F-block

The f-block is formally defined by the Aufbau principle: elements in which the last electron added occupies an f-orbital. In the periodic table, these are the 4f series (lanthanides, La–Yb or Ce–Lu depending on convention) and the 5f series (actinides, Ac–Lr). However, the actual ground-state configurations often deviate from simple Aufbau predictions due to inter-electron repulsion and relativistic effects. For example, lanthanum has a 5d¹ configuration rather than 4f¹, and actinium similarly has 6d¹. These anomalies are well-documented in spectroscopic data¹.

The f-orbitals themselves are deeply buried within the atom, shielded by outer s and p electrons. This shielding leads to the characteristic chemistry of the f-block: strong similarities across the series, high coordination numbers, and a preference for the +3 oxidation state.

The lanthanides (cerium to lutetium) are often called 'rare earths' though they are not particularly rare; cerium is more abundant than copper. Their chemistry is dominated by the +3 oxidation state, but cerium also forms Ce⁴⁺ and europium forms Eu²⁺, which are exploited in redox applications such as oxygen storage in catalytic converters and europium-doped phosphors in LED lighting².

The lanthanide contraction — the steady decrease in ionic radius across the series — arises from imperfect shielding of the 4f electrons. This effect has profound consequences: it makes the post-lanthanide transition metals (like hafnium) nearly identical in size to their earlier congeners (zirconium), which is why they are so difficult to separate. The contraction also influences the basicity of lanthanide hydroxides, which decreases across the series, a principle used in industrial separation processes.

The actinides (thorium to lawrencium) are all radioactive, with the early members (Th, Pa, U) having natural isotopes. Their 5f orbitals are more diffuse and less shielded than 4f orbitals, leading to a greater variety of oxidation states: uranium can adopt +3 to +6, and plutonium +3 to +7. This variability underpins the complex solution chemistry of nuclear fuel reprocessing, such as the PUREX process, which relies on the selective extraction of U and Pu in different oxidation states.

Relativistic effects become significant for the heavy actinides, causing contraction of s and p orbitals and expansion of d and f orbitals. These effects explain the unexpected stability of the 7s² shell in nobelium and the volatility of some actinide compounds, which has been exploited in gas-phase chemical studies of elements like seaborgium (though seaborgium is a d-block element, its chemistry is compared to that of actinides).

Beyond the textbook, the f-block holds many surprises. For instance, the element promethium has no stable isotopes and was only identified in 1945; it is used in luminous paint and as a beta source in thickness gauges. Another curiosity is the 'tetrad effect' in lanthanide distribution patterns, observed in geological samples, which reflects subtle variations in ionic radii and is used to trace the origin of rocks and ores³.

In the actinide series, the element einsteinium was discovered in the debris of the first hydrogen bomb test in 1952. Its chemistry is notoriously difficult to study because it decays rapidly, but recent experiments have measured its bond lengths, revealing the influence of relativistic effects. Also, the f-block is not limited to the two rows: the superheavy elements 121–138 are predicted to form a 'g-block' with 5g orbitals, but these have not yet been synthesized.

Lanthanide contraction

The steady decrease in ionic radius across the lanthanide series due to poor shielding of 4f electrons.

Relativistic effects

Corrections to electron behavior at high atomic numbers due to speeds approaching the speed of light.

Oxidation state

The degree of oxidation of an atom, reflecting the number of electrons lost or gained.

PUREX process

Plutonium Uranium Reduction Extraction, a method for reprocessing nuclear fuel.