It's opinionated, really. You have to think about the element in that sense - do a tad of research first. For example, I made Tar dark because it was a deathtrap for animals in the pleistocene. However water is considered good because it sustains life on Earth.
So start looking into some elements on the periodic table - see what properties they have and what they are used in, and then decide if this could be labelled as "good" or "bad".