The Foundations of Chemistry

Metal +

anode

(target)

X-rays

e–

ATOMIC NUMBER

Only a few years after Rutherford’s scattering experiments, H. G. J. Moseley (1887–1915)
studied X-rays given off by various elements. Max von Laue (1879–1960) had shown that
X-rays could be diffracted by crystals into a spectrum in much the same way that visible
light can be separated into its component colors. Moseley generated X-rays by aiming a
beam of high-energy electrons at a solid target made of a single pure element (Figure 5-6).
The spectra of X-rays produced by targets of different elements were recorded photo-
graphically. Each photograph consisted of a series of lines representing X-rays at various
wavelengths; each element produced its own distinct set of wavelengths. Comparison of
results from different elements revealed that corresponding lines were displaced toward
shorter wavelengths as atomic weights of the target materials increased, with a few excep-
tions. Moseley showed that the X-ray wavelengths could be better correlated with the
atomic number. On the basis of his mathematical analysis of these X-ray data, he con-
cluded that

each element differs from the preceding element by having one more positive charge

in its nucleus.

For the first time it was possible to arrange all known elements in order of increas-
ing nuclear charge. A plot illustrating this interpretation of Moseley’s data appears in
Figure 5-7.
We now know that every nucleus contains an integral number of protons exactly equal
to the number of electrons in a neutral atom of the element. Every hydrogen atom contains

5-5

In a modern technique known as

“X-ray fluorescence spectroscopy,” the

wavelengths of X-rays given off by a

sample target indicate which elements

are present in the sample.

5-5 Atomic Number 183

Figure 5-6 A simplified representation of the production of X-rays by bombardment of a
solid target with a high-energy beam of electrons.

H. G. J. Moseley was one of the

many remarkable scientists who

worked with Ernest Rutherford. In

1913, Moseley found that the

wavelengths of X-rays emitted by an

element are related in a precise way

to the atomic number of the

element. This discovery led to the

realization that atomic number,

related to the electrical properties of

the atom, was more fundamental to

determining the properties of the

elements than atomic weight. This

put the ideas of the periodic table on

a more fundamental footing.

Moseley’s scientific career was very

short. He enlisted in the British

army during World War I and died

in battle in the Gallipoli campaign in

1915.