Cobra Effect · Chemistry
The table that predicted missing elements
How a table of the elements left gaps, and the gaps were filled as predicted.
7 cards, read aloud in 3:32, with a test and sources.
By the 1860s, chemists knew about sixty elements, and several were hunting for a pattern among them.
Some elements behaved alike: sodium and potassium, for instance, or chlorine and bromine. Chemists had measured how heavy the atoms of each element were, compared with one another. Several people tried to arrange the elements so their family likenesses showed. A Russian chemist, Dmitri Mendeleev, found the arrangement that lasted.
Mendeleev arranged the elements by weight, and families lined up.
He listed the elements in order of their atomic weight. Every so often, properties repeated: an element would behave much like one further back in the list. By starting new rows at the right places, he made elements with similar behaviour fall into the same columns. He published his periodic table in 1869.
Mendeleev left gaps, and said they belonged to elements not yet found.
Where no known element fitted the pattern, he left the space empty rather than force something in. He predicted that elements would be discovered to fill those spaces. He gave them temporary names, such as eka-aluminium and eka-silicon, meaning one place below aluminium and silicon. He even predicted how heavy they would be, and how they would behave.
Where weights and properties disagreed, he trusted the properties.
By weight, tellurium should have come after iodine. But iodine behaves like chlorine and bromine, and tellurium like sulfur and selenium. So Mendeleev placed tellurium first, to keep the families together. He suspected the weights were wrong. The real reason for the order was found much later, in the structure of atoms.
In 1875 a new metal, gallium, was found, and it fitted the gap for eka-aluminium.
The French chemist Paul-Emile Lecoq de Boisbaudran discovered it. Mendeleev had predicted an atomic weight of about sixty eight. Gallium’s turned out to be close to seventy. He had predicted a density of about six. Gallium’s is just under six. Gallium melts below thirty degrees, so a piece can melt in a warm hand.
Scandium in 1879 and germanium in 1886 filled two more of his gaps.
Scandium matched the element Mendeleev had called eka-boron. Germanium matched eka-silicon, grey and dense, much as he had described. He had predicted a density of five and a half for eka-silicon. Germanium’s is about five point three. After successes like these, chemists took his table seriously.
So when a pattern has holes in it, ask what could fill them.
Mendeleev did not just sort what was known. He used the pattern to predict what was missing. Not every prediction came true. Some elements he suggested were never found. But the ones that were found made his table one of the most useful tools in science. Today it holds more than a hundred elements, still arranged in families.
Sources
- Mendeleev’s predicted elements, Wikipedia. The gaps he left, the elements that filled them, and the predictions that failed.
- Mendeleev’s first periodic table, Classic Chemistry, Le Moyne College. The table of 1869, with notes on how Mendeleev arranged it.
- Gallium and its surprises, ChemTalk. The metal that filled one of Mendeleev’s gaps, and melts in a warm hand.
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