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J.J. Thomson Discoverer of the Electron

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J.J. Thomson. Discoverer of the Electron. Background Information. Cathode Rays Form when high voltage is applied across electrodes in a partially evacuated tube. Originate at the cathode (negative electrode) and move to the anode (positive electrode) Carry energy and can do work - PowerPoint PPT Presentation

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Page 1: J.J. Thomson

J.J. Thomson

Discoverer of the Electron

Page 2: J.J. Thomson

Background Information

Cathode Rays• Form when high voltage is applied across

electrodes in a partially evacuated tube.• Originate at the cathode (negative electrode)

and move to the anode (positive electrode)• Carry energy and can do work• Travel in straight lines in the absence of an

external field

Page 3: J.J. Thomson

Source ofElectricalPotential

Metal Plate

Gas-filledglass tube Metal plate

Stream of negativeparticles (electrons)

A Cathode Ray Tube

Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 58

Page 4: J.J. Thomson

Cathode Ray Experiment

1897 Experimentation

• Using a cathode ray tube, Thomson was able to deflect cathode rays with an electrical field.

• The rays bent towards the positive pole, indicating that they are negatively charged.

Page 5: J.J. Thomson

The Effect of an Obstruction on Cathode Rays

Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 117

Highvoltage

cathode

source ofhigh voltage

yellow-greenfluorescence

shadow

Page 6: J.J. Thomson

Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 117

The Effect of an Electric Field on Cathode Rays

Highvoltage

cathode

source ofhigh voltage

positiveplate

negative plate

anode

_

+

Page 7: J.J. Thomson

Cathode Ray Experiment

Deflectionregion

Drift region

Displacement

+

-Anodes / collimators

Cathode

Volts

Page 8: J.J. Thomson

Thomson’s CalculationsCathode Ray Experiment

• Thomson used magnetic and electric fields to measure and calculate the ratio of the cathode ray’s mass to its charge.

Magneticdeflection

charge ofray particle

magneticfield

length ofdeflection region

length of drift region

mass of rayparticle

velocity ofray particle

x x x

x=

Electricdeflection

charge ofray particle

electricfield

length ofdeflection region

length of drift region

mass of rayparticle

velocity ofray particle

x x x

x=

2

magnetic deflection

electric deflection

magnetic field

electric fieldx velocity=

Page 9: J.J. Thomson

Conclusions

• He compared the value with the mass/ charge ratio for the lightest charged particle.

• By comparison, Thomson estimated that the cathode ray particle weighed 1/1000 as much as hydrogen, the lightest atom.

• He concluded that atoms do contain subatomic particles - atoms are divisible into smaller particles.

• This conclusion contradicted Dalton’s postulate and was not widely accepted by fellow physicists and chemists of his day.

• Since any electrode material produces an identical ray, cathode ray particles are present in all types of matter - a universal negatively charged subatomic particle later named the electron

Page 10: J.J. Thomson

So what does J.J. Thomson have to do with mass spec?

• Just as J.J. Thomson used a magnetic field to affect charged particles, so does a mass spectrometer.

• The machine sorts ions according to their mass to charge ratio, something Thomson was able to calculate for the electron using the results of his cathode ray experiments.

Highvoltage

cathode

source of

high voltage

positiveplate

negative plate

anode

_

+

Page 11: J.J. Thomson

What is mass spectrometry?Mass spectrometry is a technique used to separate a substance into ions based on their mass.

Molecules are bombarded by high energy particles that cause them to lose one electron and carry a +1 charge. These ions undergo further fragmentation producing smaller positive ions.

The spectrum produced plots intensity (abundance of ions) against the ions’ mass-to-charge ratio.

