I expect that most folks are at least somewhat familiar with the Periodic Table of the Elements. This elucidation of this table has provided an exceptional amount of information for the development of chemistry, and it one of the most elegant intellectual developments in science.
We will very briefly describe the development of the table, some of the reasons that it a wonder that it was developed as early as it was, and, finally, "why" the table works. More after the fold.
A decent representation of the Periodic Table is at this link, and it is printable if you want a hard copy:
http://en.wikipedia.org/...
Back when science was just splitting away from alchemy (indeed, science was referred to as "natural philosophy" at the time), it was observed that several basic materials seemed to occur in groups of three with similar properties, such as copper, silver, and gold. This started the thought that it might be possible to arrange known substances (elements were not known to be elementary at the time) into groups to try to explain similarities in properties. That did not work out, but the idea persisted.
Time passed and slowly it was learned that fire, water, air, and earth were not elements, but just myths. Science began to recognize that elements were chemical substances that could not be resolved into simpler materials. While some mistakes were made because some metallic oxides are very to difficult decompose, this worked pretty well. At the turn of the 19th century and afterwards, tremendous advances were made and numerous elements were discovered, such as sodium, potassium, calcium, nitrogen, oxygen, carbon, chlorine, bromine, iodine, and a host of others.
The list of elements was getting long, but no sense was being made. There were a couple of reasons for that. One was that it was not understood that one element can combine with another in more than one ratio. Thus, water was thought to be HO because it simply had not been thought of that a 2:1 ratio might be better. Another reason was that only crude methods were available to determine atomic weights, and the concept of atomic numbers was decades away.
Nevertheless, using a tremendous amount of theoretical development by the likes of Gay-Lussac, Avogadro, Boyle, Charles, and many others, a Russian scientist by the name of Mendeleev put order into the system in 1869. He arranged, as best he could, all the known elements in order of increasing atomic weight. As he listed them, he also wrote the known properties of each into the long list. Then he detected a pattern.
He noticed similarities in properties in several elements, and drew a grid so that those elements would be arranged into vertical columns, the elements with higher atomic weight appearing below the lighter ones. Thus, sodium appeared above potassium, and magnesium above calcium. He continued until he had arranged the elements in rows by increasing atomic weight by an average of one unit, and by columns with in increase in atomic weight by an average of eight units. It was a pretty good system, except for two major things: There were reversals. In other words, the properties of several elements suggested that they should belong in a different column, but the atomic weight suggested. Mendeleev guessed (incorrectly) that the atomic weights were incorrect, and placed the ones with common properties in the same column. That was the right conclusion for the wrong reason, and I will describe later. The second problem was that some of the elements just did not seem to fit anywhere, iron for instance, so he put off working on these to focus on the ones that did. Another fortuitous guess.
The last problem was that there were some gaps. He correctly deduced that several elements simply had not been discovered yet, and just blanks for those as yet undiscovered ones. The results were astounding.
One of the missing elements is what we now call germanium was a "blank" with the placeholder name of eka-silicon, meaning that it was next down the column after silicon. He predicted the following properties (actual values for germanium in parentheses). Atomic weight 72 (72.64). Density 5.5 (5.35). Melting point: High (947). Color: Gray (Gray). Not too shabby for 1869 and with no concept of electronic structure.
So Mendeleev accounted for imperfections in the determination of atomic weights, the possibility of yet undiscovered elements, the repeating similarities in properties, and even predicted, based on data contained in the table regarding known elements, the properties of undiscovered ones.
Now another fly came into the ointment. Ramsey discovered the "noble" gasses. It was not that much of a problem, because all that had to be done to accommodate them was to add another column for them, and the atomic weights filled in several blanks. If it had not been for the conviction of courage for Mendeleev to question the accuracy of atomic weights, the system would had been in disarray. Now for the rest of the story.
It was not until 1910 that J.J. Thompson and Francis Aston discovered isotopes (two or more identical chemical elements differing only in atomic weight). This revolutionized the understanding of the atom, and two new particles were postulated to account for them: the proton (carrier of positive charge and mass), and the neutron (carrier of no charge but with mass). Now everything was beginning to make sense. The electron had been discovered, and it was known that each carries a single negative electrical charge, but hardly any mass.
Mendeleev was vindicated, because if his table is restated in order of atomic number, rather than atomic weight, the assumptions that he made about inaccurate measurements go away. He had the foresight to overlook the gnats and assemble the elephants, even though there was no real reason to do it except the similarities in properties in a column (also called a "family") of elements.
Now the hunt was on for the association between atomic number and chemical properties. Since the atomic number also defines the number of electrons in a neutral atom, there was intense interest in correlation of the two. Folks like Niels Bohr, Paul Adrian Maurice Dirac, Werner Heisenberg, and many others did that, whist my grandmother was not even experiencing puberty.
Now we will look at the "why" this works. It is really comparatively simple: Chemical properties are determined solely by the electron arrangement of any given element, and those arrangements are solely determined by the number of protons (the atomic number), as implemented by the laws of quantum physics. That is why Mendeleev was a genius: he did not allow pesky isotopic variations to interfere with his great idea. Perhaps, if there is enough interest, I will continue with this series with particular families of elements. Warmest regards, Doc.