The modern form of the Periodic Table, often called the “long form”, is an ordering of elements by increasing Atomic Number, which is systematically broken into rows, called “Periods”, which are then and stacked on top of one another to make columns, called “Groups”, as shown in Table 1. However, the system used to break up the sequence of increasing Atomic Numbers into these Periods does not produce rows of Elements of equal length. The first row is only two elements long, then the following rows come in pairs which are respectively 8, 18, and 30 elements long. This gives a Table large enough to include all the known and probable elements.

However, the later rows are not made longer by adding Elements to the end of the row but by expanding the row in the middle. As a result, each time this row expansion occurs, it squeezes a new sequence of Elements into the row, which repeats itself in all later rows. These inserted sets of elements are called “Blocks”, and over time, they have been given working names in order to catalogue the Elements into convenient “families”. Together the Elements of the first two and last six Groups form the Main Block, the set of ten Elements inserted in the fourth Period is called the Transition Block. Within the Main Block the change from metallic to non-metallic character occurs through intermediary Elements called the metalloids, which are found in a “Diagonal Sequence” as shown in Figure 1. The set of fourteen inserted in the sixth Period is called the Lanthanide Block. The corresponding set in the seventh period is called the Actinide Block

This long form of the periodic Table was pieced together, from an ever in increasing data base of empirical information, during the whole 19^th century. Indeed, for the first 70 years of that century, there was no formal Periodic Table at all. At first, the idea that the existing Elements were chemically unchangeable substances had to be made clear. Then using this criterion, these known Elements were slowly characterized by an essentially random search for their occurrence in all substances in nature. This was a long, labourious task because of the large number of known Elements and the difficulties of the extraction procedures needed to isolate many of them.

As this characterization of the bulk and atomic properties of Elements went on, Table 1, the large amounts of empirical data made it very difficult to see any overall picture. However, following the work of the Swedish analytical chemist Berzelius from 1810 to 1840, the “atomic weights” of the Elements became well known and were an obvious independent basis for cataloguing the Elements in a linear sequence. However, by the 1860's, the chemical properties of enough Elements had been determined to show that they could be grouped into repetitive Behaviour patterns along this sequence. This grouping lead many chemists to try to place the Elements into some convenient scheme which would clearly illustrate this “periodic’ Behaviour. The idea which gained widest acceptance was that this pattern could be shown most clearly by breaking the atomic weight sequence into a new Period each time the Behaviour changed from “non-metallic” to “metallic”. Since Helium and its heavier Group 8 Elements were unknown at that time, this break apparently occurred after seven Elements in each of the first two Periods. However, for the next two Periods an additional 10 Elements, representing a more gradual range from metallic to nonmetallic Behaviours, appeared in the middle of the Period. This required setting up a compromise scheme for cataloguing the Elements, which we call the “short form” of the Periodic table. It was first devised by Dimitri Mendeleev in 1869 and followed rapidly by many versions, including the more familiar “long form” used today.

The importance of this empirical, two-dimensional catalogue of elemental properties was quickly recognized and exploited. In spite of the absence of any theory explaining why the Behaviours repeated themselves in this way, the catalogue was well enough defined to show many blank positions in the Periods where Elements “must exist”. Thus since 1870, the hunt for these predicted Elements has been pursued vigorously. In the true spirit of competition and national pride, no opportunity was lost in claiming the discoveries of the missing Elements. The consequences of this race can be seen in the names given to these Elements. Initially, the stable, reasonably abundant heavy Elements that were missing from the Main Block were found; Gallium (in France) and Germanium (in Germany). Later the more complicated problem of the missing transition Elements was addressed. The search was only completed with the discovery of the purely artificial Element Technetium in Italy in1937.The most difficult Elements to recognize were those at the beginning of Transition Element rows, including Yttrium (Ytterby, a suburb of Stockholm, Sweden), Lutetium, (Latin for Paris, France) and Hafnium (Latin for Copenhagen, Denmark), because they all form extremely stable oxides which make them difficult to isolate as pure metals.

However, the full power of this empirical model of nature was only revealed when two completely new sets of Elements were predicted and discovered. The first set of these new Elements filled the Period gap of metallic Elements between Barium and Lutetium with 14 “Rare Earth” Elements. The equivalent gap in the next Period, between the radioactive Elements Radium and Lawrencium, was filled by the 14 “Actinides”. Work to discover all of these Elements and to claim their identities ended only in 1958 with the confirmation of the artificial nuclear synthesis of Nobelium. More than 130 years after the Periodic Table was proposed, research continues today on actinically synthesising and naming the highly unstable Elements beyond Lawrencium.

The second set of new Elements filled the Group at the ends of all the Periods. The existence of this set of 5 new Elements was predicted because of the large mass jump between the most non-metallic Halide Element at the end of each Period and the most metallic Alkali Element at the beginning of the next Period. However, it was not obvious what kind of physical and chemical properties should be expected to connect these non-metallic and metallic Behaviours. The answer turned out to be essentially no properties at all. Thus, discovery of this Group was delayed until 1910. The lightest Element of this Group was discovered first, by its spectrum in the atmosphere of the Sun and consequently called Helium. Once it was understood that these Elements would all be unreactive monatomic gases, the heavier members of the new Group 8 were quickly found together as gaseous “impurities” of air and identified by the colours in their emission spectra.

