F Block Lanthanoids Actinoids

The Hidden Block Beneath the Periodic Table

If you look at the bottom of the periodic table you will see two long strips, separated from the rest. These are the f-block or inner transition elements — fourteen lanthanoids (4f, Ce → Lu) and fourteen actinoids (5f, Th → Lr). They earn their place outside the main grid because their valence electrons enter the deeply buried 4f or 5f orbitals.

8.17 The Lanthanoids (4f Series)

8.17.1 Electronic Configurations

All atoms of the lanthanoid series share an outer 6s² shell. The number of 4f electrons varies progressively from 0 (in La itself) up to 14 (in Lu). The general configuration is:

The 5d¹ slot is occupied for a few special members (La, Ce, Gd, Lu) but most lanthanoids place all electrons in 4f and leave 5d empty. The most stable ions of all lanthanoids are Ln³⁺, which have the simple form 4f^n (n = 0 to 14):

Eu has anomalously large atomic radius (199 pm) because its 4f⁷ shell is half-filled — extra-stable, reluctant to delocalise into bonding.

8.17.2 Atomic and Ionic Sizes — The Lanthanoid Contraction

Look at the trend of Ln³⁺ radius: from La³⁺ (106 pm) to Lu³⁺ (~85 pm) the size shrinks by about 21 pm — a steady decrease across the entire series. This is the famous lanthanoid contraction.

Cause. As we go from Ce to Lu, each successive electron enters the 4f sub-shell. The 4f orbitals have an unusual angular shape — they are rather diffuse and shield other 4f electrons very poorly from the nucleus. As nuclear charge increases by one each step, the 4f electrons feel almost the full extra pull, and so the entire 4f sub-shell (and the surrounding 5s5p shells) contracts.

Consequences of the Lanthanoid Contraction

1. 4d ≈ 5d sizes. The third transition series elements (Hf, Ta, W, Re...) are nearly the same size as the corresponding second series (Zr, Nb, Mo, Tc...). Famous case: Zr (160 pm) and Hf (159 pm) have almost identical radii — they occur together in nature (in zircon ore) and are notoriously difficult to separate.
2. Increasing covalent character of M–O bonds from La to Lu — smaller, more polarising Ln³⁺.
3. Increasing basicity of hydroxides decreases from La(OH)₃ (most basic) to Lu(OH)₃ (least basic).
4. Identical chemical behaviour within the lanthanoid series, making mixed-rare-earth chemistry routine but mutual separation extremely difficult.

8.17.3 Oxidation States

Almost universally, lanthanoids show +3 as their characteristic oxidation state. A few elements show +2 or +4 in addition, driven by the stability of empty (f⁰), half-filled (f⁷) or fully filled (f¹⁴) configurations:

• +4 states: Ce⁴⁺ (4f⁰, noble-gas configuration — the most important; used as the analytical oxidant Ce(SO₄)₂). Pr⁴⁺, Nd⁴⁺, Tb⁴⁺ (4f⁷ — half-filled, oxidising), Dy⁴⁺ — but only in the solid state (oxides MO₂).
• +2 states: Eu²⁺ (4f⁷ — half-filled), Yb²⁺ (4f¹⁴ — full), Sm²⁺ (4f⁶ — close to half-filled). Eu²⁺ is a strong reducing agent, reverting to Eu³⁺.

Cerium(IV)/Cerium(III) has E° = +1.74 V — high enough to oxidise water, but the reaction is so slow that Ce(IV) is a usable analytical reagent.

8.17.4 General Characteristics & Chemical Reactions

All lanthanoids are silvery-white, soft metals that tarnish rapidly in air. Hardness rises across the series (Sm is steel-hard). Melting points lie between 1000 K and 1200 K. The chemistry is broadly comparable to the alkaline-earth metals at the start of the series and to aluminium by the end:

Colours and magnetism. Most Ln³⁺ ions (except 4f⁰: La³⁺, Ce⁴⁺ and 4f¹⁴: Yb²⁺, Lu³⁺) are coloured because of f-f transitions; the absorption bands are very narrow (the 4f orbitals are well-shielded so ligand-field effects are small). All ions other than f⁰ and f¹⁴ are paramagnetic.

8.17.5 Uses of the Lanthanoids

• Mischmetall (≈95% Ln + 5% Fe + traces of S, C, Ca, Al) — used in lighter flints, bullets, magnesium-based alloys.
• Ln-oxide catalysts in petroleum cracking.
• Individual Ln-oxides as phosphors in colour TV / monitor screens (Eu³⁺ red, Tb³⁺ green).
• Nd-Fe-B alloy for the world's strongest permanent magnets.

8.18 The Actinoids (5f Series)

8.18.1 Members and Radioactivity

The actinoids run from Ac (Z = 89) to Lr (Z = 103). All are radioactive. Earlier members (Th, U) have long half-lives and occur in nature; later members (Es, Fm, Md, No, Lr) have half-lives ranging from days to a few minutes — they exist only as nanogram quantities synthesised in nuclear reactors or particle accelerators. This makes their chemistry both difficult to study and dangerous to handle.

8.18.2 Electronic Configurations & Ionic Sizes

All actinoids carry 7s² with variable occupation of 5f and 6d. The 14 added electrons formally enter the 5f sub-shell, but irregularities (e.g. Am [Rn]5f⁷7s², Cm [Rn]5f⁷6d¹7s²) reflect the extra stability of f⁰, f⁷ and f¹⁴ occupancies.

An actinoid contraction exists analogous to the lanthanoid contraction, but is greater per element — because 5f electrons shield the nuclear charge even more poorly than 4f electrons. The 5f orbitals are not as deeply buried as the 4f, so 5f electrons can participate in bonding to a far greater extent.

8.18.3 Oxidation States

Although +3 is common to all actinoids, the early members exhibit a remarkable range. The maximum oxidation state climbs with Z up to Np (+7), then falls again:

Why so many states? The 5f, 6d and 7s sub-shells of the actinoids lie close in energy, so different numbers of electrons can be lost without much extra cost. By contrast, the lanthanoids have 4f buried much more deeply below 5d/6s, so only the outer electrons participate — limiting them to the +3 state with rare exceptions.

8.18.4 General Characteristics & Comparison with Lanthanoids