The f-block and beyond

The vibration motor in a phone, the motor of an electric car and the generator of a wind turbine all depend on neodymium–iron–boron magnets; the red of old colour-television screens and the glow of euro banknotes under ultraviolet light come from europium. These “rare earths” are everywhere, yet from the discovery of the first rare-earth oxide in 1794 it took chemists more than a century to tell all fifteen lanthanides apart. Why are they so hard to separate?

The lanthanides have configurations [Xe]4f$^n$6s² (La, Ce, Gd and Lu also carry one 5d electron). The 4f orbitals are not low in energy, but spatially they lie inside the filled 5s²5p⁶ shells and are nearly invisible from outside: they take no part in bonding and hardly feel the ligands. So every lanthanide is predominantly +3, their chemistry is strikingly similar, and $E^\circ(\ce{Ln^3+/Ln})$ sits near −2.3 V almost throughout.

The exceptions follow from half-filled or full 4f shells: $\ce{Eu^2+}$ (4f⁷) and $\ce{Yb^2+}$ (4f¹⁴) are fairly stable, with $E^\circ(\ce{Eu^3+/Eu^2+})$ only −0.36 V; $\ce{Ce^4+}$ (4f⁰) survives in water as a strong oxidant (+1.72 V) — ceric ammonium nitrate is a common oxidising reagent.

Because 4f electrons shield the nuclear charge poorly, the ionic radius shrinks steadily with atomic number: the lanthanide contraction. $\ce{Y^3+}$ (101.9 pm) falls right between $\ce{Dy^3+}$ and $\ce{Ho^3+}$, which is why yttrium always turns up with the heavy lanthanides; the contraction also makes hafnium, tantalum and tungsten of period 6 almost the same size as zirconium, niobium and molybdenum of period 5 (see trends across the d-block).

Those undisturbed 4f levels make the lanthanides stars of functional materials:

• Magnets: 4f electrons carry large magnetic moments, and neodymium–iron–boron ($\ce{Nd2Fe14B}$, invented in 1984) and samarium–cobalt are the strongest permanent magnets.
• Phosphors and lasers: $\ce{Eu^3+}$ emits red light near 611 nm and $\ce{Tb^3+}$ green near 545 nm, both in narrow lines; neodymium-doped yttrium aluminium garnet lasers emit at 1064 nm, and erbium-doped fibre amplifiers work in the 1550 nm telecom band.
• Others: $\ce{Gd^3+}$, with seven unpaired electrons, is an MRI contrast agent; $\ce{CeO2}$ stores and releases oxygen in car catalytic converters.

Every isotope of every actinide is radioactive. Only thorium and uranium occur in quantity: their most stable isotopes have half-lives of billions of years ($\ce{^{238}U}$ 4.47 billion, $\ce{^{235}U}$ 704 million), comparable to the age of the Earth. The transuranium elements (neptunium, plutonium, …) are made artificially in reactors or accelerators. The 5f orbitals are less deeply buried than 4f, so the early actinides show more oxidation states: uranium ranges from +3 to +6, most commonly as the yellow-green, fluorescent uranyl ion $\ce{UO2^2+}$.

Only 0.720 % of natural uranium is $\ce{^{235}U}$, the isotope that slow neutrons can split; power-reactor fuel is “enriched” to 3–5 %. Because $\ce{^{235}U}$ decays faster than $\ce{^{238}U}$, uranium in the past was richer in it — two billion years ago at Oklo, in today’s Gabon, a uranium deposit ran a chain reaction on its own: a natural nuclear reactor.

Elements beyond atomic number 100 can only be made atom by atom in accelerators — element 118, for example, by firing $\ce{^{48}Ca}$ at $\ce{^{249}Cf}$. Their half-lives are often milliseconds, so their chemistry must be inferred from a handful of atoms. In 2016 elements 113, 115, 117 and 118 were named nihonium (Nh), moscovium (Mc), tennessine (Ts) and oganesson (Og), completing period 7. Their inner electrons move at a sizeable fraction of the speed of light, and relativistic effects shift their orbital energies: calculations predict that oganesson may not behave like a noble gas at all, but as a solid semiconductor.