Protactinium

A rare, highly radioactive metal whose name means before actinium, the element it decays into.

At a glance

Symbol: Pa
Atomic number: 91
Average atomic mass: 231.04
Group: none; it sits in the actinide row below the table
Period: 7
Family: actinide
Electron configuration: [Rn] 5f² 6d¹ 7s²
Electronegativity: 1.50
Common ions: +5 most commonly; also +4
Isotopes: none are stable; ²³¹Pa, with a half-life of about 33,000 years, is the longest-lived

On the table

Period 7, in the actinide row set below the main table. The other actinides are tinted, and every cell opens its own article.

Electrons and bonding

Radioactive, like every actinide. It gives up electrons to form positive ions.

Properties

State at 25 °C: solid
What it looks like: bright, silvery, radioactive metal
Melting point: 1572 °C
Boiling point: not known
Density: 15.4 g/cm³

Where it is found

One of the rarest natural elements, present in uranium ores at about a third of a gram per ton of uranium.

Discovery and name

Kasimir Fajans and Oswald Göhring found a short-lived isotope in 1913 and called it brevium. In 1917 and 1918 Otto Hahn and Lise Meitner, and separately Frederick Soddy and John Cranston, found the long-lived isotope. It was named protoactinium, before actinium, which was shortened to protactinium in 1949.

What it is used for

None outside research. The ratio of protactinium-231 to thorium-230 in ocean sediments helps date them and trace ocean currents of the past.

In compounds

Pa₂O₅protactinium pentoxidePaCl₅protactinium pentachloride

Handle with care

Highly radioactive and toxic.