What Is Terbium?
Terbium, atomic number 65 and symbol Tb, belongs to the lanthanides — the row of rare earth elements beneath the main periodic table.
A silvery-white rare earth metal used in green phosphors.
As a lanthanide in Period 6 and Group 3 (Transition Metals), terbium's placement on the table reflects its electron configuration of [Xe] 4f⁹ 6s², which governs its bonding behavior and the type of compounds it typically forms.
Position on the Periodic Table
Terbium is located in Period 6, an unusually long row that includes the entire lanthanide series.
Within that row, terbium 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
Terbium has an atomic mass of 158.925 u and a density of 8.23 g/cm³. Its melting point is 1356°C and its boiling point is 3230°C.
Terbium displays the typical lanthanide combination of a silvery appearance and chemistry dominated by its f-orbital electrons.
Electron Configuration Explained
Terbium's electron configuration is [Xe] 4f⁹ 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 terbium's chemical reactivity and the type of bonds it forms with other elements.
History & Discovery
Terbium was formally discovered in 1843, C.G. Mosander, 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 Terbium Used For?
Terbium's specific properties make it useful in several applications, including:
- Green phosphors (LEDs)
- Sonar systems
- Fuel cells
- Terfenol-D alloy
- X-ray phosphors
These uses reflect terbium'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 Terbium
- Terbium is one of four elements named after the Swedish village of Ytterby.
- Terbium is element number 65 on the periodic table, in Period 6 and Group 3.
- Its standard atomic mass is 158.925 u, calculated from the natural abundance of its isotopes.
Frequently Asked Questions
What is Terbium's atomic number?
Terbium has atomic number 65.
What is Terbium's chemical symbol?
Terbium's symbol is Tb.
What group and period is Terbium in?
Terbium is in Period 6, Group 3 (Transition Metals).
What is Terbium's atomic mass?
Terbium's standard atomic mass is 158.925 u.