Lanthanides

Fifteen near-identical metals hidden below the main table, and the secret ingredient in magnets and screens.

Look below the main body of the periodic table and you will find two rows that seem to have fallen off. The upper one is the lanthanides: fifteen metals from lanthanum to lutetium that belong in period 6 between barium and hafnium.

They occur mixed together in the same minerals and are so alike that separating them took chemists generations. Together with scandium and yttrium they are often called the rare earth elements, although most of them are not rare at all. Cerium, the most common, is more plentiful in the Earth's crust than copper. Their reputation for rarity comes from how hard they are to separate, not from how little there is.

What they have in common

The lanthanides are the first elements to fill a set of orbitals called the 4f subshell. Those f electrons sit deep inside the atom, tucked beneath the outer shells, so adding one more makes almost no difference to how the element behaves. That is why fifteen elements in a row share nearly the same chemistry. Almost all of them form ions with a charge of plus three.

As metals they are silvery-white, fairly soft and reasonably reactive. They tarnish in air, some quite quickly, and they react slowly with cold water and briskly with dilute acids, giving off hydrogen. Their melting points are moderate for metals, from about 920 degrees Celsius for lanthanum up to about 1652 degrees for lutetium.

Where the lanthanides truly stand out is in their interaction with light and magnetism. The buried f electrons give many of them sharp, pure colours in solution and make them glow in precise colours when excited, a property called luminescence. Several are also strongly magnetic because of the way those f electrons align. These two traits are behind most of the modern uses of the family.

How they change across the row

The main trend across the lanthanides is a slow, steady shrinking. As each proton is added to the nucleus, the extra f electron does a poor job of shielding the outer electrons from the increased positive charge, so the atoms and their plus-three ions get gradually smaller from lanthanum to lutetium. This is known as the lanthanide contraction. Each step is tiny, which is why they are so hard to tell apart.

Because the ions shrink, they hold on to water molecules and other partners a little more tightly as you move across the row. Modern separation methods exploit this by passing a mixture through columns that let the slightly smaller ions lag behind or run ahead of the larger ones.

Melting points and hardness generally increase across the row, though not smoothly. Densities also rise overall. The exceptions are europium and ytterbium, which are noticeably lighter and softer than their neighbours and melt at lower temperatures, about 826 and 824 degrees Celsius respectively.

Where you meet them

Neodymium is the lanthanide you are most likely to have in your pocket. Alloyed with iron and boron, it makes the strongest permanent magnets available, small enough to fit in earphones and phone speakers and powerful enough to drive electric cars and wind turbines. Dysprosium is added to help those magnets keep working when they get hot.

Europium and terbium glow red and green when struck by electrons or ultraviolet light, and for decades they provided the colours in television screens and energy-saving lamps. Cerium polishes glass and lenses, sits in catalytic converters, and combined with other lanthanides makes the sparking flint in gas lighters. Lanthanum goes into camera lenses and rechargeable batteries. Gadolinium compounds are injected into patients to sharpen MRI scans. Erbium is added to the glass fibres that carry internet traffic, where it amplifies the light signal along the way.

Odd ones out

Promethium is the only lanthanide with no stable isotopes. Every atom of it is radioactive, and the amounts formed naturally in the Earth's crust are so small that it is effectively a synthetic element, made in nuclear reactors.

Europium and ytterbium break the plus-three rule by readily forming plus-two ions, and cerium breaks it in the other direction by forming plus-four ions. These exceptions come from the special stability of a half-full or completely full f subshell. Lutetium at the end of the row is the hardest, densest and highest-melting member and is sometimes argued to belong with the transition metals instead. Lanthanum at the start has no f electrons at all in its atom, which leads some chemists to say the row should really begin with cerium. Both arguments show how blurry the edges of this family are.

The numbers

Melting points rise down the list, from Lanthanum at 920 °C to Lutetium at 1652 °C.

Density in g/cm³ (g/L for gases). Values for synthetic elements are predictions.

Did you know?

• Cerium, the most abundant lanthanide, is more plentiful in the Earth's crust than copper.
• Neodymium magnets are the strongest permanent magnets available and are found in earphones, motors and wind turbines.
• Promethium is the only lanthanide with no stable isotopes and barely exists in nature.
• The lanthanides are so chemically alike that separating them from one another took chemists over a century.
• Europium and terbium supplied the red and green colours in older television screens and low-energy lamps.
• Gadolinium compounds are injected into patients to make MRI scans clearer.

Frequently asked questions

Written by Anthony. Data from Periodic-Table-JSON (CC BY-SA).