Period 5

Period 5 looks, at a glance, like a copy of period 4 shifted one shell outward. Eighteen elements, two in the s-block, ten in the d-block, six in the p-block, and the same journey from a soft electropositive metal at one end to an unreactive gas at the other. Almost every introductory account treats it that way.

The resemblance does not survive contact with the details. This row contains more exceptions to the orbital-filling sequence than any other, one element that has no stable isotope for a reason nothing chemical can explain, and a set of transition metals that differ from their heavier counterparts by a factor of two in density. It is the row that demonstrates that the rules taught alongside the periodic table are approximations, and it does so more often than the rest of the table combined.

Six exceptions out of ten

The filling order that works acceptably in period 4 — 5s before 4d, by the same n + l argument that puts 4s before 3d — fails repeatedly here. Six of the ten 4d elements have ground-state configurations that the rule does not predict. Niobium, molybdenum, ruthenium and rhodium each keep a single 5s electron rather than two. Silver does the same. Palladium keeps none at all.

The energy separating a 4d electron from a 5s electron is smaller than the separation between 3d and 4s, so the balance tips more easily. A 4d orbital is larger and more diffuse than a 3d orbital, which reduces the repulsion between electrons occupying it; at the same time the penetration advantage that lets 5s dip inside the krypton core is worth slightly less at this size. Between the two effects, the decision about where the last electron goes is settled by differences of a few tens of kilojoules per mole, and in six cases out of ten it goes the other way.

The honest conclusion is not that these are quirks to memorise but that the filling rule was never a law. It is a pattern extracted from where electrons happen to end up, and the atom minimises its total energy without consulting it.

Palladium keeps nothing in its outer shell

The most extreme case deserves separating out. Palladium's ground state is [Kr]4d¹⁰ — a filled d subshell and an entirely empty 5s. No other element in the periodic table has a ground state with a vacant outermost s subshell.

The consequence shows up in behaviour that is genuinely unusual. Palladium absorbs hydrogen in extraordinary quantities, taking up hundreds of times its own volume of the gas into the interstices of its lattice, and it is permeable to hydrogen in a way that lets a palladium membrane purify it by letting nothing else through. Its catalytic reach across organic chemistry — the coupling reactions that earned the 2010 Nobel Prize in Chemistry for Richard Heck, Ei-ichi Negishi and Akira Suzuki are almost all palladium-catalysed — rests on the ease with which it moves between the zero and +2 oxidation states, which a closed d shell with nothing outside it makes cheap.

Two elements that cannot be stable

Every element from hydrogen to bismuth has at least one stable isotope, with exactly two exceptions. One of them is technetium, at atomic number 43, sitting in the middle of this row between molybdenum and ruthenium. The other is promethium at 61.

The explanation is nuclear and it is a rule rather than an accident. Josef Mattauch pointed out in 1934 that two nuclides with the same mass number and adjacent atomic numbers cannot both be stable — one will always be able to beta-decay into the other. Odd-numbered elements are already restricted to at most two stable isotopes by pairing effects, and for technetium every mass number that might have worked already has a stable isobar occupied by molybdenum below or ruthenium above. There is nowhere left for it to be stable.

Mendeleev had left a gap at 43 and called the missing element ekamanganese, and the gap attracted a long series of claims that could not be reproduced — nipponium, masurium and others. It was finally produced rather than found, in 1937, when Emilio Segrè and Carlo Perrier examined a piece of molybdenum foil that had been bombarded with deuterons in Ernest Lawrence's cyclotron at Berkeley and posted to Palermo. It was the first element created artificially, and its name says so.

That accident of nuclear structure produced the most useful radioisotope in medicine. Technetium-99m is a metastable nuclear isomer with a half-life of about six hours, which decays by emitting a single 140 keV gamma photon and almost nothing else — energetic enough to leave the body and be detected, gentle enough not to deposit much dose, and short-lived enough to be gone within a day. It is produced on site from a molybdenum-99 generator, and it accounts for the large majority of diagnostic nuclear imaging procedures performed worldwide. An element that cannot exist stably anywhere in nature runs a global medical infrastructure.

The row before the squeeze

Compare the 4d metals with the 5d metals directly below them and the difference is startling. Ruthenium, rhodium and palladium have densities around 12 g/cm³; osmium, iridium and platinum, the same three groups one row down, are all above 21. Nearly twice the mass in the same volume.

The reason is not in this row at all — it is that period 6 has fourteen f-block elements inserted before its d-block resumes, and the poor shielding those f electrons provide pulls the whole 5d series inward. Period 5 is what a transition series looks like without that squeeze, which makes it the baseline against which the effect is measured. The lighter platinum-group metals are correspondingly softer, cheaper and easier to work than the heavier ones, and the split between "light" and "heavy" platinum group metals in the trade is a direct consequence.

Silver is the row's other superlative. It has the highest electrical conductivity, the highest thermal conductivity and the highest optical reflectivity of any element, which is a rare case of one element holding three unrelated records — and it loses out to copper in wiring on price alone.

A clock that opens the period

There is a small pleasure in the way this row starts. Rubidium and strontium, the two s-block elements that open period 5, are a radioactive parent and its daughter standing next to each other — the only place in the table where a decay pair sits in adjacent positions at the head of a row, and the basis of one of geology's standard dating techniques.

The geochemistry does the sorting. Rubidium is near enough to potassium in charge and radius to take its seat in feldspar and mica, strontium is near enough to calcium to take its seat in plagioclase and apatite, so a cooling magma parcels parent and daughter out into different minerals of the same rock and each mineral then keeps its own time. Everything after that is waiting, and the wait is long enough that the technique is aimed at Archean gneiss and meteorites rather than at anything of human date. The details belong on the elements' own pages; what belongs here is that period 5 opens with a clock already running.

Beyond the d-block, period 5 resumes ordinary behaviour: indium and tin, then antimony and tellurium on the metalloid staircase, iodine, and xenon closing the row at 5s²5p⁶. It is a conventional ending to a distinctly unconventional row.