History

As in the 19th century many elements were discovered chemists started to note similarities in their properties. The elements in a regular fashion, in early attempts to order, were hampered by several difficulties, particularly the fact (only later realized) that atomic masses do not increase regularly with atomic number. In the year 1869, Mendeleev published the first satisfactory form of the periodic table and even though several details of layout have evolved since then, his general idea has been reserved, of ordering elements horizontally in periods so that they fall in vertical groups with identical chemical properties.

Mendeleev was forced to leave several gaps for elements that are not yet discovered and his ability to predict their properties vindicated his approach.

The first satisfactory determination of atomic number (as opposed to atomic mass) appears from Moseley's studies of X-ray spectra in the year 1917. By determining the wavelength, and therefore frequency, of X-rays emitted from distinct elements, Moseley observed dissimilar series of X-ray lines. In every series the frequency (ν) of each line varied with atomic number (Z) according to the formula (1) where C and σ are constants for a given series. From the quantum theory of many electron atoms Moseley's law can be understood. When atoms are bombarded with high-energy electrons the X-rays are produced. These knock out electrons from filled orbitals, so providing 'vacancies' into which electrons can move from other orbitals and emit X ray photons. Distinct series of lines come from different vacancies; for instance, the highest-energy K series is excited when a 1s electron is removed. Equation 1 then expresses the energy variation between two kinds of orbital, with C depending on the values of n involved and σ on the screening constants.

Using Moseley's law allowed the remaining doubts in the structure of the periodic table to be resolved. At about the similar time the theoretical ideas of the quantum theory allowed the structure of the table to be understood. Bohr's aufbau (or building up) principle was developed before the final version of the theory was presented; following Schrödinger's equation (1926) the understanding was complete. The periodic table with its hypothetical/theoretical background remains one of the principal conceptual frameworks of inorganic chemistry.