We obtain the energy we need from the food we eat, which contains chemical energy. Chemical energy is a form of potential energy held in the chemical bonds between pairs of atoms in a molecule. Recall from Chapter 2 that a covalent bond results from the sharing of electrons between two atoms. Covalent bonds form when the sharing of electrons between two atoms results in a more stable configuration than if the orbitals of the two atoms did not overlap.
The more stable configuration will always be the one with lower potential energy. As a result, energy is required to break a covalent bond because going from a lower energy state to a higher one requires an input of energy. Conversely, energy is released when a covalent bond forms.
Some bonds are stronger than other ones. A strong bond is hard to break because the arrangement of orbitals in these molecules is much more stable than the two atoms would be on their own. As a result, strong bonds do not contain very much chemical energy, similar to the potential energy of a ball at the bottom of a flight of stairs (Fig. 6.3). This may at first seem counterintuitive, but makes sense when you consider that strong bonds have a very stable arrangement of orbitals and therefore do not require a lot of energy to remain intact. Examples of molecules with strong covalent bonds that contain relatively little chemical energy are carbon dioxide (CO2) and water (H2O).
Conversely, some covalent bonds are relatively weak. These bonds are easily broken because the arrangement of orbitals in these molecules is only somewhat more stable than if the two atoms did not share any electrons. As a result, weak covalent bonds require a lot of energy to stay intact and contain a lot of chemical energy, similar to the potential energy of a ball at the top of a flight of stairs (Fig. 6.3). Organic molecules such as carbohydrates, lipids, and proteins contain relatively weak covalent bonds, including many carbon–
119