Chemistry - A Molecular Science

Observation

: The rays were deflected by

the electric and magnetic fields.

Conclusion

:

Cathode rays were not light rays, because light is not deflected by electric or magnetic fields. He concluded that

the rays had to be charged particles.

Observation:

The direction of the deflection was aw

ay from the negative plate and toward

the positive plate.

Conclusion:

The particles were negatively charged.

Observation:

The extent of the deflection (

in Figure 1.2) varies directly with the strength Δ

of the applied external field and the magnitude of the negative charge on the particles, but inversely with the mass of

the particle (a bowling ball would be

deflected far less than a Ping-Pong

® ball when struck with the same force).

Conclusion:

By measuring how far the particle was

deflected, Thomson determined that the

charge-to-mass ratio of the particle was

q/m = -1.76x

11 C

.kg

-1.*

The charge-to-mass ratio (

q/

m

) as determined by Thomson was much larger than had

been determined for any other charged particle

known at the time. In order for the ratio to

be so large, either

q (the charge) was very large, or

m

(the mass) was very small. He

assumed that the magnitude of the charge on the particles could not be that much greater than that measured for other charged particles, which meant that the mass of the particles must be much smaller. Indeed, he estimated the mass to be less than 1/1000th the mass of the hydrogen atom; contrary to Dalton’s atomic theory, a hydrogen atom was not the smallest particle. Thomson was shocked! Th

ese negatively charged particles were later

named ‘electrons’ because they were found to be the source of electricity.

* The minus sign results because the charge is negative.

Figure 1.3 Millikan’s oil drop experiment

1. Fine mist of oil is sprayed

into oil drum apparatus

2. Oil droplets

fall throughhole in plate 3. X-rays knockelectrons fromsurrounding air,which stick todroplet

4. Electrically charged
   plates influencedroplet's motion

X-ray Source

(+) ()

5. Observer times

droplet's motionand controlselectric field

droplet

Although Thomson had determined the char

ge-to-mass ratio and estimated a probable

mass for this new particle, another experiment was needed

to determine the charge and

mass separately. This experiment woul

d be performed fourteen years later.

R. A. MILLIKAN AND THE CHARGE OF THE ELECTRON (1909) Robert Millikan, an American physicist, was

the first to determine accurately the charge

on the electron. In an experiment at the University of Chicago, he welded two plates into an oil drum (Figure 1.3). The upper plate had a small hole drilled into it. Above the hole, he created a fine mist of oil droplets. Individual oil droplets passed randomly through the hole one-by-one to descend toward the lower plat

e. He used a microscope to observe the

fall of an individual droplet. Initially, the dr

op accelerated due to gravity, but eventually

the resistance due to the air stopped its acceler

ation and the drop began to fall at a constant

speed called its

terminal speed.

He determined the droplet's terminal speed and used it to

Chapter 1 The Early Experiments

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