Protactinium (Pa)
Actinide · f-block · period 7
Bright, silvery, malleable radioactive metal that keeps its lustre for a while.
Protactinium (symbol Pa) is chemical element number 91, a actinide in the actinide series (f-block), period 7 of the periodic table. Its standard atomic weight is 231.04 and its electron configuration is [Rn] 5f2 6d1 7s2. At room temperature it is a solid, and it is radioactive. Its name means 'before actinium' — it decays into actinium.
Properties
| Atomic number | 91 |
|---|---|
| Atomic mass | 231.04 |
| Category | Actinide |
| Block | f-block |
| Group · Period | f-block series · 7 |
| Electron configuration | [Rn] 5f2 6d1 7s2 |
| Electrons per shell | 2, 8, 18, 32, 20, 9, 2 |
| State at 25 °C | solid |
| Melting point | 1841 K |
| Boiling point | 4300 K |
| Density | 15.37 g/cm³ |
| Electronegativity (Pauling) | 1.5 |
| Radioactive | Yes — no stable isotope |
| Origin | decay |
| Discovered | 1913 · Fajans & Göhring |
Key facts about Protactinium
- Its name means 'before actinium' — it decays into actinium.
- One of the rarest and most expensive natural elements; extracting 125 g once took two years and 60 tonnes of waste.
- Both highly radioactive and chemically toxic.
- Its ratio to thorium in ocean sediments is used to date the last 175,000 years of climate.
What Protactinium is used for
Almost none — used only for scientific research and for dating marine sediments.
Where it sits in the periodic table
Protactinium is in the actinide series, period 7, of the f-block. Its electron configuration [Rn] 5f2 6d1 7s2 means its outermost shell holds 2 electrons, which is what places it here and explains much of its chemistry. See Protactinium on the interactive table →
Protactinium — frequently asked questions
What is Protactinium used for?
Almost none — used only for scientific research and for dating marine sediments.
Is Protactinium radioactive?
Yes. Protactinium has no stable isotope, so every form of it is radioactive.
What is the electron configuration of Protactinium?
Protactinium has the electron configuration [Rn] 5f2 6d1 7s2, which is 2, 8, 18, 32, 20, 9, 2 electrons per shell.
What group and period is Protactinium in?
Protactinium is in the actinide series (f-block), period 7, in the f-block. Its outer shell holds 2 electrons.
Who discovered Protactinium?
Protactinium was discovered in 1913 by Fajans & Göhring.