The simplest equation for reforming ATP from ADP is ADP+Pi+free energy→ATP+H2O.
ADP (adenosine diphosphate) combines with a phosphate group to form ATP (adenosine triphosphate). It’s the addition of a phosphate group that differentiates the two molecules.
The reaction is reversible. ATP gets converted back to ADP by breaking off one of its phosphate groups during hydrolyses. The equation for forming ADP from ATP is ATP+H2O→ADP+Pi+ free energy.