Physics 2000 Science Trek The Periodic Table

Valences and the Periodic Table

I understand how Mendeleev could arrange the elements in order of atomic weight, but the way he decided on the different columns isn't so easy to see. Were the differences between columns really that clear-cut?

The concept that Mendeleev found most helpful in laying out his table was the notion of valences, proposed in 1852 by the English chemist Sir Edward Frankland. The idea was this: almost all the elements known at the time would combine with either hydrogen or oxygen, so the valence of an element was related to the number of atoms of hydrogen or oxygen that combined with that element.
Hydrogen and oxygen form water, H2O, so hydrogen was given a valence of 1 and oxygen a valence of 2. For any other element, the valence was defined to be the number of hydrogen atoms, or twice the number of oxygen atoms, that would combine with one atom of that element. For example, nitrogen and hydrogen form ammonia, NH3, so nitrogen has a valence of 3; carbon dioxide, CO2, gives carbon a valence of 4.

So Mendeleev put elements with the same valence into the same column?

Yes. When he looked at the list of elements in order of atomic weight, he saw a pattern in their valences; furthermore, elements with the same valence tended to have other very similar properties.

To be fair, Mendeleev was not the first person ever to notice this pattern. His great insight, though, was this: if the "next" element in terms of weight had the wrong valence or other properties that didn't match, he left a gap in the table, awaiting the discovery of a new element that would fit the pattern. As I mentioned earlier, he correctly predicted the properties of some of these missing elements; when they were later discovered as promised, Mendeleev's credibility improved greatly.


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