Hmm... a lot of them don't really fit the chemical elements, but...
Easy ones:
Air: Nitrogen and oxygen, seems a no-brainer to me
Earth: Silicon is the most abundant element in the earth (it's in sand, as well as other minerals) so I'd go with that one
Fire: It isn't strictly made of elements, but I'd go with hydrogen and oxygen and carbon, as those are the three primary gases involved in combustion.
Life: Interestingly enough, I'd go with the same three as fire; carbon, oxygen, and hydrogen, due to them being everywhere in organic compounds. You could easily add another dozen that are crucial to life as we know it existing, such as nitrogen, iron, calcium, and others.
Water: And yeah, this one's hydrogen and oxygen. Hydronium and deuterium, as well as tritium, are still made up of those same elements.
Death: Arsenic seems a good choice :p I suppose anything that's poisonous fits here, though you could get carried away since technically any gaseous element can suffocate you in the absence of oxygen.
Hard ones:
Aether: I'd go with hydrogen here. It's the lightest element, and it's also the primary component of stars.
Darkness: Not sure about this one. Any element that has a naturally dark colour could fit here.
Entropy: Another pretty tough one. I'd go with carbon, though, because organic compounds are some of the most complex (and thus chaotic) that we know of.
Gravity: Anything heavy, I guess. Lead is the stereotypical heavy element, but there are dozens heavier.
Light: Anything brightly coloured or reflective could go here.
Time: This one's a toughie as well, but I'd go with something like Cesium, because they use that to construct some of the world's most accurate clocks. (based on radioactive decay) That one's admittedly a bit of a stretch... :p
