bei48482_FM

Many-Electron Atoms 257

periodic table could be established. The ordering of the elements by atomic number

(which is what matters) is not always the same as their ordering by atomic mass, which

until then was the method used. Atomic number was originally just the number of an

element in the list of atomic masses. For instance, Z27 for cobalt and Z28 for

nickel, but their respective atomic masses are 58.93 and 58.71. The order dictated by

atomic mass could not be understood on the basis of the chemical properties of cobalt

and nickel.

In addition, Moseley found gaps in his data that corresponded to Z43, 61, 72,

and 75, which suggested the existence of hitherto unknown elements that were later

discovered. The first two, technetium and promethium, have no stable isotopes and

were first produced in the laboratory many years later. The last two, hafnium and

rhenium, were isolated in the 1920s.

In the operation of this x-ray spectrometer, a stream of fast electrons is

directed at a sample of unknown composition. Some of the electrons

knock out inner electrons in the target atoms, and when outer electrons

replace them, x-ray are emitted whose wavelengths are characteristic of

the elements present. The identity and relative amounts of the elements

in the sample can be found in this way.

Example 7.8

Which element has a K x-ray line whose wavelength is 0.180 nm?

Solution

The frequency corresponding to a wavelength of 0.180 nm 1.80  10 ^10 m is

1.67 1018 Hz

3.00 108 m/s



1.80 10 ^10 m

c



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