What Is Promethium?
Promethium, atomic number 61 and symbol Pm, belongs to the lanthanides — the row of rare earth elements beneath the main periodic table.
The only lanthanide with no stable isotopes. Extremely rare in nature.
As a lanthanide in Period 6 and Group 3 (Transition Metals), promethium's placement on the table reflects its electron configuration of [Xe] 4f⁵ 6s², which governs its bonding behavior and the type of compounds it typically forms.
Position on the Periodic Table
Promethium is located in Period 6, an unusually long row that includes the entire lanthanide series.
Within that row, promethium 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
Promethium has an atomic mass of (145) u and a density of 7.26 g/cm³. Its melting point is 1042°C and its boiling point is 3000°C.
Promethium displays the typical lanthanide combination of a silvery appearance and chemistry dominated by its f-orbital electrons.
Electron Configuration Explained
Promethium's electron configuration is [Xe] 4f⁵ 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 promethium's chemical reactivity and the type of bonds it forms with other elements.
History & Discovery
Promethium was formally discovered in 1945, Marinsky, 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 Promethium Used For?
Promethium's specific properties make it useful in several applications, including:
- Nuclear batteries
- X-ray sources
- Thickness gauges
- Research
- Luminous paint
These uses reflect promethium'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 Promethium
- The total amount of promethium in Earth's crust at any time is less than 500 grams.
- Promethium is element number 61 on the periodic table, in Period 6 and Group 3.
- Its standard atomic mass is (145) u, calculated from the natural abundance of its isotopes.
Frequently Asked Questions
What is Promethium's atomic number?
Promethium has atomic number 61.
What is Promethium's chemical symbol?
Promethium's symbol is Pm.
What group and period is Promethium in?
Promethium is in Period 6, Group 3 (Transition Metals).
What is Promethium's atomic mass?
Promethium's standard atomic mass is (145) u.