Unit 9
The Relationship Between Electricity
and the Atom
•Electrolysis (3.1)
•Cathode Ray Tubes (3.1)
•Mass-to-Charge Ratio for Electron (3.1)
•Positive Particles (3.1)
•Determination of Electron Charge (3.1)
The First Battery (3.1)
•Alessandro Volta (1800) invents a cell similar to
today’s battery
•This was around the time a body
of evidence was beginning to
suggest that matter had electrical
properties
•The battery proved to be useful in
exploring these properties and in
further understanding the atom
Experiments with Electricity (3.1)
•Humphry Davy (early 1800’s) used powerful
batteries to break apart compounds and discover
new elements
•Michael Faraday, a protegè of Davy, further
developed a field called electrochemistry.
•Electrochemistry recognizes that charged particles
must be present to conduct electricity
Cathode Ray Tubes (3.1)
•William Crookes (1875) developed a low pressure
gas discharge tube as illustrated below.
Apply voltage
across here
Metal electrodes
Screen coated
with zinc sulfide
– will fluoresce
Most of the air is removed. Application of a voltage across the electrodes leads to
a fluorescence on the screen as seen in the right hand image (simulated beam).
The beam is called a cathode ray, where the term cathode refers to one of the
electrodes. The ray emanates from the cathode.
Image from www.crtsite.com
The Nature of Cathode Rays (3.1)
•J.J. Thomson (1897) applied an electric field to the cathode ray in
a Crooke’s tube with a result similar to that shown in the figure.
•The cathode ray was deflected away
from the negative plate suggesting the
cathode ray has properties of
negatively charged particles – named
electrons by Thomson.
•Based on the curvature of the
deflection, Thomson could determine the
ratio of the mass of the negative particle to its electric charge.
•Characteristics of the electrons were independent of the metal
used as electrodes – suggests that the electron is common to
atoms of all elements. Notice this is contrary to Dalton’s indivisible
atom concept from nearly 100 years earlier.
Complementary Positive Particles(3.1)
•Eugen Goldstein (1886) performed
experiments with a cathode ray tube
which was perforated as in the image
•Found the usual electrons going away
Image from http://chemed.chem.purdue.edu/
from the cathode toward the anode
(right to left in the image) but also found positive particles
moving left to right
•The positive particles had varying masses depending upon
the metal used to make the electrodes in the cathode ray
tube.
Millikan’s Oil Drop Experiment (3.1)
•Oil droplets are sprayed into the upper chamber. As they leave the
atomizer, they will acquire negative charges just like static
electricity. Alternatively, X-rays or a similar source may be used to
produce charges
•As the charged particles enter the chamber, they fall due to gravity
•The bottom electrical plate is negatively charged so it repels the
falling charged particles. By
adjusting the voltage, the charged
particles may be made to “hang” in
the air. The mass may be determined
from the voltage required to suspend
the charged particles.
•By looking at the smallest possible
difference in charges, the mass of the
electron may be determined.
Image from http://library.thinkquest.org/