury isotopes are:for mercury isotopes are:
Hg-196 0.146%Hg-196 0.146% Hg-198 10.02%Hg-198 10.02% Hg-199 16.84%Hg-199 16.84% Hg-200 23.13%Hg-200 23.13% Hg-201 13.22%Hg-201 13.22% Hg-202 29.80%Hg-202 29.80% Hg-204 6.85%Hg-204 6.85%
(The photographic record has been converted to a scale of relative number of atoms)
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 64.93 amu.
... )B"" )(massB"" (% )A"" )(massA"" (% (AAM) massatomic Average
Copper for amu 63.548 A.A.M.
amu 20.06337 amu 43.48463 A.A.M.
amu) .93(0.309)(64 amu) .93(0.691)(62 A.A.M.
Cu29
63.548
IsotopeIsotopePercentPercent
AbundanceAbundanceMassMass
Cu-63 69.1 62.93
Cu-65 64.93
43.48463
20.0633730.9
63.548
Protons Neutrons Electrons Mass number
Cu-65 A B 29 C
Argon D E F 40
Ba2+ 56 G H I
A. B. C.
D. E. F.
G. H. I.
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% abundantisotope “B” has a mass of 120.12 amu and is 12.36% abundant
Find the mass of isotope “C”. Show work for credit.
Extra Credit: What is a cation?
amu 119.7932 X
0.005 0.005
amu X 0.005 0.598966
amu X 0.005 0.598966
amu) (0.005)(X amu 14.846832 amu 102.764202 amu 118.21
amu) (0.005)(X amu) 20.12(0.1236)(1 amu) 17.93(0.8714)(1 amu 118.21
)C"" )(massC"" (% )B"" )(massB"" (% )A"" )(massA"" (% Mass Atomic Average
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”. Show work for credit.
The percent natural abundances The percent natural abundances for mercury isotopes are:for mercury isotopes are:
Hg-196 0.146%Hg-196 0.146% Hg-198 10.02%Hg-198 10.02% Hg-199 16.84%Hg-199 16.84% Hg-200 23.13%Hg-200 23.13% Hg-201 13.22%Hg-201 13.22% Hg-202 29.80%Hg-202 29.80% Hg-204 6.85%Hg-204 6.85%
(0.00146)(196) + (0.1002)(198) + (0.1684)(199) + (0.2313)(200) + (0.1322)(201) + (0.2980)(202) + (0.0685)(204) = x
0.28616 + 19.8396 + 33.5116 + 46.2600 + 26.5722 + 60.1960 + 13.974 = x
x = 200.63956 amu
Hg200.59
80
(% "A")(mass "A") + (% "B")(mass "B") + (% "C")(mass "C") + (% "D")(mass "D") + (% "E")(mass "E") + (% F)(mass F) + (% G)(mass G) = AAM
ABCDEFG
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
charge
8 8 8
Potassium 39 +1
Br 45 -1
30 35 30
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.
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IsotopeIsotopePercentPercent
AbundanceAbundanceMassMass
Mg-24 78.99 23.9850
Mg-25 10.00 24.9585
Mg-26 25.9826
24.304 amu
18.94575
2.49585
2.8606811.01