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
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.