The Red Line Of Rubidium

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The name rubidium derives from "deep red" in Latin. Robert Bunsen (yes, that Bunsen) named the element that after he and physicist Gustav Kirchoff discovered it burned reddish-purple in a flame. They also found caesium which lies below (rhymes with) rubidium and which burns bright blue. Burning elements is still important in fireworks and also in modern analytical chemistry techniques such as ICP-MS. Also, a fair number of elements are named after colors: chlorine, rubidium, caesium, chromium, rhodium, indium, iridium, iodine, etc.

The depiction above implies how weakly rubidum holds its outermost electron and why it so readily gives it up to become Rb⁺. Most anything can pluck it off. Here's a spectacular video of rubidium hitting water:

Naturally occurring rubidium has two isotopes: the stable ⁸⁵Rb (72.2%) and the radioactive ⁸⁷Rb (27.8%).  ⁸⁷Rb is considered only "slightly" radioactive--despite its abundance (it is naturally present in seawater)--because of its extremely long half-life of 10¹⁰ years.  Remember that something has to actually decay in order for it to emit radiation.

Radioactivity is not something which has occurred naturally in the previous 36 elements except in very trace amounts (though artificial isotopes of lighter elements of course exist). Radiochemistry is a fascinating topic unto itself and becomes increasingly important for heavier elements which pretty much means from here on out.