I think that Chemsitry.se could benefit from either a tiny periodic table or at least a link to a periodic table in the right hand navigation column.
WebElements page about chromium (and a number of resources) agree with the comment by @Philipp:
The ground state electron configuration of ground state gaseous neutral chromium is $\ce{[Ar]}3d^54s^1$
Which can be written as $\ce{[Ar]}4s^13d^5$
According to the Royal Society of Chemistry...
Europium(III) oxide, alongside other chemicals, is phosphorescent and
is used in the anti-counterfeiting details in Euro banknotes. Europium
(symbol Eu) is one of the rare earth elements and belongs to the class
of “lanthanides”.
Most of the trivalent rare earth elements are lumine...0
As already noted by the OP, many rare earths fluoresce under ultraviolet light and there seems to be no particular reason why Europium is a better choice than any other element, purely based on its fluorescent properties.
The European Central Bank keeps the exact nature of the compounds used a s...
