What Is Thulium?
Thulium is a chemical element with the symbol Tm and atomic number 69, part of the lanthanide series of rare earth elements.
The least abundant naturally occurring lanthanide. Named after mythical Thule.
As a lanthanide in Period 6 and Group 3 (Transition Metals), thulium's placement on the table reflects its electron configuration of [Xe] 4f¹³ 6s², which governs its bonding behavior and the type of compounds it typically forms.
Position on the Periodic Table
Thulium is located in Period 6, an unusually long row that includes the entire lanthanide series.
Within that row, thulium 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
Thulium has an atomic mass of 168.934 u and a density of 9.32 g/cm³. Its melting point is 1545°C and its boiling point is 1950°C.
Thulium displays the typical lanthanide combination of a silvery appearance and chemistry dominated by its f-orbital electrons.
Electron Configuration Explained
Thulium's electron configuration is [Xe] 4f¹³ 6s², 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 thulium's chemical reactivity and the type of bonds it forms with other elements.
History & Discovery
Thulium was formally discovered in 1879, P.T. Clève, 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 Thulium Used For?
Thulium's specific properties make it useful in several applications, including:
- Portable X-ray devices
- Surgical lasers
- High-temperature superconductors
- Nuclear medicine
- Research
These uses reflect thulium's underlying classification as a lanthanide, since the same properties that define its category in chemistry also determine where it becomes practically useful.
Fun Facts About Thulium
- Thulium-170 is used in portable X-ray machines that need no electricity.
- Thulium is element number 69 on the periodic table, in Period 6 and Group 3.
- Its standard atomic mass is 168.934 u, calculated from the natural abundance of its isotopes.
Frequently Asked Questions
What is Thulium's atomic number?
Thulium has atomic number 69.
What is Thulium's chemical symbol?
Thulium's symbol is Tm.
What group and period is Thulium in?
Thulium is in Period 6, Group 3 (Transition Metals).
What is Thulium's atomic mass?
Thulium's standard atomic mass is 168.934 u.