This needs more work. ''bar'' bar isn't a thing. 'closely related to gold' doesn't work either. Other rounds were very obscure

I think very few people other than astronomers could possibly know the specific link for the 3rd group. Group one also extremely specific, and as noted 'bar bar' doesn't work - if you're thinking of the rhyme that would 'baa baa' (as would the Barbarians rugby team nickname).

Fine if you accept "Bar Bar" is a thing, or if you are a Phd in Chemistry or Astrophysics.......otherwise not so much

Thanks for grid... a toughie for sure!