The bond energy (or bond association energy) is the energy required to break one mole of the bond in the gas phase. Breaking a bond is always an endothermic process, which requires energy. We can represent the process of breaking the bond as shown. For example, the energy required to break a single bond between the two chlorine atoms in Cl2 gas is 243 kilojoules per mole. To break a single bond between hydrogen and chlorine, it requires 431 kilojoules per mole. The exact energy of a bond depends not just on the two atoms involved in the bond, but also the rest of the molecule. For example, to break a carbon-hydrogen single bond in CHF3 requires 446 kilojoules per mole, but to break a carbon-hydrogen single bond in CHCl3 requires 401 kilojoules per mole. The molecular context of the bond affects the bond energy. However, we can determine an average bond energy for a bond between two atoms across many molecules. Acknowledging that the actual bond energy may be slightly different depending on the molecular context of the bond, the average bond energy is nonetheless useful for comparing types of bonds. From this table of average bond energies, we can see that as we share more and more electrons between two atoms, moving from a single to a double to a triple bond, the bond energy increases. We see this pattern when comparing carbon-carbon single, double, and triple bonds or nitrogen-nitrogen single, double, or triple bonds. Bonds not only get stronger as more electrons are shared between two atoms, but they also get shorter. Bond length is the distance between the two nuclei in a bond. From comparing carbon-carbon bonds, we can see that between two given atoms, stronger bonds are shorter bonds. Let's apply these concepts in this practice problem. Our problem reads, "Put the following in order of increasing carbon to carbon bond length: C2H4, C2H6, C2H2, and C6H6." We'll start by drawing the Lewis structure for each of these molecules. C2H4 has a double bond between the two carbon atoms, C2H6 has a single bond between the two carbon atoms, C2H2 has a triple bond between our two carbon atoms, and C6H6 is benzene, which exists as a pair of resonance structures with alternating single and double carbon-to-carbon bonds. This means the actual structure of benzene has six identical carbon-carbon bonds somewhere between a single and double bond. If we put these in order of increasing bond lengths, our shortest bond is also our strongest bond. The triple bond in C2H2 should be our shortest and strongest bond. Next should be our double bond in C2H4. Then in C6H6, our carbon-to-carbon bonds are somewhere between a double bond and a single bond, so it goes next. And lastly, our single bond in C2H6 is the longest.