Protactinium (Pa)

Protactinium is one of the rarest and most expensive naturally occurring elements: the UK Atomic Energy Authority extracted about 125 grams from 60 tonnes of uranium processing waste in 1961, at a cost that worked out around $500,000, and that batch supplied most of the world's research for decades. The name means "before actinium", because protactinium-231 decays into it. It is awkward to handle for reasons beyond radioactivity — it hydrolyses and sticks to the walls of any container that is not carefully chosen.

Properties

Atomic number

91

Symbol

Pa

Standard atomic weight

231.04 u

Category

Actinide

Period

7

Block

f-block

Electron configuration

[Rn] 7s² 5f² 6d¹

Electrons per shell

2, 8, 18, 32, 20, 9, 2

Melting point

1,845 K — 1571.8 °C — 2861.3 °F

Density

15.37 g/cm³

Electronegativity

1.5 (Pauling)

Oxidation states

+5, +4

Discovered

1913 by Kasimir Fajans and Oswald Göhring

Electron configuration

Full

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶ 7s² 5f² 6d¹

Noble gas shorthand

[Rn] 7s² 5f² 6d¹

Shells

K: 2 · L: 8 · M: 18 · N: 32 · O: 20 · P: 9 · Q: 2

What it is used for

• Protactinium-231 and thorium-230 ratios for dating marine sediments and corals

Hazards

• Highly radioactive and highly toxic; protactinium-231 has a 32,700-year half-life

Anomalous electron configuration: [Rn] 7s² 5f² 6d¹ rather than the predicted 7s² 5f³. The boiling point is omitted: published values around 4,300 K are extrapolations, not measurements.

Neighbours in the table