To
tear apart one mole of H2
molecules into two moles of H atoms requires 103.24
kcal of energy; thus we say that the bond energy of the H-H bond is
103.24 kcal/mole. We can represent H2
molecules and H atoms on an energy-level diagram as shown on the right
hand side of this page, where the vertical direction symbolizes increasing
energy (and less stability). When we tear H2
molecules apart, we store energy in the atoms in the same way that
we store potential energy in a boulder when we roll it uphill. This
energy is released when the atoms form a bond, or when the boulder
rolls downhill.