Q.Why do actinides have lower first ionisation enthalpy as compared to the lanthanides (specially for the early elements)?
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Actinoids: The Heavy, Radioactive Inner Transition Metals
Imagine you are walking through the periodic table, row by row. You know the lanthanoids — the 14 elements from cerium to lutetium that sit below the main table, where the 4f subshell is being filled. Now go one row lower. The elements that follow actinium (atomic number 89) are the actinoids: thorium (90) through lawrencium (103). Here, the 5f subshell is being filled.
The name comes from actinium, the first element of the series. But the real story begins with uranium and plutonium — the elements that made nuclear energy and atomic bombs possible. These are not just academic curiosities; they are the workhorses of nuclear physics.
The Precise Statement
Actinoids are the 14 elements from thorium (Z=90) to lawrencium (Z=103) in which the 5f orbitals are progressively filled. They are all radioactive, exhibit a wide range of oxidation states, and show a steady decrease in ionic radii across the series — the actinoid contraction.
Why They Are Special
1. Radioactivity is the rule, not the exception. Every actinoid is radioactive. Some, like uranium-238, have half-lives comparable to the age of the Earth. Others, like lawrencium, exist for only seconds. This radioactivity is not a side effect — it is the defining property. It means their chemistry is often studied with trace amounts, using radiation detectors rather than test tubes.
2. Multiple oxidation states — more than you expect. Unlike the lanthanoids, which mostly stick to +3, the actinoids show a rich variety. Uranium, for example, exists as U3+, U4+, UO2+ (U5+), and UO22+ (U6+). The early actinoids (Th, Pa, U, Np, Pu) are especially versatile because the 5f, 6d, and 7s orbitals are close in energy. As you move right, the +3 state becomes more stable — curium and beyond behave more like lanthanoids.
3. The actinoid contraction. As you go from Th to Lr, the nuclear charge increases by 14 protons. The 5f electrons are poor at shielding each other, so the effective nuclear charge felt by the outer electrons rises steadily. The result: ionic radii shrink smoothly across the series. This is exactly analogous to the lanthanoid contraction, and it has the same consequence — the chemistry of later actinoids becomes very similar to their lanthanoid counterparts, making separation difficult.
The actinoid contraction is why americium and curium are so hard to separate from the lanthanoids in nuclear waste. Their ionic radii are nearly identical.
A Quick Comparison with Lanthanoids
| Property | Lanthanoids (4f) | Actinoids (5f) |
|---|---|---|
| Radioactivity | Mostly stable (except Pm) | All radioactive |
| Common oxidation state | +3 (strongly dominant) | +3, +4, +5, +6 (early ones) |
| 5f vs 4f character | 4f deeply buried, little bonding | 5f more exposed, can participate in bonding |
In the actinides the 5f orbitals are less penetrating and poorly shielding, so the outer electrons are held less tightly (and are more easily removed) than the 4f electrons of the lanthanides. …
The 5f orbitals of the actinides are more diffuse and less penetrating than the 4f orbitals of the lanthanides; they shield poorly and their electrons are loosely held, giving the early actinides a lower first ionisation enthalpy.
First ionisation enthalpy depends on how firmly the outermost electron is held, i.e. on the effective nuclear charge it experiences.
- In the lanthanides, the differentiating electrons enter the compact 4f orbitals. These are relatively well buried, and the outer electrons experience a fairly high effective nuclear charge. …
- CBSE 2026Set V11 markMCQQ.Match the following given in List I with List II.Choose the correct option :
List I (Elements) List II (Their maximum oxidation states) i) Thorium (Th) a) +7 ii) Protactinium (Pa) b) +6 iii) Lawrencium (Lr) c) +3 iv) Uranium (U) d) +5 v) Plutonium (Pu) e) +4 (a) i – e, ii – d, iii – a, iv – b, v – c(b) i – e, ii – d, iii – c, iv – b, v – a(c) i – b, ii – a, iii – c, iv – e, v – d(d) i – b, ii – e, iii – c, iv – d, v – a›Reveal solutionSolution
Matching each actinoid to its maximum oxidation state gives Th–+4, Pa–+5, Lr–+3, U–+6, Pu–+7, i.e. option (b).
Early actinoids show a rise in maximum oxidation state as the number of available 5f/6d electrons increases, peaking around U–Np–Pu, then falling again toward the heavier members (which behave much like lanthanoids with a stable +3 state).
