Answer: The bonds that are made and broken in a reaction release energy when forming products that contain less total energy than the reactants.
Explanation: Elements form bonds with each other when the total energy of the resulting products is less than the starting energy. The term "energy" refers to the energy of the electrons that are being held in an atom by the nucleus. Keeping the positively charged nucleus happy means that the electrons are trying to get closer, but they repel each other as they get closer to the nucleus. This results in orbits, preferred areas around the nucleus where the electrons spend their day in a delicate balance with the nucleus and other electrons. These orbits offer housing for the electrons that involve discreet energies for the electrons. When elements react, the resulting bonding pattern often results in an overall reduction in total energy. The energy reduction results in heat, light, and/or sound. The release is termed "exothermic."