Where chemistry meets the nucleus
Everything you have learned about the f-block so far has lived in the electron cloud. The earlier guides in this rung showed you the lanthanides, where buried 4f electrons barely touch the bonding, and the actinides, where the more exposed 5f orbitals let early members like uranium reach a whole staircase of oxidation states instead of the lanthanides' single dominant +3. That is electron chemistry, and it is real chemistry. But the actinides carry a second identity that no other part of the periodic table shares: their nuclei are unstable. To understand them fully you have to look not at the orbitals but at the dense knot of protons and neutrons they wrap around.
A heavy nucleus is a tug-of-war. The strong nuclear force glues neighboring protons and neutrons together but reaches only across a few neighbors, while the electrical repulsion between protons pushes on every pair at once, no matter how far apart. Pile on enough protons and the long-range repulsion starts to win. That is why there are no stable elements beyond bismuth, why uranium and thorium are radioactive, and why everything heavier than uranium has decayed away on Earth and must be made deliberately. The chemistry of the actinides and the instability of their nuclei are not two separate subjects bolted together — they are the same elements seen from two sides, and this guide is where the two sides meet.
Fission: splitting uranium and plutonium
Some heavy nuclei are not merely radioactive — they are fissile, meaning a single slow neutron can split them clean in two. The textbook case is uranium-235. When a wandering neutron sticks to a U-235 nucleus, the lump becomes so swollen and wobbly that it tears apart into two medium-sized fragments (say barium and krypton, though the split varies), releases a burst of energy, and — crucially — spits out two or three fresh neutrons. Those new neutrons can find other U-235 nuclei and split them too. That is a chain reaction: a self-sustaining cascade, controlled and gentle in a power reactor, prompt and runaway in a weapon. Natural uranium is over 99 percent U-238, which is not fissile in the same easy way, so reactor fuel is usually enriched to lift the U-235 fraction to a few percent.
Plutonium-239 is the other great fissile fuel, and its origin is a beautiful piece of nuclear bookkeeping. When the abundant, non-fissile U-238 swallows a neutron it does not split; it becomes U-239, which is unstable and beta-decays. In beta decay a neutron turns into a proton and ejects an electron, so the proton count climbs by one with each step. U-239 decays to neptunium-239, which decays again to plutonium-239 — two clicks up the periodic table, from element 92 to 93 to 94. So a reactor running on uranium quietly breeds plutonium inside its own fuel. Pu-239 is itself fissile, so part of the energy from a uranium reactor actually comes from plutonium made and burned on the spot.
The fuel cycle and reprocessing
Zoom out from the single nucleus and you get the nuclear fuel cycle — the cradle-to-grave path of uranium, and the stage where the actinides' inorganic chemistry does the real work. Mined uranium ore is leached and purified into a uranium oxide concentrate (the famous yellowcake), converted to the volatile fluoride UF6 for enrichment, made back into solid UO2 pellets, and sealed into the hafnium-free zirconium cladding you met in the d-block guide. After a few years powering a reactor, the spent fuel comes out a chemical menagerie: leftover uranium, freshly bred plutonium, and a cocktail of intensely radioactive fission fragments spanning much of the periodic table.
Pulling the still-useful uranium and plutonium back out of that mess is reprocessing, and it is one of the proudest demonstrations of separation chemistry ever built. The dominant method, PUREX, dissolves the spent fuel in hot nitric acid and then exploits the actinides' oxidation-state ladder. Uranium sits as the linear uranyl ion UO2^2+ (uranium in its +6 state) and plutonium can be coaxed to Pu(IV); both of these higher-charged species are gripped by an organic extractant called tributyl phosphate dissolved in kerosene, so they slip out of the water and into the oil. The fission products, mostly lower-charged ions, stay behind in the acid. Then a clever trick splits the two: chemically reduce the plutonium to the +3 state, which the extractant barely holds, and it falls back into the water while the uranyl ion stays in the oil. Pure uranium in one stream, pure plutonium in the other.
- Dissolve the chopped spent fuel in hot concentrated nitric acid; uranium becomes uranyl UO2^2+, plutonium is adjusted to Pu(IV), and fission products dissolve as a mix of ions.
- Shake the acid against kerosene carrying tributyl phosphate; the high-charged U(VI) and Pu(IV) prefer the organic oil, while the lower-charged fission products stay in the water.
- Chemically reduce plutonium to Pu(III), which the extractant scarcely holds; plutonium drops back into a fresh water phase while uranyl stays in the oil — the two actinides are now separated.