Substances can be identified by their characteristic fragment ions represented on a mass spectrum

Page 12: J.J. Thomson

Mass spectrometers that break up molecules into fragments that can be characterized by electrical methods. [image link]

Detectorplate

Least massive ionsIon-accelerating

electric field

Magnetic field

Heating device to vaporize sample

Positive ions

Sample

Electron beam

acceleratedIon beam

Most massive

ions

Slits

Page 13: J.J. Thomson

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

Page 14: J.J. Thomson

Inlet - ensures that the sample enters the machine with minimal loss

Source - sample components are ionized (the method by which this is done depends on the specific mass spectrometer being used.)

Analyzer - accelerates ion and separates them

Detector - records the charge induced when an ion passes by or hits a surface.

Signal Processor - produces a mass spectrum, a record of the m/z's at which ions are present.

*A vacuum must be used to maintain a low pressure. A low pressure reduces the collisions among the ions.

Components of a Mass Spectrometer

InletSignal

processor

Source Analyzer Detector

Vacuum

Page 15: J.J. Thomson

The general operation of a mass spectrometer is: 1. create gas-phase ions 2. separate the ions based on their mass-to-charge ratio 3. measure the quantity of ions of each mass-to-charge ratio

Electron Beam

Ion AcceleratingArray

MolecularSource

Magnetic FieldBends Path of Charged

Particles

Collector Exit Slit

Ho

Page 16: J.J. Thomson
Page 17: J.J. Thomson

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

Page 18: J.J. Thomson

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)

Page 19: J.J. Thomson

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

Page 20: J.J. Thomson

• 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

Page 21: J.J. Thomson

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

Page 22: J.J. Thomson

An electric or magnetic field can deflect charged particles.

The particles have kinetic energy as they move through a magnetic field (KE=1/2mv2).

The particles’ inertia depends on their mass.

A mass analyzer can steer certain masses to the detector based on their mass-to-charge ratios (m/z). by varying the electrical or magnetic field.

Typically ions in a mass spectrometer carry a +1 charge so the m/z ratio is equivalent to the ion’s mass.

What’s mass got to do with it?

Page 23: J.J. Thomson

What does a mass spectrum look like?

Intensity or ion abundance is plotted on the y-axis.The m/z ratio is plotted on the x-axis.The base beak is from the ion that is the most abundant and is assigned an intensity of 100%.The molecular ion peak, M+, is the peak due to the parent ion (the original molecule minus one electron).

Page 24: J.J. Thomson

40

30

20

10

50

90

80

70

60

100

0

5 10 15 20 25 30 35 40 45 50m/z

% R

ELA

TIV

E I

NT

EN

SIT

Y

Mass spectrum of carbon dioxide, CO2 molecular ion is seen at m/z 44.

12 16 28C+ O+

CO+

CO2+ M+

Page 25: J.J. Thomson

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

Page 26: J.J. Thomson

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

Page 27: J.J. Thomson

Where’s Waldo?

C-13

Page 28: J.J. Thomson

M +1 = 17[C13H4]+.

1.11

M+ = 15C12H3

+

86100 Base peak

M+ = 16Molecular ion

[C12H4]+.

12 13 14 15 16 17m/z

The Mass Spectrum of Methane

16

83

[C12H2]+.

[C12]+.

C12H+

Page 29: J.J. Thomson
Page 30: J.J. Thomson

Why is the Mass Spectrometer an Important Analytical Instrument?

Mass Spectrometers have been used in:

1) Forensics

2) Organic synthesis laboratories

3) The analysis of large biomolecules: proteins and nucleic acids

4) Drug Test

5) Determination of isotopic abundance

6) Identification of impurities in pharmaceutical products

7) Diagnosis of certain diseases.

Page 31: J.J. Thomson

• http://www.aist.go.jp/RIODB/SDBS/• http://www.infochembio.ethz.ch/links/en/

spectrosc_mass_lehr.html • http://dbhs.wvusd.k12.ca.us/AtomicStructure/Disc-of-

Electron-Intro.html• http://wps.prenhall.com/wps/media/objects/

340/348272/Instructor_Resources/Chapter_12/47

References