- i) Thorium (Th): maximum oxidation state =+4 → (e)
- ii) Protactinium (Pa): maximum oxidation state =+5 → (d) …
- CBSE 2025Set ANNUAL1 markMCQQ.Element of actinoid series is -(a) Thorium(b) Cerium(c) Cadmium(d) Chromium
›Reveal solutionSolution
Thorium (Th, Z = 90) belongs to the actinoid series; the other three options are not actinoids.
Actinoids are the 14 elements from Th (Z=90) to Lr (Z=103) that follow actinium, in which the 5f orbitals are progressively filled.
Checking each option:
- Thorium (Z=90): an actinoid.
- Cerium (Z=58): a lanthanoid (4f block), not an actinoid. …
- CBSE 2024Set B1 markQ.Write True or False: Lanthanoids is more reactive than actinoids.
›Reveal solutionSolution
Because of their larger size and lower ionisation enthalpies (and comparable energies of 5f, 6d, 7s orbitals), actinoids are more reactive than lanthanoids, especially in the finely divided state.
Lanthanoids (4f series) are comparatively less reactive, tarnishing slowly in air, while actinoids (5f series) — especially the early actinoids like thorium, uranium, plutonium — are much more reactive: they combine readily with most non-metals, tarnish rapidly, and some (like plutonium) are pyrophoric in powdered form. This higher reactivity of actinoids is attributed to their larger atomic/ionic radii and t …
- CBSE 2023Set ANNUAL1 markMCQQ.The maximum oxidation state exhibited by actinides is:(a) +7(b) +5(c) +4(d) +8
›Reveal solutionSolution
Actinides can show oxidation states from +3 up to a maximum of +7, because the 5f, 6d and 7s electrons are close in energy and all can participate in bonding.
+3 is the most common and stable oxidation state throughout the actinide series (like +3 is typical for lanthanides), but because the 5f orbitals extend further from the nucleus than the 4f orbitals of lanthanides and are less effectively shielded, more of the outer electrons of an actinide atom can be involved in bonding. This lets some early actinides reach higher oxidation states than lanthanides ever d …
- CBSE 2023Set ANNUAL1 markQ.Why do actinoids show larger number of oxidation states as compared to the lanthanoids ?
›Reveal solutionSolution
The near-equal energies of 5f/6d/7s in actinoids let more electrons participate in bonding than the more energetically separated 4f/5d/6s orbitals of lanthanoids allow.
In lanthanoids, the 4f orbitals are deeply buried beneath the filled 5s and 5p subshells and are well shielded from the nucleus; they lie considerably lower in energy than the 5d and 6s orbitals. As a result, 4f electrons rarely take part in bonding, and lanthanoids show an overwhelmingly dominant +3 oxidation state, with only occasional +2/+4 states (when they give a specially stable f⁰, f⁷ or f¹⁴ configuration).
…
- CBSE 2022Set E1 markMCQQ.Which of the following is not an actinide ?(a) Curium(b) Californium(c) Uranium(d) Terbium
›Reveal solutionSolution
Terbium belongs to the lanthanide (4f) series; curium, californium and uranium are actinides (5f).
Actinides are the 5f-series elements (Z = 90-103), which include uranium (92), curium (96) and californium (98).
…
- CBSE 2019Set ANNUAL1 markQ.What is the most common oxidation state of the Actinoids?
›Reveal solutionSolution
Like the lanthanoids, actinoids show a wide range of oxidation states, but +3 is the most common/characteristic one.
Actinoids exhibit a greater range of oxidation states than lanthanoids (owing to the closer energies of 5f, 6d and 7s orbitals), but the most common and characteristic oxidation state throughout the series, as with the lanthanoids …
- CBSE 2018Set ANNUAL1 markQ.Write name and symbol of one transurenic element.
›Reveal solutionSolution
A transuranic element is any element with atomic number greater than 92 (uranium); Neptunium (Np, Z = 93) is one example.
Elements with atomic number beyond that of uranium (Z = 92) are called transuranic (or transuranium) elements. They are all synthetic/man-made, produced in nuclear reactors or particle accelerators by bombarding heavy nuclei with neutrons or light nuclei, and all are radioactive. Examples include Neptunium (Np, Z = 93), Plutonium (Pu, Z = 94), …
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