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Mendeleev’s periodic table of 1869 contained 17 columns, with two nearly complete periods (sequences) of elements, from potassium to bromine and rubidium to iodine, preceded by two partial periods of seven elements each (lithium to fluorine and sodium to chlorine), and followed by three incomplete periods. In an 1871 paper Mendeleev presented a revision of the 17-group table, the principal improvement being the correct repositioning of 17 elements. He, as well as Lothar Meyer, also proposed a table with eight columns obtained by splitting each of the long periods into a period of seven, an eighth group containing the three central elements (such as iron, cobalt, nickel; Mendeleev also included copper, instead of placing it in Group I), and a second period of seven. The first and second periods of seven were later distinguished by use of the letters “a” and “b” attached to the group symbols, which were the Roman numerals.

With the discovery of the noble gases helium, neon, argon, krypton, radon, and xenon by Lord Rayleigh (John William Strutt) and Sir William Ramsay in 1894 and the following years, Mendeleev and others proposed that a new “zero” group to accommodate them be added to the periodic table. The “short-period” form of the periodic table, with Groups 0, I, II,…VIII, became popular and remained in general use until about 1930.

Based on an earlier (1882) model of T. Bayley, J. Thomsen in 1895 devised a new table. This was interpreted in terms of the electronic structure of atoms by Niels Bohr in 1922. In this table there are periods of increasing length between the noble gases; the table thus contains a period of 2 elements, two of 8 elements, two of 18 elements, one of 32 elements, and an incomplete period. The elements in each period may be connected by tie lines with one or more elements in the following period. The principal disadvantage of this table is the large space required by the period of 32 elements and the difficulty of tracing a sequence of closely similar elements. A useful compromise is to compress the period of 32 elements into 18 spaces by listing the 14 lanthanoids (also called lanthanides) and the 14 actinoids (also called actinides) in a special double row below the other periods.

Other versions of the periodic table

Alternate long forms of the periodic table have been proposed. One of the earliest, described by A. Werner in 1905, divides each of the shorter periods into two parts, one at either end of the table over the elements in the longer periods that they most resemble. The multiple tie lines connecting the periods in the Bayley-type table are thus dispensed with. This class of table, too, can be greatly simplified by removing the lanthanoid and actinoid elements to a separate area. By the mid-20th century this version of the table had become the most commonly used.

Discovery of new elements

The great value of the periodic law was made evident by Mendeleev’s success in 1871 in finding that the properties of 17 elements could be correlated with those of other elements by moving the 17 to new positions from those indicated by their atomic weights. This change indicated that there were small errors in the previously accepted atomic weights of several of the elements and large errors for several others, for which wrong multiples of the combining weights had been used as atomic weights (the combining weight being that weight of an element that combines with a given weight of a standard). Mendeleev was also able to predict the existence, and many of the properties, of the then undiscovered elements eka-boron, eka-aluminum, and eka-silicon, now identified with the elements scandium, gallium, and germanium, respectively. Similarly, after the discovery of helium and argon, the periodic law permitted the prediction of the existence of neon, krypton, xenon, and radon. Moreover, Bohr pointed out that the missing element 72 would be expected, from its position in the periodic system, to be similar to zirconium in its properties rather than to the rare earths; this observation led G. de Hevesy and D. Coster in 1922 to examine zirconium ores and to discover the unknown element, which they named hafnium.

Significance of atomic numbers

In spite of the corrections made by the redetermination of atomic weights, some of the elements in the Mendeleev and Lothar Meyer periodic tables of 1871 were still required by their properties to be put in positions somewhat out of the order of atomic weights. In the pairs argon and potassium, cobalt and nickel, and tellurium and iodine, for example, the first element had the greater atomic weight but the earlier position in the periodic system. The solution to this difficulty was found only when the structure of the atom was better understood.

About 1910 Sir Ernest Rutherford’s experiments on the scattering of alpha particles by the nuclei of heavy atoms led to the determination of the nuclear electrical charge. The ratio of the nuclear charge to that of the electron was noted to be roughly one-half the atomic weight. In 1911 A. van den Broek suggested that this quantity, the atomic number, might be identified with the ordinal number of the element in the periodic system (following the lead of Newlands, it had become customary to number the elements according to their position in the table). This suggestion was brilliantly confirmed in 1913 by H.G.J. Moseley’s measurements of the wavelengths of the characteristic X-ray spectral lines of many elements, which showed that the wavelengths did indeed depend in a regular way on the atomic numbers—identical with the ordinal numbers of the elements in the table. There is no longer any uncertainty about the position of any element in the ordered series of the periodic system.

That the exact atomic weight of an element is of small significance for its position in the periodic system is shown by the existence of isotopes of every element—atoms with the same atomic number but different atomic weights. The chemical properties of the isotopes of an element are essentially the same, and all the isotopes of an element occupy the same place in the periodic system in spite of their differences in atomic weight.

Elucidation of the periodic law

Detailed understanding of the periodic system has developed along with the quantum theory of spectra and the electronic structure of atoms, beginning with the work of Bohr in 1913. Important forward steps were the formulation of the general rules of the old quantum theory by William Wilson and Arnold Sommerfeld in 1916, the discovery of the exclusion principle by Wolfgang Pauli in 1925, the discovery of the spin of the electron by George E. Uhlenbeck and Samuel Goudsmit in 1925, and the development of quantum mechanics by Werner Heisenberg and Erwin Schrödinger during the same year. The development of the electronic theory of valence and molecular structure, beginning with the postulate of the shared electron pair by Gilbert N. Lewis in 1916, also played a very important part in explaining the periodic law (see chemical bonding).