The Foundations of Chemistry

out to be integral multiples of the same number. He assumed that this smallest charge

was the charge on one electron. This value is 1.60218 10 
^19 coulomb (modern value).

The charge-to-mass ratio, e/m1.75882 108 C/g, can be used in inverse form to

calculate the mass of the electron:

m1.60218 10 
^19 C

9.10940 10 
^28 g

This is only about 1/1836 the mass of a hydrogen atom, the lightest of all atoms. Millikan’s

simple oil-drop experiment stands as one of the cleverest, yet most fundamental, of all

classic scientific experiments. It was the first experiment to suggest that atoms contain

integral numbers of electrons; we now know this to be true.

CANAL RAYS AND PROTONS

In 1886, Eugen Goldstein (1850–1930) first observed that a cathode-ray tube also gener-

ates a stream of positively charged particles that moves toward the cathode. These were

called canal raysbecause they were observed occasionally to pass through a channel, or

“canal,” drilled in the negative electrode (Figure 5-3). These positive rays,or positive ions,

are created when the gaseous atoms in the tube lose electrons. Positive ions are formed

by the process

Atom88ncatione
 or X88nXe
 (energy absorbed)

Different elements give positive ions with different e/mratios. The regularity of the e/m

values for different ions led to the idea that there is a unit of positive charge and that it

resides in the proton.The proton is a fundamental particle with a charge equal in magni-

tude but opposite in sign to the charge on the electron. Its mass is almost 1836 times that

of the electron.

5-3

1 g



1.75882 108 C

The charge on one mole (Avogadro’s
number) of electrons is 96,485
coulombs.

18 0CHAPTER 5: The Structure of Atoms

Figure 5-3 A cathode-ray tube with a different design and with a perforated cathode. Such

a tube was used to produce canal rays and to demonstrate that they travel toward the

cathode. Like cathode rays, these positiverays are deflected by electric or magnetic fields, but

in the opposite direction from cathode rays. Canal ray particles have e/mratios many times

smaller than those of electrons, due to their much greater masses. When different elements

are in the tube, positive ions with different e/mratios are observed.

+

+

+

+

+

+

+

+

Cathode, with holes

(pierced disk) –

Positive ion

Electron

Anode

The proton was observed by
Rutherford and James Chadwick in
1919 as a particle that is emitted by
bombardment of certain atoms with
-particles.

The value of e/mobtained by
Thomson and the values of eand m
obtained by Millikan differ slightly
from the modern values given in this
text because early measurements were
not as accurate as modern ones.