What Is Thorium?
Thorium is a chemical element with the symbol Th and atomic number 90, part of the radioactive actinide series.
A weakly radioactive actinide. More abundant than uranium. Alternative nuclear fuel candidate.
As a actinide in Period 7 and Group 3 (Transition Metals), thorium's placement on the table reflects its electron configuration of [Rn] 6d² 7s², which governs its bonding behavior and the type of compounds it typically forms.
Position on the Periodic Table
Thorium is located in Period 7, the bottom row of the standard periodic table, consisting largely of radioactive and synthetic elements.
Within that row, thorium 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
Thorium has an atomic mass of 232.038 u and a density of 11.72 g/cm³. Its melting point is 1750°C and its boiling point is 4788°C.
Thorium is radioactive, like every other member of the actinide series, with a nucleus prone to spontaneous decay.
Electron Configuration Explained
Thorium's electron configuration is [Rn] 6d² 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 thorium's chemical reactivity and the type of bonds it forms with other elements.
History & Discovery
Thorium was formally discovered in 1829, Berzelius, 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 Thorium Used For?
Thorium's specific properties make it useful in several applications, including:
- Nuclear fuel (future)
- Gas mantles
- Tungsten alloys
- Catalysts
- Nuclear research
These uses reflect thorium'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 Thorium
- Thorium is 3–4 times more abundant than uranium and produces less long-lived nuclear waste.
- Thorium is element number 90 on the periodic table, in Period 7 and Group 3.
- Its standard atomic mass is 232.038 u, calculated from the natural abundance of its isotopes.
Frequently Asked Questions
What is Thorium's atomic number?
Thorium has atomic number 90.
What is Thorium's chemical symbol?
Thorium's symbol is Th.
What group and period is Thorium in?
Thorium is in Period 7, Group 3 (Transition Metals).
What is Thorium's atomic mass?
Thorium's standard atomic mass is 232.038 u.