Protactinium at a glance
- Atomic number
- 91
- Symbol
- Pa
- Atomic mass
- 231.04 u
- Category
- Actinide
- Period
- 7
- Electron configuration
- [Rn] 5f 2 6d 1 7s 2
- State at 25 °C
- Solid
- Melting point
- 1568 °C
- Boiling point
- 4027 °C
- Density
- 15.37 g/cm³
- Electronegativity
- 1.5
- 1st ionization energy
- 568 kJ/mol
- Oxidation states
- +5
- Discovered
- 1913
Atomic structure
A neutral protactinium atom has 91 protons and 91 electrons, and its most common isotope carries about 140 neutrons. One atom weighs about 3.84 × 10⁻²⁵ kg, so a mole of protactinium (6.022 × 10²³ atoms) weighs 231.04 g. That nucleus holds 1.54 neutrons for every proton, typical of heavy nuclei, which need extra neutrons to hold together against the protons' repulsion. The electrons fill 7 shells (K 2, L 8, M 18, N 32, O 20, P 9, Q 2), which gives the configuration [Rn] 5f 2 6d 1 7s 2. As a f-block element, both its outer s electrons and its inner f electrons can take part in bonding, which is why it shows more than one oxidation state. Its configuration breaks the simple filling order: one electron moves so that a subshell becomes half-full or full, which is more stable. All of its isotopes are radioactive, so any protactinium on Earth is either freshly made or part of a decay chain.
Physical properties
At room temperature (25 °C) protactinium is a solid. It melts at 1568 °C and boils at 4027 °C, so it stays liquid across a 2459-degree range. It stays solid in molten iron (1538 °C) but would melt in a welding arc. Its density is 15.37 g/cm³, 15.4 times as dense as water. Its atoms have a radius of about 180 pm (Thorium next door is 180 pm). In the solid, its atoms pack into a tetragonal lattice.
Chemical behaviour
Its electronegativity is 1.5 on the Pauling scale. Pulling off its outermost electron takes 568 kJ/mol. A bond between protactinium and chlorine would be polar covalent, with an electronegativity difference of 1.7. Its common oxidation states are +5, so in compounds you will usually meet it as Pa⁵⁺. It shares much of its chemistry with the other actinides, especially its neighbours Actinium, Thorium, Uranium and Neptunium. Handle with care: as a pure element it is radioactive.
Where it comes from
It makes up 0.0000014 mg/kg of Earth's crust, the 84th most abundant element there. It was first identified in 1913, making it the 86th element to be discovered (counting the 11 known since antiquity together).