What Is Protactinium?
Protactinium is a chemical element with the symbol Pa and atomic number 91, part of the radioactive actinide series.
A dense, silvery, highly radioactive metal. One of the most expensive natural substances.
As a actinide in Period 7 and Group 3 (Transition Metals), protactinium's placement on the table reflects its electron configuration of [Rn] 5f² 6d¹ 7s², which governs its bonding behavior and the type of compounds it typically forms.
Position on the Periodic Table
Protactinium is located in Period 7, the bottom row of the standard periodic table, consisting largely of radioactive and synthetic elements.
Within that row, protactinium sits in Group 3 (Transition Metals), alongside elements that share a similar number of valence electrons and, by extension, similar chemical behavior.
This position is not arbitrary — the periodic table was deliberately organized so that an element's row and column reveal its expected reactivity, bonding pattern, and physical state at a glance.
Physical & Chemical Properties
Protactinium has an atomic mass of 231.036 u and a density of 15.37 g/cm³. Its melting point is 1568°C and its boiling point is 4027°C.
Protactinium is radioactive, like every other member of the actinide series, with a nucleus prone to spontaneous decay.
Electron Configuration Explained
Protactinium's electron configuration is [Rn] 5f² 6d¹ 7s², describing how its electrons are distributed across shells and subshells around the nucleus.
Reading the configuration in order shows electrons filling shells of increasing energy, following the standard Aufbau principle that explains how atomic structure builds up across the periodic table.
The outermost (valence) electrons shown in this configuration are what ultimately determine protactinium's chemical reactivity and the type of bonds it forms with other elements.
History & Discovery
Protactinium was formally discovered in 1913, Fajans & Göhring, identified through the careful isolation and analytical techniques typical of that era's chemical research.
Many elements from this period were anticipated by Dmitri Mendeleev's periodic table, which successfully predicted gaps corresponding to undiscovered elements well before laboratory instruments existed to confirm them directly.
What Is Protactinium Used For?
Protactinium's specific properties make it useful in several applications, including:
- Scientific research
- Ocean dating
- Nuclear physics
- No commercial uses
- Radiometric dating
These uses reflect protactinium's underlying classification as a actinide, since the same properties that define its category in chemistry also determine where it becomes practically useful.
Fun Facts About Protactinium
- Protactinium costs around $280,000 per gram — one of the most expensive materials on Earth.
- Protactinium is element number 91 on the periodic table, in Period 7 and Group 3.
- Its standard atomic mass is 231.036 u, calculated from the natural abundance of its isotopes.
Frequently Asked Questions
What is Protactinium's atomic number?
Protactinium has atomic number 91.
What is Protactinium's chemical symbol?
Protactinium's symbol is Pa.
What group and period is Protactinium in?
Protactinium is in Period 7, Group 3 (Transition Metals).
What is Protactinium's atomic mass?
Protactinium's standard atomic mass is 231.036 u.