When this full form of the Periodic Table emerged from its century of empirical discovery, four types of correlation between the Behaviour of the Elements and their positions in the Table became clear. Initially these correlations were used to help in the discovery of Elements because they made it possible to predict the Behaviours that an Element should have. Later they were used to predict more the subtle Behaviour of which the Elements should display when acting as a constituent of a condensed phase, such as a salt, a molecule or an alloy. Today, each Element is characterized by many properties, used for predicting their Structures or Behaviours in any new situation. These correlations are summarized in Table 2 to define the strongest common Behaviour which identifies each trend.

Direction	Name	Behaviour Correlation
Horizontal	Period	Metal to Non-metal
Vertical	Group	Non-metal to Metal
Diagonal	Sequence	Constant Properties
Nested	Block	Similar Properties

Periods are one-dimensional correlations, which follow the change in Elemental Behaviour horizontally across the Table from the limits of metallic to non-metallic behaviour. Physically, the Elements go from conductors to insulators. Chemically, they go from forming cations to anions.

Groups are again one-dimensional correlations, which follow the opposite change in Elemental Behaviour vertically down the Table, from the limits of non-metallic in the light Elements at the top to metallic behaviour in the heaviest Elements at the bottom.

Sequences are also one-dimensional correlations which follow the constant metallic or non-metallic properties of Elements on diagonal lines going from left to right and down simultaneously.

Blocks are two dimensional correlations of Elements which define regions of the Table in which the Elements all display very similar metallic or non-metallic properties. Within blocks, the horizontal, vertical and diagonal one-dimensional trends of the whole table are repeated locally, as if the blocks were Periodic sub-Tables. This is so clearly obvious in the Transition block that the Groups are often labelled with the same numbering system as the Main block Elements. This occurs less obviously in the Rare Earth block but is still effective for correlating data. In modern use of the Table, all four types of correlation are exploited to help design physical and chemical systems for specific purposes.

The almost exclusive use of empirical methods which was used to establish the Periodic Table during the 19^th century was not the preferred choice of the chemists trying to discover and catalogue the behaviour of the Elements. Indeed, right from the beginning of that century, the potential importance of theoretical methods was clearly recognized in Dalton’s “Atomic Theory”. The basic hypothesis of this theory was that there must be a unique type of “atom” for each Element. This meant that the atoms of each Element must have a specific internal Structure which determined both its characteristics “atomic weight”and its particular type of bonding to other atoms. However, there was no physical model capable of describing this internal Structure of atoms or showing how these atoms might bond to each other. For this reason, the most important initial benefit of Dalton’s model was simply to use of the atomic weight concept as the theoretical basis for the discovery of the Elements. The more detailed hypothesis of unique internal Structures remained unproven until atoms were detected, then dissected by physical experiments almost 100 years later. Then, the “quantum mechanical” model, which was developed initially to predict the physical Behaviours of atoms, was exploited by chemists to explain their chemical Behaviours.

The reason for this interest in theoretical methods was that, since the original empirical form of the Periodic table was proposed in 1869, many inconsistencies in cataloguing the Elements satisfactorily had been recognized. Among the most troublesome were ;

The inconsistences in the ordering of Chemical properties ( eg; Ni before Co )

It was very clear from these inconsistencies that the empirical ordering of Elements by atomic weight alone did not provide any means of explaining why the Behaviours were Periodic or how to predict the geometries and magnitudes of unmeasured Behaviours.

In fact, the problems in empirical Tables fall into two categories. The first concerns the proper identification and sequencing of Elements. In the quantum theoretical model, the atomic weight criterion is replaced by the “atomic number”, Z, which represents the number of protons in the nucleus of each Elemental atom and therefore, the number of electrons in the neutral atom. This defines the number of unknown Elements in the large atomic weight gaps of the Periodic Table and eliminates the inconsistent ordering of known Elements. Specifically, ordering by Z showed that exactly 1 Element remained undiscovered at the end of each Period, in the “Rare Gas” Group within the Main Block. It equally defined the need to identify exactly 14 Elements in each of the Periods containing an atomic weight gap for the “Rare Earth” Block . It also showed that Co occurs before Ni rather than after as indicated by their atomic weights.

The second type of limitation in the empirical Periodic Table is the lack of any obvious reason for the strange pattern of growth of successive Periods. In quantum theory, this was explained by the relative importance of one-electron and two-electron energies in the Structure of the atom. As in most discovery processes, this theoretical model of Periodic Structures was induced from the empirical, spectroscopic Behaviours of atoms. The strategy for this induction process relied on the Daltonian assumption that any reproducible Behaviour must occur inside a well defined Structure. If this is true, then a QSAR must exist and the minimum necessary Structure capable of supporting the observable Behaviour can be defined by applying “Occam’s Razor”. This is a basic research tactic which says that the best way to define this Structure is to assume that there is one uniquely simple Structure which can support the observable Behaviours of each type of atom.