CHEM1006 Lecture Notes - Lecture 9: Crookes Tube, Fluorescence, Johann Wilhelm Hittorf

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1 Aug 2016
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Lecture 9
Crookes found that as he pumped more air out of the tubes, the Faraday dark space spread
down the tube from the cathode toward the anode, until the tube was totally dark.
At the anode end of the tube, the glass of the tube itself began to glow.
What was happening was that as more air was pumped from the tubes, the electrons could
travel farther, on average, before they struck a gas atom.
By the time the tube was dark, most of the electrons could travel in straight lines from the
cathode to the anode end of the tube without a collision.
With no obstructions, these low mass particles were accelerated to high velocities by the
voltage between the electrodes.
When they reached the anode end of the tube, they were traveling so fast that, although they
were attracted to it, they often flew past the anode and struck the back wall of the tube.
When they struck atoms in the glass wall, they excited their orbital electrons to higher energy
levels, causing them to fluoresce.
A Crookes tube is a rarefied tube evacuated to a pressure below 10−6 atm.
Later researchers painted the inside back wall with fluorescent chemicals such as zinc sulfide, to
make the glow more visible.
Cathode rays themselves are invisible, but this accidental fluorescence allowed researchers to
notice that objects in the tube in front of the cathode, such as the anode, cast sharp-edged
shadows on the glowing back wall.
In 1869, German physicist Johann Hittorf was first to realize that something must be traveling in
straight lines from the cathode to cast the shadows.
J.J. Thomson studied cathode ray tubes and came up with the idea that the particles in the
cathode beams must be negative because they were repelled by negatively charged items and
attracted by positively charged items.
He called these super tiny pieces of the atom, "Electrons." Through his experiments, Thomson
disproved Dalton's atomic theory, because Dalton's atomic theory stated that atoms were the
smallest piece of the matter in the universe and they were indivisible.
Clearly, the presence of electrons negated these portions of Dalton's atomic theory.
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