Adenosine TRI phosphate (ATP) is just like adenosine DI phosphate (ADP), except that ATP has a third phosphate group stuck on the end of the molecule. Think about what that phosphate group does for the molecule. Phosphates are big, bulky moieties, so it is difficult to cram a third group onto an ADP molecule that already has two. Also, they have a strong net negative charge - so that makes it doubly hard to push negative charges together. So the bond between the third phosphate group and the rest of the ATP molecule is very high energy - it takes a lot of energy to form it.