Rubidium at a glance
- Atomic number
- 37
- Symbol
- Rb
- Atomic mass
- 85.468 u
- Category
- Alkali metal
- Group / period
- 1 / 5
- Electron configuration
- [Kr] 5s 1
- State at 25 °C
- Solid
- Melting point
- 39.31 °C
- Boiling point
- 687.9 °C
- Density
- 1.532 g/cm³
- Electronegativity
- 0.82
- 1st ionization energy
- 403 kJ/mol
- Oxidation states
- +1
- Discovered
- 1861
Atomic structure
A neutral rubidium atom has 37 protons and 37 electrons, and its most common isotope carries about 48 neutrons. One atom weighs about 1.42 × 10⁻²⁵ kg, so a mole of rubidium (6.022 × 10²³ atoms) weighs 85.468 g. That nucleus holds 1.3 neutrons for every proton. The electrons fill 5 shells (K 2, L 8, M 18, N 8, O 1), which gives the configuration [Kr] 5s 1. With 1 electron in its outer shell, it tends to lose electrons and form positive ions. It has only one stable isotope, Rb-85, plus the long-lived Rb-87. No other element has a stable isotope with mass number 85, so Rb-85 has no stable isobars; its neighbours on that line decay into it or away from it.
Physical properties
At room temperature (25 °C) rubidium is a solid. It melts at 39.31 °C and boils at 687.9 °C, so it stays liquid across a 648.5-degree range. It stays solid in your hand (body temperature) but would melt in boiling water. Its density is 1.532 g/cm³, 1.53 times as dense as water. Its atoms have a radius of about 235 pm (Strontium next door is 200 pm). In the solid, its atoms pack into a body-centred cubic lattice.
Chemical behaviour
Its electronegativity is 0.82 on the Pauling scale, one of the lowest, so it gives up electrons very easily. Pulling off its outermost electron takes 403 kJ/mol, less than Potassium above it (419 kJ/mol). A bond between rubidium and chlorine would be mostly ionic, with an electronegativity difference of 2.3. It releases 47 kJ/mol when it gains an extra electron. Its common oxidation states are +1, so in compounds you will usually meet it as Rb⁺. Group 1 also holds Hydrogen, Lithium, Sodium, Potassium, Caesium and Francium, which share its outer-shell pattern. Its Lewis symbol is Rb surrounded by 1 dot. Handle with care: as a pure element it is flammable or reacts with water.
Where it comes from
It makes up 90 mg/kg of Earth's crust, the 22nd most abundant element there. In the human body it accounts for about 0.00046 % of body mass. It was first identified in 1861, making it the 60th element to be discovered (counting the 11 known since antiquity together). When excited it glows at its own set of colours; its brightest visible lines are at 420.2 nm, 421.6 nm and 572.4 nm. In a flame test its compounds colour the flame red-violet.