Atomic number consistent with the modern practice of ordering the elements by proton number, "Z", but that number was not known or suspected at the time. A simple numbering based on periodic table position was never entirely satisfactory, however. Besides the case of iodine and tellurium, later several other pairs of elements (such as argon and potassium, cobalt and nickel) were known to have nearly identical or reversed atomic weights, thus requiring their placement in the periodic table to be determined by their chemical properties. However the gradual identification of more and more chemically similar lanthanide elements, whose atomic number was not
Effective atomic number possible to obtain a much more accurate single-valued Z by weighting against the spectrum of the source. The effective atomic number for electron interactions may be calculated with a similar approach; see for instance Taylor "et al." 2009 and Taylor 2011. The cross-section based approach for determining Z is obviously much more complicated than the simple power-law approach described above, and this is why freely-available software has been developed for such calculations (Taylor "et al." 2012). Effective atomic number Effective atomic number has two different meanings: one that is the effective nuclear charge of an atom, and one that calculates