Odd one out.

[MissBoots]

In the 1850's the new spectroscope made possible the discovery of helium, nebulium, coronium and geocoronium.

Can anyone tell me which one is the odd one out in this list, and why?

I've spent all evening 'googling' and am now totally confused! Many thanks for any help.

[kempie]

Helium exists as an element - the other three were hypothetical elements that never panned out; their observed "forbidden lines" were indicative of ionized ordinary elements:

Nebulium was ionized oxygen (missing two electrons)
Coronium was highly ionized iron (13 electrons missing)
Geocoronium was radioactive nitrogen.

or...

The Helium, Nebulium and Coronium spectral lines originated in distant bodies while Geocoronium originated in the Earth's upper atmosphere.

http://astronomyhistory.suite101.com/article.c fm/discovery_of_helium_in_the_sun

http://elements.vanderkrogt.net/elem/he.html

I often dismay at the ambiguity of "Odd One Out" questions/puzzles. It is mostly the case that one "correct" answer is expected but invariably a valid argument exists for each item to be shown as "odd".

[MissBoots]

Thank you kempie for such a comprehensive answer. I too, could have chosen any one as 'odd 'one out but you have helped me focus on the 'best' choice. Thanks again for taking the time.