Mendeleyev and his Periodic Table

Back when Dimitri Mendeleyev came up with his famous Periodic Table of elements, only atomic weights were known, not atomic numbers. And yet, when Mendeleyev arranged the elements in increasing order of atomic weights, writes Paul Strathern in Mendeleyev’s Dream:

“(He found) their properties repeated in a series of periodic intervals.”

But there were anomalies:

-       The elements with similar properties didn’t always fall in increasing order of atomic weights. But:

“Mendeleyev questioned the atomic weight of the element, suggesting that it had been calculated incorrectly.”

-       In some cases, no element seemed to fit the “next” slot. Undeterred:

“Where no element fit the pattern, he left a gap. He predicted that these gaps would one day be filled by elements which had not yet been discovered.”

He went even further by predicting the properties of such yet to be discovered elements.

Not everyone was sold on this:

“Whoever heard of a scientific theory which relied upon scientific errors?… How could you base a scientific law upon discoveries which had not yet been made?”

A German scientist, Julius Meyer, discovered “almost an identical pattern among the elements”. So why does Mendeleyev get all the credit? Because unlike Meyer:

“(Mendeleyev) was willing to back his chemical insight in the face of all the ‘facts’.”

Here’s one example of how far Mendeleyev was willing to back his beliefs. Paul Lecoq discovered one of the elements that Mendeleyev had predicted. That new element (gallium) “exactly matched the properties Mendeleyev had predicted”. Except for its specific gravity (4.7 v/s 5.9). Did this mean that Mendeleyev’s “other ‘predictions’ had been nothing more than a series of lucky guesses”? Here is how Mendeleyev reacted:

“He dispatched a letter to Lecoq informing him that his sample of gallium was insufficiently pure, suggesting that he repeat the experiment with another sample.”

Lecoq obliged and voila! The specific gravity indeed matched Mendeleyev’s predicted value of 5.9. Other new elements too “fit” Mendeleyev’s predictions.

No wonder then that Strathern writes:

“With the Periodic Table, chemistry came of age. Like the axioms of geometry, Newtonian physics and Darwinian biology, chemistry now had a central idea upon which an entire new range of science could be built.”