What Is Thallium?
With atomic number 81 and chemical symbol Tl, Thallium sits among the post-transition metals just right of the main transition block.
A soft, malleable toxic metal. Compounds are odorless and tasteless.
As a post-transition metal in Period 6 and Group 13 (Boron Group), thallium's placement on the table reflects its electron configuration of [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p¹, which governs its bonding behavior and the type of compounds it typically forms.
Position on the Periodic Table
Thallium is located in Period 6, an unusually long row that includes the entire lanthanide series.
Within that row, thallium sits in Group 13 (Boron Group), 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
Thallium has an atomic mass of 204.383 u and a density of 11.85 g/cm³. Its melting point is 304°C and its boiling point is 1473°C.
Thallium is generally softer and lower-melting than the transition metals positioned to its left on the periodic table.
Electron Configuration Explained
Thallium's electron configuration is [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p¹, 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 thallium's chemical reactivity and the type of bonds it forms with other elements.
History & Discovery
Thallium was formally discovered in 1861, Crookes, 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 Thallium Used For?
Thallium's specific properties make it useful in several applications, including:
- Infrared optics
- Medical imaging
- Electronics
- Research
- Alloys
These uses reflect thallium's underlying classification as a post-transition metal, since the same properties that define its category in chemistry also determine where it becomes practically useful.
Fun Facts About Thallium
- Thallium sulfate was used as rat poison and by Agatha Christie as a murder weapon in detective stories.
- Thallium is element number 81 on the periodic table, in Period 6 and Group 13.
- Its standard atomic mass is 204.383 u, calculated from the natural abundance of its isotopes.
Frequently Asked Questions
What is Thallium's atomic number?
Thallium has atomic number 81.
What is Thallium's chemical symbol?
Thallium's symbol is Tl.
What group and period is Thallium in?
Thallium is in Period 6, Group 13 (Boron Group).
What is Thallium's atomic mass?
Thallium's standard atomic mass is 204.383 u.