Average mass of an atom in unified atomic mass units (1 u is 1/12 of the mass of a carbon-12 atom). Accounts for the natural mix of the element’s isotopes.
Number of the vertical column, 1 to 18 in IUPAC numbering. Elements in a group share the same outer-shell layout, so their properties are alike.
Number of the horizontal row. Usually equal to the number of electron shells in the atom; palladium is the exception, with four shells in period 5.
The subshell (s, p, d or f) that receives the last electrons in the usual filling order; it divides the table into four blocks. Real atoms sometimes differ: lanthanum starts the f-block but has no f electrons.
[Kr] 5s¹
How electrons are arranged in subshells. The bracket stands for the previous noble gas; the superscript is the number of electrons in a subshell.
2, 8, 18, 8, 1
Number of electrons in each shell, from the nucleus outwards.
39.3 °C (312.46 K)
Temperature at which the solid turns liquid at standard pressure. Helium, carbon and arsenic do not melt at normal pressure: helium needs about 25 atm, carbon and arsenic turn straight into vapour.
688 °C (961 K)
Temperature at which the liquid boils at standard pressure. For carbon and arsenic, the temperature at which the solid turns straight into vapour (sublimation).
1.53 g/cm³
Mass of one cubic centimetre of the substance. For gases, at 0 °C and standard pressure.
0.82 Pauling
How strongly an atom pulls electrons towards itself in a chemical bond. On the Pauling scale it runs from 0.79 for caesium to 3.98 for fluorine.
4.177 eV
Energy needed to remove the most loosely bound electron from an atom. The lower it is, the more easily the element gives up an electron, which is typical of metals.
0.486 eV
Energy released when a neutral atom gains an electron. Highest for the halogens. Noble gases, nitrogen and some metals form no stable negative ion.
220 pm
Half the length of a single covalent bond between two atoms of the element. It shows the size of an atom inside a molecule or crystal: 76 pm for carbon, 132 pm for iron.
+1
Charges the atom would have in its compounds if all its bonds were ionic; the main ones are listed.
Rubidium (Rb) is a chemical element with atomic number 37. It sits in group 1 and period 5 of the periodic table, in the s-block. It is an alkali metal. At room temperature it is a solid. Its standard atomic weight is 85.468.
Origin of the name
Robert Bunsen and Gustav Kirchhoff discovered rubidium in 1861 with their newly invented spectroscope: light from the mineral lepidolite showed two deep red lines no one had seen before. They named the element after them, from the Latin rubidus, ‘deep red’.
Where is rubidium found?
Rubidium is more common in the Earth’s crust than was once thought, but it forms no ores of its own. It is obtained from minerals such as lepidolite and pollucite, partly as a by-product of producing lithium.
What is rubidium used for?
Rubidium keeps time in atomic clocks. It is also used in photocells and to remove the last traces of gas from vacuum tubes.
Interesting facts
Rubidium bursts into flame in air and reacts violently with water, so it is stored under oil or in an inert gas.
Natural rubidium is slightly radioactive: rubidium-87 slowly turns into strontium, and geologists use this to find the age of rocks.