Page 228 - 9780077418427.pdf

/Users/user-f465/Desktop
tiL12214_ch08_203-228.indd Page 205 9/1/10 9:45 PM user-f465
tiL12214_ch08_203-228.indd Page 205 9/1/10 9:45 PM user-f465 /Users/user-f465/Desktop

Magnet
–
+

Charged
plates
Detecting
screen

Cathode
rays

FIGURE 8.3 A cathode ray passed between two charged plates
is deflected toward the positively charged plate. The ray is also
deflected by a magnetic field. By measuring the deflection by both,
J. J. Thomson was able to calculate the ratio of charge to mass. He
was able to measure the deflection because the detecting screen
was coated with zinc sulfide, a substance that produces a visible
light when struck by a charged particle.

made by the magnet with the deflections made by the electric
FIGURE 8.1 This is a computer-generated model of a beryllium
field, Thomson could determine the ratio of the charge to mass
atom, showing the nucleus and electron orbitals. This configuration
can also be predicted from information on a periodic table (not to for an individual particle. Today, the charge-to-mass ratio is
11
scale). considered to be 1.7584 × 10 coulombs/kilogram (see p. 143 ).
A significant part of Thomson’s experiments was that he found
the charge-to-mass ratio was the same no matter what gas was
(positive terminal). Since this mysterious beam seemed to come in the tube or of what materials the electrodes were made.
out of the cathode, it was said to be a cathode ray. Thomson had discovered the electron, a fundamental particle
The English physicist J. J. Thomson figured out what the of matter.
cathode ray was in 1897. He placed charged metal plates on A method for measuring the charge and mass of the electron
each side of the beam (Figure 8.3) and found that the beam was was worked out by an American physicist, Robert A. Millikan,
deflected away from the negative plate. Since it was known that around 1906. Millikan used an apparatus like the one illus-
like charges repel, this meant that the beam was composed of trated in Figure 8.4 to measure the charge on tiny droplets of
negatively charged particles. oil. Millikan found that none of the droplets had a charge less
The cathode ray was also deflected when caused to pass
between the poles of a magnet. By balancing the deflections

High voltage
Oil
sprayer
Cathode + + + +
– Cathode rays Viewing
scope
Anode
Oil
droplets
To vacuum – – – –
pump +

FIGURE 8.4 Millikan measured the charge of an electron by
balancing the pull of gravity on oil droplets with an upward electri-
cal force. Knowing the charge-to-mass ratio that Thomson had
FIGURE 8.2 A vacuum tube with metal plates attached to a calculated, Millikan was able to calculate the charge on each drop-
high-voltage source produces a greenish beam called cathode rays. let. He found that all the droplets had a charge of 1.60 × 10 –19
These rays move from the cathode (negative charge) to the anode coulomb or multiples of that charge. The conclusion was that this
(positive charge). had to be the charge of an electron.

8-3 CHAPTER 8 Atoms and Periodic Properties 205