The Periodic Table

Chm 1311 Lecture for 8 June 2000

Organizing the Elements Mendele'ev MS Encarta '97	OK, you're too young to remember the Cold War, but a favorite Russian phrase then was "We invented it first!" Well, in the case of the Periodic Table of the Elements, they were right! Dmitri Mendele'ev was Professor of General Chemistry at University of St. Petersberg when he hit upon an organizing principle for the then known atoms. As they were listed in A (atomic weight) order, the cyclic nature of their chemistry suggested the kind of columnar organization above where the chemistry (reactions and properties) of atoms in a given column were all suspiciously similar despite great differences in their weights. Later we'll see why that has to be so, and I wouldn't want to spoil the surprise, now would I? So we'll content ourselves here with admiring the fact of such periodicity as a mnemonic to chemical prediction.
	While Mendele'ev thought it a fine way to present the myriad of the chemical elements to his students (and to you!), his Periodic Table turned out to be central in the discovery of new elements. For example, the spot now occupied by 31Ga, Gallium (after the Latin Gallia name for France), was unknown at the time Mendele'ev organized the elements. But by the trend in the properties of Boron, B, and Aluminum, Al, he predicted the kind of chemistry that Eka-Aluminum, as he called it in 1869. Ga was discovered via those hints only 6 years later by a French chemist. Notice that while he didn't discover any elements himself, Mendele'ev is immortalized in the table as the element mendeleevium, 101Md!
	The similarities of chemistry of elements in the same column has given rise to common names for many of them. For example, those in the first column (except hydrogen which excepts itself from many rules) are called the Alkali Metals because these metals react strongly with water to create highly alkaline (chemists would say "basic") solutions. Our bodies use Na+ and K+ ions in our nerve cells where their motions through cell membranes constitute the electrical signals by which we think and sense our environment.

The second columnar elements also produce alkaline water solutions, but they are far, far less reactive than the Alkali Metals. This second column contains what are called the Alkaline Earths. We use the representative alkaline earth element calcium as a construction element in our own bones as do shellfish in their shells.

On the other side of the table from the highly reactive alkali metals lie the terribly inert (unreactive) Noble Gases. Only in very recent years has anyone persuaded any of these atoms to engage in any chemistry at all! We chemists appreciate that these elements provide perfect inert atmospheres in which to keep otherwise highly reactive chemicals. But the only time we find any noble gas in our bodies is when we inhale a lungful of air, which consists of about 1% Argon! Divers use Helium instead of Nitrogen in the air they carry with them because it doesn't dissolve in their blood (as does nitrogen) and bubble out in deadly bends when they ascend from deep dives.

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