bei48482_FM

Henry G. J. Moseley(1887–1915)

was born in Weymouth, on Eng-

land’s south coast. He studied

physics at Oxford, where his father

had been professor of anatomy. Af-

ter graduating in 1910, Moseley

joined Rutherford at Manchester,

where he began a systematic study

of x-ray spectra that he later contin-

ued at Oxford. From the data he was

able to infer a relationship between the x-ray wavelengths of an

element and its atomic number, a relationship that permitted him

to correct ambiguities in then-current atomic number assignments

and to predict the existence of several then-unknown elements.

Moseley soon recognized the important link between his discov-

ery and Bohr’s atomic model. By then World War I had broken

out and Moseley enlisted in the British Army. Rutherford unsuc-

cessfully tried to have him assigned to scientific work, but in 1915

Moseley was sent to Turkey on the ill-conceived and disastrous

Dardanelles campaign and was killed at the age of twenty-seven.

the energy differences between shells. Let us look at what happens when an energetic

electron strikes the atom and knocks out one of the K-shell electrons. The Kelectron

could also be raised to one of the unfilled upper states of the atom, but the difference

between the energy needed to do this and that needed to remove the electron com-

pletely is insignificant, only 0.2 percent in sodium and still less in heavier atoms.

An atom with a missing Kelectron gives up most of its considerable excitation en-

ergy in the form of an x-ray photon when an electron from an outer shell drops into

the “hole” in the Kshell. As indicated in Fig. 7.20, the Kseriesof lines in the x-ray

spectrum of an element consists of wavelengths arising in transitions from the L,M, N,

... levels to the Klevel. Similarly the longer-wavelength Lseriesoriginates when an
Lelectron is knocked out of the atom, the Mserieswhen an Melectron is knocked
out, and so on. The two spikes in the x-ray spectrum of molybdenum in Fig. 2.17 are
the K and K lines of its Kseries.
It is easy to find an approximate relationship between the frequency of the K x-ray
line of an element and its atomic number Z. AK photon is emitted when an L(n2)
electron undergoes a transition to a vacant K(n1) state. The Lelectron experiences
a nuclear charge of Zethat is reduced to an effective charge in the neighborhood
of (Z1)eby the shielding effect of the remaining Kelectron. Thus we can use
Eqs. (4.15) and (4.16) to find the K photon frequency by letting ni2 and nf1,
and replacing e^4 by (Z1)^2 e^4. This gives

   cR(Z1)^2   

Kx-rays  (7.21)

where Rme^4  820 ch^3 1.097  107 m^1 is the Rydberg constant. The energy of a

K x-ray photon is given in electronvolts in terms of (Z1) by the formula

E(K )(10.2 eV)(Z1)^2 (7.22)

In 1913 and 1914 the young British physicist H. G. J. Moseley confirmed Eq. (7.21)

by measuring the K frequencies of most of the then-known elements using the dif-

fraction method described in Sec. 2.6. Besides supporting Bohr’s newly formulated atomic

model, Moseley’s work provided for the first time a way to determine experimentally the

atomic number Zof an element. As a result, the correct sequence of elements in the

3 cR(Z1)^2



4

1



22

1



12

1



ni^2

1



nf^2

m(Z1)^2 e^4



820 h^3

256 Chapter Seven

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