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Chemical bonds are the forces that hold atoms together, forming molecules. Chemical bonds may be divided into two groups: 1) covalent (strong) bonds in which atoms share electrons, and; 2) a variety of weak bonds, including electrostatic attractions (ionic bonds, hydrogen bonds), hydrophobic interactions, and Van der Waals forces.

The strength of a chemical bond is expressed in -kcal/mole, the energy released upon bond formation. Consider bond formation between two atoms: A + B --> A:B. The concentrations of the unbonded reactants and the bonded products at equilibrium defines K(eq): K(eq)=[A:B]/[A][B]. From deltaG=-RTlnK(eq), we see that the greater the free energy released (in negative kcal/mole) upon bond formation, the stronger the chemical bond {i.e the greater K(eq) is}.  The energy of bond formation (-deltaG) equals the amount of energy required to break a given bond. A table that lists the bond strengths (energies) of various chemical bonds follows:
 
 

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