- Strip the uranyl back out of the oil into clean water, then purify each stream further; what remains is the high-level waste of fission products to be vitrified and stored.
It is worth being honest that reprocessing is as contested as it is elegant. Recovering plutonium recycles fuel and shrinks the volume of the longest-lived waste, but separated plutonium is also weapons-usable, so reprocessing sits at the center of hard questions about proliferation. Some countries reprocess; others have chosen a once-through cycle and direct disposal instead. The chemistry tells you what is possible; it does not by itself tell you what is wise. Notice too that the whole scheme rides on the very feature that defines actinide chemistry — that uranium, neptunium, and plutonium each offer several accessible oxidation states whose charges you can dial up and down to make them stick to an extractant or let go.
Building elements that no longer exist on Earth
The same beta-decay arithmetic that breeds plutonium is the doorway to the transuranium elements — everything past uranium, none of which survives in nature in any meaningful amount. The recipe for the lighter ones is patient neutron feeding. Park plutonium in an intense neutron flux and it absorbs neutrons one by one, climbing in mass; every so often one of those swollen nuclei beta-decays, nudging the proton count up a notch and minting the next element. Step by step this builds americium (95), curium (96), berkelium (97), californium (98) and on. Most of the smoke detectors in the world contain a speck of americium-241 made exactly this way — an element that did not exist on Earth until we made it, now sitting on the ceiling.
Notice how neatly this nuclear story rhymes with the electronic one. The actinides are filling the 5f subshell across the row, and a clean parallel holds: in the early actinides those 5f electrons are loosely held and chemically active, which is exactly why uranium and plutonium flaunt several oxidation states — and why reprocessing works. By the back half of the row the 5f electrons have been pulled in tight, much like the buried 4f electrons of the lanthanides, and the late actinides settle into a dominant +3 state, looking and behaving more and more like their lanthanide cousins. The contraction in size across the actinides echoes the lanthanide contraction you met one row up. The electron chemistry and the nuclear chemistry are reading the same f-block from two angles.
Superheavy elements, one atom at a time
Neutron feeding runs out of road around fermium (100): the nuclei become so fragile that they fission before they can climb any higher. To reach the superheavy elements beyond, chemists and physicists switch to brute collision — superheavy element synthesis. You coat a target with a heavy actinide, say curium or californium, and fire a beam of a lighter nucleus such as calcium-48 at it in an accelerator. Once in a great while, two nuclei meet just gently enough to stick rather than shatter, fusing into a single superheavy nucleus. This is how elements up to oganesson (118) have been made and named — among them flerovium, moscovium, and tennessine.
Resist any image of vials of these elements. The yields are almost unimaginably small — often a single atom produced over weeks of running, surviving for milliseconds or less before it decays. Their existence is confirmed not by weighing or by any beaker chemistry but by detecting the precise chain of alpha decays each atom emits as it falls apart, a fingerprint that pins down what was briefly made. Saying we have studied the chemistry of these elements means, at most, that a handful of atoms were chased through a few fast reactions to ask whether element 112 still behaves like the mercury sitting above it in the table. This is genuine inorganic chemistry pushed to its most extreme and humble limit: chemistry on a sample of one.
What the f-block leaves you with
Step back and the whole f-block rung resolves into one idea seen from many sides. The f orbitals are the subtle, buried, late-arriving newcomers of the periodic table, and their reluctance to engage explains both faces of these elements. In the lanthanides the 4f electrons stay so deep that the metals form a near-uniform family good for magnets and phosphors. In the actinides the early 5f electrons come out to play, giving the rich oxidation-state chemistry that runs reprocessing, before retreating in the later members. And straddling all of it, the actinide nuclei are heavy enough to be unstable, which is why this corner of inorganic chemistry is also the chemistry of the nuclear age.
Hold onto the honest boundary one last time, because this guide straddles it more than any other. Oxidation state, complex formation, solvent extraction, color — these are chemistry, decided by electrons, and chemistry cannot separate two isotopes or change one element into another. Fission, decay, fusion, and the half-lives behind the island of stability are nuclear physics, decided by protons and neutrons, and they cannot be sped up or slowed by any reagent in a flask. The actinides are extraordinary precisely because they make you fluent in both languages at once. With that, you have finished the f-block — the quiet, subtle bottom rows where inorganic chemistry runs right up to the edge of what the atom itself will allow.