The issue had a little more complicated that what it looked like. There was another bug that triggered when when trying direct fix.

So. this fixes the original issue

julia> ChemicalSpecies(:Si) == :Si
true

julia> :Si == ChemicalSpecies(:Si) # now fixed
true

and the other issue that was

julia> ChemicalSpecies(:H) == :not_atom
ArgumentError

and now is

julia> ChemicalSpecies(:H) == :not_atom
false

For the record the latter was triggered by

tmp = ChemicalSpecies(:C12; atom_name=:MyC)
@test atom_name(tmp) != atomic_symbol(tmp)

which raised an error for ChemicalSpecies(:MyC).

Because this is a bigger fix this needs some reviews too.