No chemistry textbook, classroom, lecture theatre or research laboratory is complete without a copy of the periodic table of the elements. Since the earliest days of chemistry, attempts have been made to arrange the known elements in ways that revealed similarities between them. However, it required the genius of Mendeleev to see that arranging elements into patterns was not enough; he realised that there was a natural plan in which each element has its allotted place, and this applies not only to the known elements but to some that were still undiscovered. Today we have the so-called long form of the table. This has emerged supreme from well over 100 designs that have been proposed since the time of Mendeleev. With the advantage of hindsight we can now see why this form of the table was bound to succeed.

Today there are 109 known elements, and these are usually displayed in the form of a matrix called a periodic table. The term periodic came from the regular occurrence of certain chemical properties in the list of known elements when these are arranged in order of increasing relative mass. The common form, complete with the new group numbers (1-18) were finally agreed by the International Union of Pure Applied Chemistry, IUPAC, in 1985, after years of wrangling. In truly Parkinsonian fashion, this least important of changes has probably consumed the most effort!

Since Antoine Lavoisier first defined a chemical element and drew up a table of 33 of them for his book 'Traité Elémentaire de Chimie' (Treatise on the Chemical Elements) published in 1789, there have been attempts to classify them. Lavoisier himself grouped them into four categories on the basis of their chemical properties: gases, nonmetals, metals and earths. In the first category he listed substances that we now know as oxides but which at the time had defeated all attempts at separation.

This desire to identify and classify elements continued in chemistry for another 80 years until Mendeleev stumbled upon the correct classification - the periodic table. Today the periodic table is securely based on the properties of atomic number of the nucleus and the electron energy levels which surround it. Both of these concepts postdate Mendeleev by several decades. He, however perceived them indirectly through the related properties of atomic weight and chemical valency and arrived at the periodic table in 1869.

Because atomic weight, relative atomic mass, is roughly proportional to atomic number, and because valency, which manifests itself in the chemical composition, is based on the outermost electrons of an atom, Mendeleev had chosen the two properties that in his day most nearly reflected the fundamental principles on which the table today is based. Consciously or subconsciously, he arrived at the idea that a table existed with positions that were to be occupied by the elements, rather than the other way round - that the known elements determined the arrangement of the table, as others imagined.

This being so, Mendeleev then put the 65 elements he knew into his table and at the same time pointed out the many unoccupied positions in the overall scheme. He took the further and much bolder step of predicting the properties of these missing elements. Moreover, the gap in atomic weights between cerium (140) and tantalum (182) suggested to him that a whole period of the table remained to be discovered. Later in the century many of these elements, which we now call the lanthanides, were isolated.

Historical Background

Mendeleev’s periodic table of 1869 seems all the more remarkable when we consider his relative isolation from the main centres of chemical research in Western Europe, and the rather naive attempts made by scientists in those centres to bring some sort of order to the growing list of chemical elements.

As early as 1829 Johann Döbereiner announced his law of Triads, which referred to groups of three chemically similar elements in which the properties of the middle element could be inferred from the lighter and heavier ones. Such triads as lithium, sodium and potassium, sulphur, selenium and tellurium or chlorine, bromine and iodine are clear examples. By 1843 when Leopold Gmelin published the first edition of his famous Handbuch der Chemie , three tetrads and even a pentad - nitrogen, phosphorus, arsenic, antimony and bismuth - which we now recognise as group 15 of the p-block of the periodic table.

No real progress was going to be made in classifying elements until the one essential property common to them all, their atomic weight, was settled. This was done by Stanislao Cannizzaro in 1858. Prior to this, equivalent weights were used and for many elements there were several equivalent weights, depending upon the elements oxidation state.

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