ATOMIC NUMBER, in chemistry, is the ordinal number of an element in the series of the elements arranged in accordance with the periodic law (q.v.).
The chemical elements were formerly arranged in the periodic classification in ascending order of their atomic weights, but this arrangement led to three pairs of anomalies, viz., argon and potas sium, cobalt and nickel, and tellurium and iodine, in which the order of the atomic weight obviously disagreed with the position of the elements as shown by their chemical properties. This dis crepancy was completely cleared up in 1913 by H. G. J. Moseley, who measured the wave-length of X-rays given off by elements when bombarded with cathode rays and showed that the frequen cies of these X-rays were characteristic for each element. The square root of the frequency of the principal rays increased pro portionately with the rise in atomic number, and when this relationship is traced out with a group of elements including iron, cobalt, nickel and copper, it is found that cobalt precedes nickel although of higher atomic weight, and X-ray spectra reveal a simi lar inversion in the case of tellurium and iodine. Similarly, potas sium (atomic weight 39.1) is placed next but one after chlorine (atomic weight 35.46), thus leaving the intermediate position for argon (atomic weight 4o). In Moseley's own words these results show that "there is in the atom a fundamental quantity which increases by regular steps as we pass from one atom to the next. This quantity can only be the charge on the central positive nucleus." (Moseley in Philosophical Magazine, 1913 and 1914.) Chemical atoms are composed of positive units of electricity (protons) and of an equal number of negative units (electrons), the charge on any one of such units being 7 X I electro static units. The protons are concentrated in the nucleus which also contains a portion of the electrons, the remaining electrons being extra-nuclear or planetary. Thus an element of atomic weight WW and atomic number N will have a nucleus consisting of TV protons and W—N electrons surrounded by N planetary elec trons. Accordingly the atomic number is (I ) the ordinal number of the element, (2) the positive electrical charge on the nucleus, and (3) the number of planetary electrons surrounding the nucleus.
(G. T. M.)