ATP has three phosphate groups that are attached to adenosine. The first phosphate group is attached to adenosine by a phosphoester bond. The second and third phosphate groups are attached to adenosine by different kind of bond known as phosphoanhydride bonds and all these are covalent bonds.
When phosphoanhydride bonds are broken, a greater amount of energy is released than when the phosphoester bond is broken. Therefore in energy production, it is the 2nd or 3rd phosphate group with the higher energy yielding bond that is "unhooked" from ATP to form ADP, yielding a high amount of free energy of at least 7.3 kcal/mol.