Lanthanides (rare earth elements)

Filling the 4f subshell

Across the lanthanides, electrons are added to the 4f subshell, which lies deep inside the atom, beneath the 5s and 5p electrons. Many lanthanides have the configuration [Xe] 4fⁿ 6s², such as neodymium ([Xe] 4f⁴ 6s²). Lanthanum ([Xe] 5d¹ 6s²) has no 4f electrons yet, and lutetium ([Xe] 4f¹⁴ 5d¹ 6s²) has a full 4f subshell.

Fitting all 15 into the main table would make it 32 columns wide, so they are printed separately. Chemists still debate whether lanthanum or lutetium belongs in group 3 under yttrium; this site shows both in the lanthanide row.

Properties and reactions

The lanthanides are soft, silvery metals that tarnish in air. They react slowly with cold water and faster with hot water, releasing hydrogen: 2La + 6H₂O → 2La(OH)₃ + 3H₂. Because the buried 4f electrons take little part in bonding, almost all lanthanides form 3+ ions and have very similar chemistry, although cerium can also be +4 and europium and ytterbium can be +2.

Unpaired 4f electrons give many lanthanide ions strong magnetism and sharp, pure colors of light. Promethium is the only lanthanide with no stable isotopes and is almost absent from nature.

The lanthanide contraction

From lanthanum to lutetium the 3+ ions shrink steadily, because each added proton pulls harder on the electrons while the 4f electrons shield one another poorly. Chemists exploit these small differences in size to separate the lanthanides. The contraction also explains why hafnium and the other period 6 transition metals are almost the same size as the metals above them.

Rare but useful

Despite the name, most rare earths are not especially rare: cerium is about as common in Earth’s crust as copper. They occur mixed together in minerals such as monazite and bastnäsite and are difficult to separate.

Neodymium-iron-boron magnets are the strongest permanent magnets available and are found in electric car motors, wind turbines, headphones and hard drives. Europium and terbium provide red and green light in screens and lamps, erbium amplifies signals in fiber-optic cables, gadolinium is a contrast agent for MRI scans, and cerium oxide polishes glass. Lanthanum goes into camera lenses and nickel-metal hydride batteries.