According to the Wikipedia entry on ATP,
ATP is an unstable molecule in unbuffered water, in which it hydrolyses to ADP and phosphate. This is because the strength of the bonds between the phosphate groups in ATP is less than the strength of the hydrogen bonds (hydration bonds), between its products (ADP + phosphate), and water
Why are the bonds between a water and a phosphate stronger than the bonds between two phosphates?
I'm studying first year chemistry, so an explanation that appealed to basic principles would be best.
(I've thought about this for a bit, and looked at the molecular diagrams in hope of seeing the answer. I'd include my thoughts on the matter in this question, however those thoughts amount to a half dozen inconclusive lines of speculation, so I opted to leave them out. Saying that to say, I have put effort into trying to figure out the answer, but I haven't gotten anywhere.)