Bond energy is the energy required to break a bond homolytically. These values are usually expressed in units of kcal/mol or kJ/mol and are referred to as bond dissociation energies when given for specific bonds or average bond energies when indicated for a given type of bond over many compounds. Firstly, the bond dissociation energy for a single bond is weaker than that of a double bond, which in turn is weaker than that of a triple bond. Secondly, hydrogen forms relatively strong bonds with carbon, nitrogen, and oxygen. Finally, with the exception of carbon and hydrogen, single bonds between atoms of the same element are relatively weak. Reactions between organic compounds involve the making and breaking of bonds. Hence, the strengths of bonds and their resistance to breaking are essential concepts in organic chemistry. Reactions in which bonds are broken pass through a high-energy transition state before transforming into products. In order to reach this transition state, the reactant molecules must be oriented in a suitable direction and must be supplied with certain threshold energy. The activation energy, ΔG‡, is the energy provided to the reactants to raise them to the transition state. Overall, for a reaction to occur, the reacting molecules must collide or otherwise interact. The necessary activation energy for the reactant-transition-state jump is provided by the kinetic energy of the colliding particles. Lastly, the colliding molecules must collide in a specific orientation so as to maximize the impact of the collision.