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.

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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+

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Nucleus

Neutron

Proton

3 p+

3 n02e– 1e– 3 p+

4 n02e– 1e–

6Li 7Li

+

+

+Nucleus

Electrons

Nucleus

Neutron

Proton

Lithium-6Neutrons 3Protons 3Electrons 3

Nucleus

Electrons

Nucleus

Neutron

Proton

Lithium-7Neutrons 4Protons 3Electrons 3

+

+

+

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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70

60

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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

Rel

ativ

e n

umb

er o

f at

oms

Rel

ativ

e n

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