Thomson Model of the Atom

1
Thomson Model of the Atom
J. J. Thomson - English physicist. 1897 Made a
piece of equipment called a cathode ray tube. It
is a vacuum tube - all the air has been pumped
out.
2
A Cathode Ray Tube
Zumdahl, Zumdahl, DeCoste, World of Chemistry
2002, page 58
3
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

4
A Cathode Ray Tube
Zumdahl, Zumdahl, DeCoste, World of Chemistry
2002, page 58
5
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.

6
The Effect of an Electric Field on Cathode Rays
Dorin, Demmin, Gabel, Chemistry The Study of
Matter , 3rd Edition, 1990, page 117
7
Thomsons Experiment
voltage source
-

vacuum tube
metal disks
8
Thomsons Experiment
voltage source
-

vacuum tube
metal disks
9
Thomsons Experiment
voltage source
ON
-
OFF

Passing an electric current makes a beam appear
to move from the negative to the positive end
10
Thomsons Experiment
voltage source
ON
-
OFF

11
Thomsons Experiment
voltage source
ON
-
OFF

By adding an electric field
he found that the moving pieces were negative.
12
Cathode Ray Experiment
-
Displacement
Volts
Anodes / collimators
Cathode

Deflection region
Drift region
13
Thomsons Calculations
Thomson PAPER

• Cathode Ray Experiment
• Thomson used magnetic and electric fields to
  measure and calculate the ratio of the cathode
  rays mass to its charge.

14
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 Daltons 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

15
Conclusiones

• He comparado con el valor de la masa / carga más
  ligera para la proporción de partículas cargadas.
  
• En comparación, Thomson calcula que las
  partículas de rayos catódicos pesaba 1 / 1000
  tanto como el hidrógeno, el átomo más ligero.
• Él llegó a la conclusión de que los átomos
  contienen partículas subatómicas, átomos de
  dividirse en partículas más pequeñas.
• Esta conclusión es contradicha Dalton postulado y
  no fue ampliamente aceptada por sus compañeros
  físicos y químicos de su época.
• Dado que todo el material del electrodo produce
  una idéntica de rayos, rayos catódicos partículas
  están presentes en todos los tipos de materia,
  universal negativamente cargado de partículas
  subatómicas más tarde el nombre de electrón

16
Cathode Rays
(A) The effect of an obstruction on cathode rays
shadow
source of high voltage
cathode
yellow-green fluorescence

• Cathode ray electron
• Electrons have a
• negative charge

(B) The effect of an electric field on cathode
rays
Dorin, Demmin, Gabel, Chemistry The Study of
Matter , 3rd Edition, 1990, pages 117-118
17
J.J. Thomson

• He proved that atoms of any element can be made
  to emit tiny negative particles.
• From this he concluded that ALL atoms must
  contain these negative particles.
• He knew that atoms did not have a net negative
  charge and so there must be balancing the
  negative charge.

J.J. Thomson
18
William Thomson (Lord Kelvin)

• In 1910 proposed the Plum Pudding model
• Negative electrons were embedded into a
  positively charged spherical cloud.

Zumdahl, Zumdahl, DeCoste, World of Chemistry
2002, page 56
19
Plum-Pudding Model
Zumdahl, Zumdahl, DeCoste, World of Chemistry
2002, page 56
20
Thomson Model of the Atom

• J.J. Thomson discovered the electron and knew
  that electrons could be emitted from matter
  (1897).
• William Thomson proposed that atoms consist of
  small, negative electrons embedded in a massive,
  positive sphere.
• The electrons were like currants in a plum
  pudding.
• This is called the plum pudding model of the
  atom.

electrons
-
-
-
-
-
-
-
-
21
Other pieces

• Proton - positively charged pieces
• 1840 times heavier than the electron
• Neutron - no charge but the same mass as a
  proton.
• How were these pieces discovered?
• Where are the pieces?