Isotopes

• Atoms of the same element with different mass numbers.

Mass #

Atomic #

• Nuclear symbol:

• Hyphen notation: carbon-12carbon-12Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem

12

6 C

Isotopes

+

+

+

+

+

+

Nucleus

Electrons

Nucleus

Neutron

Proton

Carbon-12Neutrons 6Protons 6Electrons 6

Nucleus

Electrons

Carbon-14Neutrons 8Protons 6Electrons 6

+

+

+

+

+

+

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

+

+

+

Isotopes

• Chlorine-37

– atomic #:

– mass #:

– # of protons:

– # of electrons:

– # of neutrons:

17

37

17

17

20

Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem

Cl37

17

37

17 Cl

Relative Atomic Mass

• 12C atom = 1.992 × 10-23 g

• 1 p = 1.007276 amu

1 n = 1.008665 amu

1 e- = 0.0005486 amu

• atomic mass unit (amu)

• 1 amu = 1/12 the mass of a 12C atom+

+

+

+

+

+

Nucleus

Electrons

Nucleus

Neutron

Proton

Carbon-12Neutrons 6Protons 6Electrons 6

Average Atomic Mass

• weighted average of all isotopes

• on the Periodic Table

• round 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= 16.00

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= 35.40 amu

100

90

80

70

60

50

40

30

20

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 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

Mass Spectrometry

- +

Photographic plate

196 199 201 204

198 200 202

Mass spectrum of mercury vaporMass spectrum of mercury vapor

Hill, Petrucci, General Chemistry An Integrated Approach1999, page 320

Stream of positive ionsStream of positive ions

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)

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

Natural uranium, atomic weight = 238.029 g/molDensity is 19 g/cm3. Melting point 1000oC.

Two main isotopes:

U238 92

U235 92

99.3%

0.7%

Because isotopes are chemically identical(same electronic structure), they cannot beseparated by chemistry.

So Physics separates them by diffusion orcentrifuge (mass spectrograph is too slow)…

Separation of Isotopes

(238 amu) x (0.993) + (235 amu) x (0.007)

236.334 amu + 1.645 amu

237.979 amu

U238

92

• Assume you have only two atoms of chlorine.• One atom has a mass of 35 amu (Cl-35)• The other atom has a mass of 36 amu (Cl-36)

• What is the average mass of these two isotopes?

35.5 amu

• Looking at the average atomic mass printed on the periodic table...approximately what percentage is Cl-35 and Cl-36?

55% Cl-35 and 45% Cl-36 is a good approximation

Cl35.453

17

Using our estimated % abundance data

55% Cl-35 and 45% Cl-36

calculate an average atomic mass for chlorine.

Cl35.453

17

Average Atomic Mass = (% abundance of isotope "A")(mass "A") + (% "B")(mass "B")

AAM = (% abundance of isotope Cl-35)(mass Cl-35) + (% abundance of Cl-36)(mass Cl-36)

AAM = (0.55)(35 amu) + (0.45)(36 amu)

AAM = (19.25 amu) + (16.2 amu)

AAM = 35.45 amu

Isotopes

Dalton was wrong.

Atoms of the same element can have different numbers of neutrons

different mass numbers

called isotopes

California WEB

C-12 vs. C-14

Naming Isotopes

• Put the mass number after the name of the element

• carbon- 12

• carbon -14

• uranium-235

California WEB

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

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

Oxygen

Bromine

Zinc

O

K

Zn

8

19

35

16

80

19

35

30

20

65

18

36

0

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

A N S W E R K E Y

Atomic Mass

• How heavy is an atom of oxygen?• There are different kinds of oxygen atoms.• More concerned with average atomic mass.• Based on abundance of each element in nature.• Don’t use grams because the numbers would be

too small

carbon atom

(12 amu)

Measuring Atomic Mass

• Unit is the Atomic Mass Unit (amu)• One twelfth the mass of a carbon-12 atom. • Each isotope has its own atomic mass we need

the average from percent abundance.

(1 amu)

(1 amu)(1 amu)

(1 amu)(1 amu) (1 amu)

(1 amu) (1 amu)

(1 amu) (1 amu)(1 amu) (1 amu)

Mass spectrums reflect the abundance of naturally occurring isotopes.

Hydrogen

Carbon

Nitrogen

Oxygen

Sulfur

Chlorine

Bromine

1H = 99.985% 2H = 0.015%

12C = 98.90% 13C = 1.10%

14N = 99.63% 15N = 0.37%

16O = 99.762% 17O = 0.038% 18O = 0.200%

32S = 95.02% 33S = 0.75%

34S = 4.21% 36S = 0.02%

35Cl = 75.77% 37Cl = 24.23%

79Br = 50.69% 81Br = 49.31%

Natural Abundance of Common Elements

For example….Methane

For carbon 1 in approximately 90 atoms are carbon-13

The rest are carbon-12 the isotope that is 98.9% abundant.

So, for approximately 90 methane molecules…1 carbon is carbon-13

Where’s Waldo?

C-13

Calculating averages

• You have five rocks, four with a mass of 50 g, and one with a mass of 60 g. What is the average mass of the rocks?

• Total mass = (4 x 50) + (1 x 60) = 260 g

• Average mass = (4 x 50) + (1 x 60) = 260 g 5 5

• Average mass = 4 x 50 + 1 x 60 = 260 g 5 5 5

California WEB

Calculating averages

• Average mass = 4 x 50 + 1 x 60 = 260 g 5 5 5

• Average mass = .8 x 50 + .2 x 60

• 80% of the rocks were 50 grams

• 20% of the rocks were 60 grams

• Average = % as decimal x mass + % as decimal x mass + % as decimal x mass +

California WEB

Isotopes

• Because of the existence of isotopes, the mass of a collection of atoms has an average value.

• Average mass = ATOMIC WEIGHT

• Boron is 20% B-10 and 80% B-11. That is, B-11 is 80 percent abundant on earth.

• For boron atomic weight= 0.20 (10 amu) + 0.80 (11 amu) = 10.8 amu

Periodic Table

• Dmitri Mendeleev developed the modern periodic table.

• Argued that element properties are periodic functions of their atomic weights.

• We now know that element properties are periodic functions of their ATOMIC NUMBERS.

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

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

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?

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?

A positively charged atom. An atom that has lost a(n) electron(s).

119.7932 amu

Protons Neutrons Electrons Mass number

Cu-65 A = 29 B = 36 29 C = 65

Argon D = 18 E = 22 F = 18 40

Ba2+ 56 G = 81 H = 54 I = 137

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.