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THE HISTORY OF AN IDEA · 1869–1947

Filling the gaps the table left open

Mendeleev left blank places and stated what should occupy them; the elements that arrived turned an arrangement into a predictive instrument, and the last two gaps had to be manufactured.

Explore the supporting references at the foot of this article.

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01

A prediction specific enough to fail

Leaving a gap is easy; saying what belongs in it is not. Mendeleev gave his hypothetical eka-aluminum and eka-silicon approximate atomic weights, densities, formulas and even expected methods of discovery, which meant a chemist finding a new element could check it against the description rather than merely note that it fitted somewhere. The predictions were falsifiable in detail, and that is what separated his table from earlier arrangements that were equally capable of listing the known elements.

RSC Periodic Table — Cesium

02

Three confirmations in twelve years

Gallium came in 1875 from a zinc , scandium in 1879 from Scandinavian , germanium in 1886 from a silver mineral whose analysis kept coming up several per cent short. Winkler’s germanium was the sharpest test: eka-silicon had been given a density near 5.5 and an oxide of a particular formula, and the measured element matched closely enough that the periodic law stopped being a proposal. Nilson’s scandium is the quieter case, because he did not immediately recognize it as eka-boron; Cleve made that connection.

LANL Periodic Table — Gallium

03

Gaps that nature does not fill

Two places resisted, and for a reason no better chemistry could remove: elements 43 and 61 have no stable , so whatever the Earth started with has . Several claims were announced and withdrawn, including the Noddacks’ masurium, before Perrier and Segrè found technetium in 1937 in a molybdenum foil that had been bombarded in a cyclotron, and promethium was identified in 1945 among products. The last two blanks in the main body of the table were filled by making the elements rather than finding them.

Germanium: history, properties and uses

What followed

Predicting a property from a position is now ordinary practice, extended to the heaviest synthesised elements where a handful of atoms must be identified by whether their behavior matches what their place implies.

RSC Periodic Table — CesiumLANL Periodic Table — GalliumGermanium: history, properties and usesLANL Periodic Table — GalliumLANL Periodic Table — Technetium

Continue the story

Follow a discovery, compare a material choice, or meet the people behind an idea.

SCIENTISTClemens Winkler ↗Read the analyst whose persistent mass deficit in a silver mineral produced germanium.SCIENTISTEmilio Segrè ↗Follow the physicist who found one missing element in a bombarded foil and another by synthesis.DISCOVERY HISTORYHow an ordering became a predictive system ↗Return to the arrangement that made these gaps meaningful.

Sources & further reading

  1. RSC Periodic Table — Cesium ↗Royal Society of ChemistryReference explanation · Source checked
  2. LANL Periodic Table — Gallium ↗Los Alamos National LaboratoryReference explanation · Source checked
  3. Germanium: history, properties and uses ↗Royal Society of ChemistryReference explanation · Source checked
  4. LANL Periodic Table — Gallium ↗Los Alamos National LaboratoryReference explanation · Source checked
  5. LANL Periodic Table — Technetium ↗Los Alamos National LaboratoryReference explanation · Source checked

Sources and further reading. What the review labels mean