The end of the seventh row

01

Two ways to fuse two nuclei

Making a superheavy element means bringing two nuclei together with just enough energy to fuse and not so much that the product boils off neutrons and falls apart. Cold fusion, developed at Dubna and exploited at GSI in Darmstadt, fires a medium-weight beam such as chromium, iron, nickel or zinc at a lead or bismuth target: the compound nucleus is formed cool and survives by emitting a single neutron. Hot fusion, which Dubna returned to for the heaviest elements, fires calcium-48 at an actinide target and accepts a hotter product that sheds three or four neutrons. Each route has a reach; neither works everywhere.

rdreview.jaea.go.jp — Rutherfordium

02

Proof when the sample is one atom

A claim rests on a decay chain. The apparatus separates a recoiling atom from the beam, implants it in a detector, and records the sequence of alpha decays and the final spontaneous fission, with each energy and time compared against expectation and, where possible, terminating in a known nuclide. A single chain is not enough; the criteria that IUPAC and IUPAP applied required reproducibility, internal consistency and, for the newest elements, cross-bombardment checks. Several early claims failed these tests, and the transfermium naming disputes of the 1970s and 1990s were about evidence at least as much as about credit.

asrc.jaea.go.jp — Rutherfordium

03

Chemistry at the limit, and what the row means

Even at a few atoms, chemical behavior can be tested by seeing where an atom sticks in a temperature-gradient column, and Hoffman’s group and others showed that relativistic effects on the innermost electrons make some superheavy elements behave less like their lighter group members than the table implies. Nihonium, moscovium, tennessine and oganesson were confirmed in 2015 and named in 2016, completing period 7. Whether an eighth row is reachable depends on whether an island of relatively stable nuclei exists near the next closed shells.

RSC Periodic Table — Rutherfordium

What followed

The seventh row is complete, and the field’s methods — separation of single recoils, decay-chain identification and one-atom chemistry — now set the standard for any claim about an element that cannot be collected in a bottle.

rdreview.jaea.go.jp — Rutherfordiumasrc.jaea.go.jp — RutherfordiumRSC Periodic Table — Rutherfordiummedia.iupac.org — Rutherfordiumpmc.ncbi.nlm.nih.gov — Rutherfordium