What Is Rubidium?
Rubidium is a chemical element with the symbol Rb and atomic number 37, classified among the alkali metals in Group 1 of the periodic table.
A soft, highly reactive alkali metal. Ignites spontaneously in air.
As a alkali metal in Period 5 and Group 1 (Alkali Metals), rubidium's placement on the table reflects its electron configuration of [Kr] 5s¹, which governs its bonding behavior and the type of compounds it typically forms.
Position on the Periodic Table
Rubidium is located in Period 5, which continues the transition metal pattern seen in Period 4 but one principal shell further out.
Within that row, rubidium sits in Group 1 (Alkali 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
Rubidium has an atomic mass of 85.468 u and a density of 1.532 g/cm³. Its melting point is 39.3°C and its boiling point is 688°C.
Rubidium is soft enough to be cut with a knife and never appears as a free element in nature, since it reacts too readily with moisture and oxygen in the air.
Electron Configuration Explained
Rubidium's electron configuration is [Kr] 5s¹, 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 rubidium's chemical reactivity and the type of bonds it forms with other elements.
History & Discovery
Rubidium was formally discovered in 1861, Bunsen & Kirchhoff, 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 Rubidium Used For?
Rubidium's specific properties make it useful in several applications, including:
- Atomic clocks
- GPS systems
- Medical imaging
- Research
- Fireworks (purple)
These uses reflect rubidium's underlying classification as a alkali metal, since the same properties that define its category in chemistry also determine where it becomes practically useful.
Fun Facts About Rubidium
- Rubidium atomic clocks lose only 1 second every 300 years — used in GPS satellites.
- Rubidium is element number 37 on the periodic table, in Period 5 and Group 1.
- Its standard atomic mass is 85.468 u, calculated from the natural abundance of its isotopes.
Frequently Asked Questions
What is Rubidium's atomic number?
Rubidium has atomic number 37.
What is Rubidium's chemical symbol?
Rubidium's symbol is Rb.
What group and period is Rubidium in?
Rubidium is in Period 5, Group 1 (Alkali Metals).
What is Rubidium's atomic mass?
Rubidium's standard atomic mass is 85.468 u.