When a bond is formed, what happens to the energy?

When atoms combine to make a compound, energy is always given off, and the compound has a lower overall energy. When a chemical reaction occurs, molecular bonds are broken and other bonds are formed to make different molecules. For example, the bonds of two water molecules are broken to form hydrogen and oxygen.

One of the biggest challenges for chemistry students is understanding whether energy is required or released when chemical bonds are broken and formed. One reason this can be confusing is that a complete chemical reaction can go either way.

Exothermic reactions release energy in the form of heat, so the sum of the energy released exceeds the amount required. Endothermic reactions absorb energy, so the sum of the energy required exceeds the amount that is released. In all types of chemical reactions, bonds are broken and reassembled to form new products. However, in exothermic, endothermic, and all chemical reactions, it takes energy to break the existing chemical bonds and energy is released when the new bonds form.