The type of reaction that breaks the bonds that join phosphate groups in an ATP molecule is called hydrolysis.

Here's why:

* Hydrolysis means "breaking with water."

* In the case of ATP, a water molecule (H₂O) is added to the bond between a phosphate group and the rest of the ATP molecule.

* This breaks the bond, releasing a phosphate group (Pi) and leaving behind ADP (adenosine diphosphate).

* The energy stored in the phosphate bond is released as heat, making this reaction exergonic (releases energy).

Here's the simplified reaction:

ATP + H₂O → ADP + Pi + energy

Let me know if you'd like more detail on this process!