Gadolinium (Gd)

Gadolinium-157 has the largest thermal neutron capture cross-section of any stable nuclide — around 250,000 barns, roughly a quarter of a million times that of ordinary hydrogen — which makes a thin gadolinium foil an extremely effective neutron shield. It is ferromagnetic below 20 °C, so a gadolinium sample held in a cold room sticks to a magnet and stops doing so once it warms in your hand. Its seven unpaired f electrons are what make gadolinium chelates the standard MRI contrast agent; the free ion is toxic, so it is always wrapped in a cage of ligands.

Properties

Atomic number

64

Symbol

Gd

Standard atomic weight

157.25 u

Category

Lanthanide

Period

6

Block

f-block

Electron configuration

[Xe] 6s² 4f⁷ 5d¹

Electrons per shell

2, 8, 18, 25, 9, 2

Melting point

1,586 K — 1312.8 °C — 2395.1 °F

Boiling point

3,546 K — 3272.8 °C — 5923.1 °F

Density

7.9 g/cm³

Electronegativity

1.2 (Pauling)

Oxidation states

+3

Discovered

1880 by Jean Charles Galissard de Marignac

Electron configuration

Full

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f⁷ 5d¹

Noble gas shorthand

[Xe] 6s² 4f⁷ 5d¹

Shells

K: 2 · L: 8 · M: 18 · N: 25 · O: 9 · P: 2

What it is used for

• MRI contrast agents, in about a third of all scans
• Neutron shielding and reactor control rods
• Magnetic refrigeration research
• Green phosphors in X-ray intensifying screens
• Improving the workability of iron and chromium alloys

Hazards

• Free gadolinium ions are toxic; some MRI contrast agents deposit trace gadolinium in tissue

Anomalous electron configuration: [Xe] 6s² 4f⁷ 5d¹ rather than the predicted 6s² 4f⁸ — a half-filled f subshell is worth an electron in the d subshell. The only element named after a person via a mineral named for Johan Gadolin.

Neighbours in the table