How did Mendeleev predict elements nobody had found yet?
In 1869 Mendeleev left gaps in his table and described what would fill them. Within 17 years, three of the missing elements had turned up, and they matched.
▶ Start the storyMendeleev predicted new elements by treating the gaps in his table as information. On 6 March 1869 he presented to the Russian Chemical Society a table of elements ordered by atomic weight and valence. As not all elements were then known, there were gaps in his table, and he used the periodic law to predict some properties of the missing elements. He wrote that many unknown elements should be expected, for example two analogous to aluminium and silicon, with atomic weights between 65 and 75.
He was not the only one with a table. Lothar Meyer published a paper in 1864 classifying 28 elements by valence, but with no predictions of new elements. Mendeleev's table went further and made predictions.
The test came in the following years. He named his three predicted elements ekaboron, ekaaluminium and ekasilicon, and they turned out to be scandium, gallium and germanium. Lecoq de Boisbaudran discovered gallium in 1875 by its spectrum, and at first measured its density as 4.7 g/cm3, the only property that failed to match Mendeleev's predictions. Mendeleev wrote to him and suggested he remeasure it, and the corrected value was 5.9 g/cm3, exactly what Mendeleev had predicted.
Gallium's density (g/cm3)
g/cm3
| Density | |
|---|---|
| First measurement | 4.7 g/cm3 |
| Mendeleev prediction | 5.9 g/cm3 |
| Remeasured | 5.9 g/cm3 |
Germanium followed in 1886, and its compounds corresponded well with his predictions, an important confirmation of periodicity. The noble gases, which he had not predicted, fitted into his scheme as an eighth main group.
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Recap
Mendeleev trusted the pattern enough to leave gaps and to doubt data, and the discovery of the missing elements proved the pattern right.
💡 A trick to remember it · A good table is a jigsaw with a few pieces missing, and the picture tells you what the missing pieces look like.
Surprising fact · Gallium's first measured density was wrong; Mendeleev told its discoverer to remeasure it, and the new value matched his prediction exactly.
Sources (5)
No source, no claim. Every fact in this lesson (19 claims) cites at least one of these.