Numbering the Elements
In 1913 British physic H. G. J., Moseley generated characteristic x rays for as many elements as he could finds-be found 38-by using them as targets for electron bombardment in an evacuated tube of his own design. By means of a trolley manipulated by strings. Moseley was able to move the individual targets into the path of an electron beam. He measured the wavelengths of the emitted x rays by the crystal diffraction method described in Section . Moseley then sought (and found) regularities in theses extra as he moved from element to element in the periodic table. In particular, he noted that if. for a given spectral line such as Ko. he plotted for each element te square root of the frequency f against the position of the element in the periodic table. a straight line resulted. shows a portion of his extensive data. Moseley’s conclusion was this: We have here a proof that there is in the atom a fundamental quantity. which increases y regular steps as we pass from one element to the next. This quantity can only be the charge on the central nucleus. Owing to Moseley’s work. the characteristic x-ray spectrum became the universally accepted signature of an element. permitting the solution of a number of periodic table puzzles. Prior to that time (19 J3). the positions of elements in the table were assigned in order of atomic weight, although it was necessary to invert this order for several pairs of elements because of compelling chemical evidence; Moseley showed that it is the nuclear charge (that is. the atomic number Z) that is there al basis for numbering the elements. In 1913 the periodic table had several empty squares, and a surprising number ui claims for new element had been advanced. The x-ray spectrum provided a conclusive test of such claims. The lan handed elements, often called the rare earth elements, had been sorted out only imperfectly because their similar chemical properties made sorting difficult. Once Moseley’s work was reported. these elements were properly organized. In more recent times. the identities of some elements beyond uranium were pinned down beyond dispute when the elements became available in quantities large enough to permit a study of their individual x-ray spectra.It is not hard to see why the characteristic x-ray spectrum shows such impressive regularities from element to element whereas the optical spectrum in the visible and near-visible region does not: The key to the identity of’ an element is the charge on its nucleus. Gold. for example, is what it is because its atoms have a nuclear charge0 of +7ge (that is. Z = 79). An at on with one more elementary charge on its nucleus is mercury; with one fewer. it is platinum. The K electrons. which play such a large role in the production of the x-ray spectrum. lie very close to the nucleus and are thus sensitive probes of its charge. The optical spectrum. on the other hand. involves transitions of the outermost electrons, which are heavily screened from the nucleus by the remaining electrons of the atom and thus are not sensitive probes of nuclear
Accounting tor the Moseley Plot
Moseley’s experimental data, of which the Moseley plot of is but a part. can be used directly to assign the elements to their proper places in the periodic table. This can be done even if no theoretical basis for Moseley’s results can be established. However. there is such a basis. According to Eq. 40-24 the energy of the hydrogen atom is.
Consider now one of the two innermost electrons in the K shell of a multielectron atom. Because of the presence of the other K-shell electron. our electron “sees” an effective nuclear charge of approximately (Z – l)e, where e is the elementary charge and Z is the atomic number of the element. The factor e4 in Eq. 41 -24 is the product the measured energies. (c) Finally, plot the deviations and comment on the trend. Tile measured energies (eV) of the Ka photo ns for these elements are as follows:
(There is actually more than one Ka ray because of the splitting of the L energy level, but that effect is negligible for the elements listed here.) SEC. 41-12 H w lasers Work 53E. Lasers can be used to generate pulses of light whose durations are as short as 10 fs. (a) How many wavelengths of light (A =500 nm) are contained in such a pulse? (b) Supply the missing quantity X (in years):