The Foundations of Chemistry

mass of an electron is very small compared with the mass of either a proton or a neutron.

The charge on a proton is equal in magnitude, but opposite in sign, to the charge on an

electron. Let’s examine these particles in more detail.

THE DISCOVERY OF ELECTRONS

Some of the earliest evidence about atomic structure was supplied in the early 1800s by

the English chemist Humphry Davy (1778–1829). He found that when he passed electric

current through some substances, the substances decomposed. He therefore suggested

that the elements of a chemical compound are held together by electrical forces. In 1832–

1833, Michael Faraday (1791–1867), Davy’s student, determined the quantitative rela-

tionship between the amount of electricity used in electrolysis and the amount of chemical

reaction that occurs. Studies of Faraday’s work by George Stoney (1826–1911) led him

to suggest in 1874 that units of electric charge are associated with atoms. In 1891, he

suggested that they be named electrons.

The most convincing evidence for the existence of electrons came from experiments

using cathode-ray tubes(Figure 5-1). Two electrodes are sealed in a glass tube containing

5-2

The process is called chemical

electrolysis. Lysismeans “splitting

apart.”

Study Figures 5-1 and 5-2 carefully as

you read this section.

178 CHAPTER 5: The Structure of Atoms

Figure 5-1 Some experiments with cathode-ray tubes that show the nature

of cathode rays. (a) A cathode-ray (discharge) tube, showing the production

of a beam of electrons (cathode rays). The beam is detected by observing

the glow of a fluorescent screen. (b) A small object placed in a beam of

cathode rays casts a shadow. This shows that cathode rays travel in straight

lines. (c) Cathode rays have negative electric charge, as demonstrated by

their deflection in an electric field. (The electrically charged plates produce

an electric field.) (d) Interaction of cathode rays with a magnetic field is also

consistent with negative charge. The magnetic field goes from one pole to

the other. (e) Cathode rays have mass, as shown by their ability to turn a

small paddle wheel in their path.

+

+

+

+

+

Anode

Cathode

Collimator

Electron beam

(cathode rays)

Cathode

Anode

Fluorescent

screen

Shadow

Charged metal

plates –

+

Magnet

S

N

Paddle wheel

(a)

(b)

(c)

(d)

(e)