What are all these?

Figure out what all these are! I'm the food for my opposite on being divided, and a life saver for people when united. What am I? The one who made me doesn't need me, the one buying me hates to be in me, and the one in need doesn't know me. What am I? What can you catch but not throw? What goes around the world but stays in a corner? You light my head, and I cry without any tears. I am dying to die and give you light. You can't look at me when I'm upon your head, and will die if you get in me for too long without water! What can I be? You'll never know what it is as you yourself are that thing! What can it be? Answers on recieveing 20 comments!