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The Discovery of Atomic Structure
Part 1: Atomic Theory and the Electron
Democritus (460-370 B.C.) atomos is “that which cannot be further cut”
John Dalton (1766-1844)
The Atomic Theory (September 1803)
The chemical elements are composed of very minute indivisible particles of matter,
called atoms, which preserve their individuality in all chemical changes.
All atoms of the same element are identical in all respects, particularly in weight.
Different elements have atoms differing in weight. Each element is characterized by the
weight of its atom.
Chemical combinations occur by the union of the atoms of the elements in simple
numerical ratios.
1880-1890’s Physicists work with Gas Discharge Tubes (also called Cathode Ray Tubes)
J.J. Thompson
(1856-1940)
1897 Discovers that the atom contains tiny particles with negative electric charges.
The electron is discovered!
Figure 1-1:
Figure 1-2:
A charge difference between the 2 electrodes (+ and -) causes the tube to
glow!
Using a screen with a hole, the glow is now a beam which travels from the
cathode to the anode and is called a cathode ray.
The findings of Cathode Ray Tubes shows that cathode rays:
1. travel in straight lines.
2. cast shadows.
Suggests that the cathode ray is like light.
3. can turn pinwheels placed in the path.
Suggests that the cathode ray is made of particles.
4. heat a metal foil placed between the electrodes.
Suggests that the cathode ray has energy.
5. are always the same regardless of the nature of the electrode material or the gas in the
tube.
Cathode rays are a fundamental part of nature.
6. can be bent toward a positively charged plate and away from a negatively charged plate.
Indicates the cathode ray particles are negatively charged.
By 1900, Thompson and others conclude:
1. Electrons must be a part of the atoms of all elements.
2. Electrons carry one unit of negative charge.
3. Electrons have a mass that is 1/1800 the mass of a proton.
Thompson knew that ordinary matter is neutral. He therefore reasons that positive charges also
exist in atoms to balance this negative charge (he never detected them though). He proposes
that an atom is positive electricity with negative charges embedded in it. He calls his model the
Plum pudding model of the atom, named after a popular English desert.
Figure 1-1:
(+).
In a cathode ray tube, electrons travel as a ray from the cathode (-) to the anode
High
Voltage
Source
Gas at low Pressure
-
+
Metal
Cathode
“-”
Figure 1-2:
Screen
with hole
To vacuum pump
Direction of
Cathode Ray
(electrons)
Metal
Anode
“+“
Cathode rays are attracted by a positively charged plate. This attraction shows
the negatively charge nature of the particles (electrons).
High
Voltage
Source
Negatively Charged Plate
-
-
+
+
Metal
Cathode
“-”
Screen
with hole
To vacuum pump
Direction of
Cathode Ray
(electrons)
Metal
Anode
“+“
Positively Charge Plate
The Discovery of Atomic Structure
Part II:
The Discoveries of the Electron
Cathode Ray Tube of J.J. Thompson
High
Voltage
Source
+
A. Electric Field
influence
+
S
B. No influence
N
C. Magnetic Field
influence
-
Using a CRT with a Magnetic Field of Known strength.
Thompson applied a charge until the beam is brought back to the
middle of the screen.
Found the Charge ratio = -1.76 x 108 Coulombs
Mass
gram
*The coulomb is a unit of electrical charge. The charge of 6.25 x 1018 electrons = 1 Coulomb.
Either, the charge of an electron is very large or, the mass is very
small for this electron. Which do you think it is?
Robert Millikan
1909
(1868-1953)
Millikan determines the charge of a single electron.
X - rays
+
Adjustable
High
Voltage
Telescope
-
•
The rate of fall of an oil droplet without the electric field can be used to
determine the mass of a drop.
•
Then, with an electric field and the oil droplets bombarded with x-rays, the rate
at which the droplets fell changed. Some fell less rapidly, and some even rose
up toward the anode (+ plate).
Millikan interprets this to mean that the oil droplet is gaining one or more
electrons (negative charges).
He repeats the experiment thousands of times with oil, glycerine and mercury.
His data:
3.24 x 10-19 Coulombs
4.83 x 10-19 Coulombs
6.44 x 10-19 Coulombs
9.62 x 10-19 Coulombs
=
=
=
=
2 x 1.61 x 10-19 Coulombs
3 x 1.61 x 10-19 Coulombs
4 x 1.61 x 10-19 Coulombs
6 x 1.61 x 10-19 Coulombs
With this and some playing with the idea that droplets are picking up electrons,
he found the amount of charge was a whole number times the amount of charge of a
single electron (1.61 x 10-19 Coulombs)