oxidation states of the actinides
If the lanthanides are a one-note family, almost always +3, the early actinides are a small jazz ensemble: uranium plays +3, +4, +5, and +6; neptunium and plutonium can have several of those states coexisting in one beaker, shifting with acidity and what is nearby. Then, further along the row, the music quiets back down and the later actinides settle into the lanthanides' single +3 note. Understanding this arc is the key to actinide chemistry.
The reason is how deep the 5f electrons sit, and it changes across the series. In the early actinides (thorium, protactinium, uranium, neptunium, plutonium) the 5f orbitals lie relatively high and extend outward enough to overlap with ligand and 6d orbitals, so 5f electrons can be coaxed into bonding. That makes a whole ladder of oxidation states accessible: uranium famously runs from U(III) up to U(VI), the high states often appearing as the linear 'yl' ions like uranyl UO2-2+ and neptunyl NpO2-+ in which the metal is multiply bonded to two oxygens. As you move right, more protons pull the 5f orbitals down and inward, locking the electrons away just as the 4f electrons are locked in the lanthanides. By americium and beyond, the higher states fade and +3 dominates again — the actinides 'become lanthanide-like' precisely because their f electrons have finally become core-like.
This matters for the whole of nuclear chemistry. The reason plutonium is so troublesome to process and so mobile in the environment is that it can sit in four oxidation states at once (Pu(III), Pu(IV), Pu(V), Pu(VI)), each with different solubility and chemistry, and even disproportionate among them. Separating uranium from plutonium in spent fuel exploits exactly these redox differences — adjust the oxidation state of one element and not the other, and they part company. The honest caveat: these states are real chemistry, but every actinide ion is also radioactive and many are scarce, so much of this redox map was painstakingly assembled from work on milligram or microgram quantities behind shielding.
Plutonium is notorious for showing four oxidation states in the same acidic solution at once: Pu3+ (blue-violet), Pu4+ (tan), PuO2-+ (Pu(V), pink), and PuO2-2+ (Pu(VI), orange). Their standard potentials are so close that plutonium readily disproportionates, turning one state into a mixture of others.
Plutonium can show four colored oxidation states in one beaker.
The wide oxidation-state range belongs to the early actinides only. By the second half of the series the 5f electrons are buried, the chemistry collapses back to +3, and the late actinides really do resemble lanthanides; do not generalize uranium's flexibility to the whole row.