Explain how ATP going to ADP is an exothermic reaction
ATP is composed of adenosine and three phosphate groups. Each phosphate group has two negative charges. When phosphate groups are very close to each other, a lot of electrical repulsion occurs because of the close proximity of all those electrons and negative charges. Electrical repulsion occurs because electrons do not want to be close to each other.
Remember, like charges repel and opposite charges attract. Electrons want to be closer to positive charges. The abundance of electron repulsion increases the potential energy of the molecule, and when the potential energy is high, the molecule is less favored. So when ATP changes to ADP one phosphate group goes away.
This means that there are less negatively charged phosphate groups next to each other, less electrical repulsion, and less potential energy. Its exothermic because the decrease in potential energy its energetically favored.