Lanthanides and Actinides: A Guide to the f-Block

The two rows set apart beneath the periodic table can look like an appendix, but they hold some of the most important elements in modern technology and energy. The magnets in phones and electric-car motors, the fuel of nuclear power stations and the power sources of deep-space probes all come from these rows. Together they form the f-block, where electrons fill the f orbitals. The upper row is the lanthanides and the lower row the actinides.

Structure of the f-block

There are seven f orbitals, holding up to fourteen electrons. The lanthanides run from lanthanum (57) to lutetium (71), and the actinides from actinium (89) to lawrencium (103), fifteen elements in each series by the usual convention of including the first element.

The 4f orbitals filled across the lanthanides lie deep inside the atom, shielded by the outer 5s and 5p electrons. The electrons that take part in chemistry are mostly the outer ones, so the lanthanides are remarkably similar to one another. Most exist as +3 ions. The exceptions, such as cerium in the +4 state and europium in the +2 state, are linked to f subshells that become empty, half-full or full.

The lanthanide contraction

Going from lanthanum to lutetium the atomic number rises by fourteen, yet the atoms and their +3 ions become slightly smaller. This is the lanthanide contraction.

The cause is the poor shielding provided by 4f electrons. Each step adds one proton and one 4f electron, but f orbitals are diffuse and awkwardly shaped, so they do little to screen the outer electrons from the growing nuclear charge. The effective nuclear charge felt by the outer electrons therefore increases, pulling the electron cloud inward.

This small shrinkage leaves marks across the whole table.

Zirconium and hafnium as twins: hafnium in period 6 is almost exactly the same size as zirconium directly above it in period 5. Normally an element one period down is larger, but the lanthanide contraction cancels the increase. The two are so chemically alike that hafnium was not discovered until 1923, hidden in zirconium ores, and they remain difficult to separate.
Very dense period 6 metals: atoms that barely grow in size while gaining a great deal of mass make osmium, iridium and platinum exceptionally dense.
Hard-to-separate lanthanides: neighboring lanthanides differ very little in size, which makes separating them chemically a painstaking job. Before ion exchange and solvent extraction methods matured in the mid-twentieth century, obtaining a pure lanthanide was extremely laborious.

Rare earths that are not so rare

The rare earth elements are the fifteen lanthanides plus scandium and yttrium, seventeen in total. Scandium and yttrium are group 3 elements, but they occur in the same minerals and their +3 ions behave similarly, so they are grouped together.

Despite the name, rare earths are not especially scarce in Earth's crust. Cerium is about as abundant as copper, and even the scarcer ones such as thulium and lutetium are more common than gold or platinum. They are called rare because economically concentrated deposits are uncommon and because separating and refining these near-identical elements is difficult. Promethium is the exception: it has no stable isotopes and occurs in nature only in minute traces.

Much of the history of the rare earths centers on Ytterby, a small village in Sweden. Minerals from a nearby quarry yielded yttrium, terbium, erbium and ytterbium, and all four were named after the village.

In modern industry small amounts of rare earths make large differences in performance.

Neodymium, praseodymium and dysprosium: neodymium-iron-boron permanent magnets in electric-vehicle motors, wind turbines, earphones and hard drives.
Samarium: samarium-cobalt magnets that keep their strength at high temperatures.
Europium and terbium: red and green phosphors in displays and fluorescent lamps.
Erbium: signal amplifiers in fiber-optic communication.
Gadolinium: MRI contrast agents.
Cerium: glass polishing powder and automotive exhaust catalysts.
Lanthanum: alloys in nickel-metal hydride batteries and high-refractive-index camera lens glass.

Mining and refining often produce acidic waste and radioactive by-products such as thorium and uranium, so the environmental burden is significant. Production and refining are also concentrated in a small number of countries, which has made supply security a policy issue in many places. That is one reason recycling rare earths from discarded electronics is attracting attention.

Actinides: all radioactive

None of the fifteen actinides has a stable isotope. Thorium and uranium, however, have such long half-lives that they have survived since Earth formed; they are primordial elements. Thorium-232 has a half-life of about 14 billion years, uranium-238 about 4.5 billion years and uranium-235 about 700 million years. These long half-lives make uranium isotopes a key tool for radiometric dating of rocks and of Earth itself.

Unlike the lanthanides, the early actinides show a wide range of oxidation states. Uranium ranges from +3 to +6 and plutonium from +3 to +7. The 5f orbitals extend farther from the nucleus than the 4f orbitals and take part in bonding more readily. That behavior led early twentieth-century chemists to treat these elements as an extension of the d-block transition metals, a view corrected by Glenn Seaborg's actinide concept in 1945.

The transuranium elements

Elements with atomic numbers higher than uranium's 92 are called transuranium elements. They are almost absent in nature, and nearly all were first made artificially.

In 1940 at Berkeley, Edwin McMillan and Philip Abelson bombarded uranium with neutrons and produced neptunium (93).
Shortly afterward Seaborg's team identified plutonium (94). Plutonium-239 fissions readily, which gave it an outsized role in the history of nuclear power and nuclear weapons.
Americium (95) and curium (96) followed around 1944, and berkelium (97) and californium (98) in 1949–1950.
Einsteinium (99) and fermium (100) were first identified in 1952 in debris from a hydrogen bomb test.
Mendelevium (101), nobelium (102) and lawrencium (103) were made in accelerators by firing light ions at heavy target nuclei.

Half-lives generally shorten as atomic number rises, and the number of atoms that can be made at once falls steeply. No element beyond fermium has ever been collected in a visible quantity.

Radioactivity and its uses

Radioactive elements are hazardous, but when their behavior is understood and controlled they are extremely useful.

Uranium-235: fuel for nuclear power reactors. It makes up only about 0.7 percent of natural uranium, so the fuel is usually enriched.
Plutonium-238: fuel for radioisotope thermoelectric generators, which turn decay heat into electricity. They have powered deep-space probes far from the Sun and rovers on Mars.
Americium-241: half-life about 432 years. A tiny amount is used in some ionization smoke detectors.
Californium-252: a strong neutron source used to start up reactors and to analyze materials.

The danger of radiation depends not just on the element but on the type of radiation (alpha, beta or gamma), the dose and the route of exposure. An alpha emitter outside the body is stopped by a sheet of paper, but inhaled or swallowed it can do serious harm. That is why actinides are handled only under strict regulation.

Summary

The lanthanides are so alike that they are hard to separate, yet their subtle differences decide the performance of magnets, phosphors and lasers. The actinides are all radioactive and are central to nuclear energy, space exploration and radiometric dating. Although the two rows sit apart from the main table, they follow the same rule as everything above them: electrons fill orbitals in order. Click through the f-block elements and compare their oxidation states and discovery dates to see the pattern for yourself.