ATP is used to store and transport chemical energy within cells because of the high energy content of the diphosphate bonds due to the charge separation and the relative stabilty of the resulting AMP (adenosine monophosphate) in the chemical reaction:

ATP + H₂O → ADP + Pi + H+ ΔG˚ = -30.54 kJ/mol (-7.3 kcal/mol)

ATP + H₂O → AMP + PPi + H+ ΔG˚ = -30.54 kJ/mol (-7.3 kcal/mol)

As ATP breaks down releasing the stored energy, ATP becomes ADP and a separate phosphate. ATP = ADP + Phosphate.

Reference

Biochemistry