2. 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
3. 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.
4. Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 117
The Effect of an Electric Field on
Cathode Rays
High
voltage
cathode
source of
high voltage
positive
plate
negative
plate
anode
_
+
5. 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
6. 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
7. Cathode
Rays
•Cathode ray = electron
•Electrons have a
negative charge
Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, pages 117-118
High
voltage
cathode
source of
high voltage
yellow-green
fluorescence
shadow
(A) The effect of an obstruction on cathode rays
(B) The effect of an electric field on cathode rays
High
voltage
cathode
source of
high voltage
positive
plate
negative
plate
anode
source of
low voltage
+
-
8. 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
9. 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
Spherical cloud of
Positive charge
Electrons
10. 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
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