Final answer:
The phosphate group is the part of an ATP molecule that breaks free when it is used to release energy, specifically the bond between the beta and gamma phosphates during hydrolysis.
Step-by-step explanation:
When an ATP molecule is used for energy, the part that breaks free from the rest is the phosphate group. More specifically, the bond between the β (beta) and γ (gamma) phosphate groups is broken, and this reaction is known as hydrolysis. It is this bond breaking that releases the energy required to power numerous cellular reactions and processes, resulting in the creation of adenosine diphosphate (ADP) and an inorganic phosphate group (Pi).