THE HISTORY OF AN IDEA · 1898–1935
Elements that arrive by decay
Several elements were first found as short-lived members of radioactive series, and explaining them forced chemists to accept that one place in the table can hold atoms of different mass.
Explore the supporting references at the foot of this article.
01
Chemistry with an invisible tracer
The Curies’ method was ordinary separation chemistry with an extraordinary indicator: after each step they measured the activity of the fractions and followed whichever one stayed active. That let them assert the presence of polonium and radium in pitchblende long before either had been weighed, in much the same way that a line had let Bunsen assert cesium. It also meant the new elements were defined at first by a physical property rather than by a reaction, which is why establishing their chemistry took years more than announcing them did.
02
A crowd of species and too few places
By 1910 the series had produced dozens of named activities — radiothorium, ionium, mesothorium, uranium X — with distinct half-lives but, in many cases, chemistry that no separation could distinguish. There were far more species than empty places in the periodic table. Soddy’s was to say that the table classifies by chemical identity and that several species can share one place; he called them . His displacement rule made the chains navigable: moves an atom two places left, emission one place right.
03
Weighing the difference
Isotopes stopped being an inference when Thomson’s positive-ray work and then Aston’s mass spectrograph separated neon into masses 20 and 22, showing that a stable, non-radioactive element could be a mixture too. Aston’s whole-number rule and his measurements of small departures from it turned atomic weight from a fundamental constant into an average over an isotopic mixture — which is why lead from a uranium-bearing has a measurably different atomic weight from lead of another origin.
What followed
Isotopic composition is now a working tool rather than a puzzle: it dates rocks, traces the origin of materials, distinguishes natural from synthetic supply and underlies the atomic-weight intervals the table reports for elements whose composition varies with source.
RSC Periodic Table — Radiumphy.anl.gov — RadiumMarie Curie: Nobel Lecture, 1911DOE — RadiumUSGS — Radium
Sources & further reading
- RSC Periodic Table — Radium ↗Royal Society of ChemistryReference explanation · Source checked
- phy.anl.gov — Radium ↗phy.anl.govReference explanation · Source checked
- Marie Curie: Nobel Lecture, 1911 ↗Nobel Prize OutreachOriginal research · Source checked Where to look: Lecture paragraph reporting metallic radium with André Debierne in 1910, within the account of polonium and radium.Retrospective first-person research lecture; the cited paragraph was checked in indexed NobelPrize.org text.
- DOE — Radium ↗United States Department of EnergyReference explanation · Source checked
- USGS — Radium ↗United States Geological SurveyReference explanation · Source checked
Sources and further reading. What the review labels mean