91
Pa
Protactinium
231
Protactinium (Pa)
ActinidePeriod 7f-block-blockRadioactive
Protactinium is a chemical element with symbol Pa and atomic number 91. It is a dense, silvery-gray metal which readily reacts with oxygen, water vapor and inorganic acids. It forms various chemical compounds where protactinium is usually present in the oxidation state +5, but can also assume +4 and even +2 or +3 states.
Latin name: Protactinium
Atomic Number
91
Atomic Mass
231 g/mol
Phase (STP)
Solid
Electron Config.
[Rn] 5f² 6d¹ 7s²
General
SymbolPa
Atomic Number91
Atomic Mass231 g/mol
CategoryActinide
Period7
Blockf-block
Electron Configuration[Rn] 5f² 6d¹ 7s²
Valence Electrons5
Coloursilvery metallic
Appearancebright, silvery metallic luster
CAS NumberCAS7440-13-3
Physical Properties
Phase (STP)Solid
Density15.37 g/cm³
Melting Point1567.85°C · 2854.13°F · 1841.00 K
Boiling Point3999.85°C · 7231.73°F · 4273.00 K
Atomic Properties
Atomic Radius243 pm
Electronegativity1.5 Pauling
Ionization Energy568 kJ/mol
Oxidation States+5, +4
Ion ChargePa4+/Pa5+
Crystal Structure
Crystal Structuretetragonal
Nuclear & Hazards
RadioactiveYes
Half-life32760 years (Pa-231)
Abundance
In Universe0.000000000001%
In Earth's Crust0.000000000001%
Discovery & History
Discovery Year1913
Discovered ByOswald Helmuth Göhring and Kasimir Fajans
Country of DiscoveryGermany/Poland
Named ByOtto Hahn
Isotopes of Protactinium
| Isotope | Abundance | Half-life | Decay |
|---|---|---|---|
| Pa-231 | — | 3.276 × 10⁴ years | α |
| Pa-233 | — | 26.967 days | β⁻ |
| Pa-234 | — | 6.70 hours | β⁻ |