The Actinoids

Actinoids are the 14 elements from thorium (Z = 90) to lawrencium (Z = 103), coming after actinium (Z = 89, configuration [Rn]$6d^1 7s^2$). Across the series, electrons progressively occupy the 5f subshell, giving the general configuration

where: [Rn] is the radon core (Z = 86), the superscript range on 5f shows how many of the 14 f-electrons have been added by that point in the series, and the 6d exponent 0 or 1 reflects the same kind of irregular d/f filling seen in some d-block anomalies. Every actinoid is radioactive. Only thorium and uranium exist in nature in appreciable amounts; everything from neptunium onward (the transuranium elements) is synthetic, produced by nuclear reactions, and several are available only as short-lived traces.

Oxidation states. As with the lanthanoids, +3 recurs throughout the series and becomes dominant for the later, heavier members (americium onward), which behave in a distinctly lanthanoid-like way. What sets actinoids apart is the earlier half of the series: thorium commonly shows +4, protactinium +5, uranium ranges from +3 to +6 (with +6, as in the uranyl ion $UO_2^{2+}$, especially important), and neptunium/plutonium can reach +6 or even +7 under special conditions. The reason is orbital energetics, not chance — the 5f, 6d and 7s subshells of the early actinoids sit close enough in energy that electrons from more than one of them are simultaneously available for ionisation, unlike the deeply separated 4f/5d/6s levels of the lanthanoids.

Lanthanoid versus actinoid — the examinable contrast:

Both series contract steadily in ionic radius across the row, because f-electrons shield the nuclear charge poorly, so each successive proton pulls the outer electrons in a little tighter.

Actinoids show their unusually wide oxidation-state range because of where their orbitals sit in energy, not because of any change to the basic rule "electrons in higher-energy orbitals are lost first." In a lanthanoid such as neodymium, the single 4f electron pool is shielded deep inside the atom by the filled 5s and 5p shells, and the 4f, 5d and 6s energy levels are well separated — so only the outermost $6s^2$ electrons (and occasionally one f-electron near a stable $f^0$, $f^7$ or $f^{14}$ configuration) are readily removed, locking in a narrow, +3-dominated chemistry. In an early actinoid such as uranium, the situation is fundamentally different. The 5f orbitals extend further from the nucleus than 4f orbitals do, so they are not buried as deeply, and — crucially — the 5f, 6d and 7s subshells lie close enough in energy that electrons from all three become roughly comparable candidates for removal. That means a variable number of electrons, drawn from more than one subshell, can be lost depending on the reaction conditions, producing several accessible oxidation states rather than one dominant one. As the series progresses toward the heavier actinoids (americium onward), the 5f orbitals contract and drop in energy relative to 6d and 7s, the gap widens again, and behaviour converges back toward the lanthanoid-like, +3-dominated pattern. This is also why 5f orbitals, being less shielded and more radially extended than 4f, participate more in covalent bonding — giving actinoid compounds noticeably more covalent character than the largely ionic lanthanoid compounds.

Write the electronic configuration of uranium (Z = 92) and use it to explain why uranium commonly shows oxidation states from +3 to +6, in contrast to a typical lanthanoid such as neodymium (Z = 60).

Step 1. Locate uranium in the actinoid series: Z = 92, following the [Rn] core (Z = 86), so its configuration is built by adding 6 electrons on top of [Rn].

Step 2. Write the configuration. Uranium's observed ground-state configuration is

an irregular filling (similar in spirit to d-block anomalies) that arises precisely because the 5f, 6d and 7s energy levels are so close together.

Step 3. Count the electrons available for ionisation: 2 from 7s, 1 from 6d, and 3 from 5f, giving 6 electrons total, all roughly comparable in energy and therefore all potentially removable.

Step 4. Because these six electrons are not sharply separated in energy the way lanthanoid 4f electrons are, uranium can lose different numbers of them depending on conditions, giving $U^{3+}$, $U^{4+}$ (the most stable oxidation state in aqueous solution), $U^{5+}$, and $U^{6+}$ (as in the uranyl ion $UO_2^{2+}$).

Step 5. Contrast with neodymium: its $4f^4 6s^2$ configuration keeps the 4f electrons shielded deep inside by the filled 5s/5p shells, with a large 4f–5d–6s energy gap, so essentially only the outer $6s^2$ electrons are lost, giving the narrow +3-dominated behaviour typical of lanthanoids. Uranium's six comparably-available electrons across 5f/6d/7s give it the +3-to-+6 range; neodymium's isolated, buried 4f electrons give it almost exclusively +3.

Competitive stretch

A related question sometimes asked: why does plutonium (Z = 94, configuration $[\text{Rn}]5f^6 7s^2$) reach +7 only under special (strongly oxidising) conditions, while uranium reaches +6 more readily? The answer follows the same logic — as Z increases across the early actinoids, the 5f orbitals gradually contract and drop below the 6d level in energy, so fewer electrons remain "equally available"; higher states become progressively harder to reach and need stronger oxidising conditions to force out the more tightly bound 5f electrons.

• Assuming actinoids behave exactly like lanthanoids because both are f-block elements — the wide oxidation-state range (especially +4, +5, +6 for Th, Pa, U) is a genuine, examinable difference, not a minor detail.
• Forgetting that only thorium and uranium occur naturally in significant quantity — every other actinoid is synthetic and radioactive, a fact frequently tested as a comparison point.
• Mixing up which subshell belongs to which series — lanthanoids fill 4f, actinoids fill 5f; writing $4f^{1-14}$ for actinoids is a common slip.
• Treating "+3 is dominant" as equally strong for both series — it is essentially the only major state for lanthanoids but just one of several accessible states for actinoids, especially the earlier members.

Ask the tutor: want a quick drill comparing the electronic configurations and oxidation states of a specific lanthanoid-actinoid pair, like neodymium versus uranium?