Protactinium (Pa)
Element 91 · Actinide · Solid at 25 °C · Period 7 (f-block)
Protactinium is one of the rarest and most expensive naturally occurring elements, with essentially no uses outside research.
On the periodic table, protactinium sits in the f-block in period 7. It is one of 15 elements classified as an actinide, and at room temperature (25 °C) it is a solid.
The actinides fill the 5f orbitals in the bottom row of the table, and every one of them is radioactive. Only thorium and uranium occur on Earth in significant quantities; the rest are made in reactors or particle accelerators. Their nuclear properties — fission above all — shaped the twentieth century.
Properties of protactinium
| Atomic number (Z) | 91 |
|---|---|
| Symbol | Pa |
| Atomic mass | 231.04 |
| Category | Actinide |
| Group | — (f-block, associated with group 3) |
| Period | 7 |
| Block | f-block |
| Metal / nonmetal | Metal |
| State at 25 °C | Solid |
| Electron configuration | [Rn] 5f2 6d1 7s2 |
| Electronegativity (Pauling) | 1.5 |
| Melting point | 1,568 °C (1,841.15 K / 2,854.4 °F) |
| Boiling point | 4,027 °C (4,300.15 K / 7,280.6 °F) |
| Density | 15.37 g/cm³ |
Position in the periodic table
Protactinium belongs to the actinides, the detached f-block row shown beneath the main table. These elements all fit between groups 2 and 4 of period 7, and are pulled out into their own row only to keep the table a manageable width.
Physical properties
Protactinium melts at 1,568 °C (1,841.15 K / 2,854.4 °F) and boils at 4,027 °C (4,300.15 K / 7,280.6 °F), giving it a liquid range of about 2,459 °C at standard pressure.
Its density is 15.37 g/cm³.
See also: the interactive periodic table, how to read the periodic table, periodic trends.