ATOMIC NUMBERS It was from the study of the X-ray spectra of various elements, all exhibiting bright lines similar in type, that H. G. J. Moseley was led to his Atomic Numbers. The vibration-frequency of these lines varied as the squares of whole numbers which advanced by one unit as the series of elements was ascended.
This series of whole numbers when applied in order to the elements in the Periodic Table, beginning with H=1, gives the number of units of positive electricity in the nucleus and the number of negative electrons round the nucleus required to neutralize them, thus fixing the number of electrons attached to each element—a number ranging from one in the hydrogen atom to 92 in the uranium atom. The atomic numbers in this system are more fundamental than the atomic weights, for the properties of the elements depend directly upon them. The Table of Atomic Numbers could, like the Periodic Table, claim the power of suc cessful prediction, for the element hafnium (celtium) (q.v.), subsequently discovered, filled the vacancy left at No. 72.
The breaking down of radioactive elements such as uranium and thorium by the successive losses of a helium atom to produce in many cases substances which appeared to have almost identical chemical characters, and the natural occurrence of groups of elements of similar character and approximately equal atomic weights, led F. Soddy and K. Fajans, quite independently of each other to formulate the theory of isotopes (q.v.), i.e., of elements having the same number in the atomic order, but hav ing different atomic weights. If, for instance, both uranium and thorium lose helium atoms until they reach a stable form in lead, it might be conjectured that two forms of lead could be found—associated with different minerals—differing only in atomic weight. Chemical analyses have shown such differences in lead from different sources, so that it is permissible to regard ordinary lead as a mixture of two isotopes. This conclusion is much strengthened by the actual separation by F. W. Aston of the positively charged atoms travelling to the negative pole in the case of many of the simpler elements, of which neon formed a striking example. Similarly Aston was able to show that ordinary chlorine contains at least two isotopes differing by two weight units. In all the cases examined, Aston found the weights of each isotope to be close to a whole number.
Lastly Rutherford, using the newly ejected helium atom as his shell, and C. T. R. Wilson's water-laden gas as marker, has shown that certain elements of low uneven atomic number yield on bombardment a swiftly moving atom which is conjectured to be hydrogen. Elements of even number are not so attacked. The inference is that elements such as nitrogen may contain hydrogen atoms in or close to the nucleus, whilst the even elements are built up of helium atoms, in each of which four hydrogen atoms are so packed that they have lost a portion of their mass.
In Bohr's theory of the genesis of atoms, it is assumed that electrons are bound on successively to a nucleus originally naked. If such a synthesis begins in the interior of a star by the coalescence of four hydrogen atoms into one helium atom weigh ing less than its four components, there must be a definite loss of mass (energy) which might maintain the heat and radiation of the star. Again, if the outer electrons failed to join up, or were stripped off in the turmoil of the stellar centre, matter far denser than any of our elements might be formed. Of such condensation of matter there is astronomical evidence in dense stars—such as the Companion of Sirius.
BIBLIOGRAPHY.-M. Berthelot, Les Origines de l'Alchimie (1885) ; Bibliography.-M. Berthelot, Les Origines de l'Alchimie (1885) ; A. Ladenburg, History of Chemistry since the time of Lavoisier (trans. L. Dobbin, 1900) ; T. E. Thorpe, History of Chemistry (1921) ; J. M. Stillman, History of Early Chemistry (1924) ; D. Mendeleeff, Principles of Chemistry (ed. T. H. Pope, 3rd ed. 1905) ; W. Ostwald, Outlines of General Chemistry (1912) ; G. F. Hood and J. A. Carpenter, Text-book of Practical Chemistry (1921) ; T. E. Thorpe, Dictionary of Applied Chemistry, 7 vols. (1921, etc.) ; F. Ullman, Enzyklopaedie der Technischen Chemie, 12 vols. (1922) ; E. J. Holmyard, The Cultivation of Gold, Al Dragi (Paris, 1923) ; Roscoe and Schorlemmer, Treatise on Chemistry, vols. i. and ii. (new edition) . See also F. A. Mason, Introduction to the Literature of Chemistry (1924), which gives a list of journals in which original papers appear, and The Chemist's Yearbook, 2 vols. yearly, ed. F. W. Atack, which contains useful information and lists of journals. (H. B. D.)