Another principle of the collision theory is related to
the behaviour of the reactant particles as they collide. According to collision theory, when particles collide at the proper angle with the proper amount of energy, they form particles with structures unlike the structures of either the reactants or products. These intermediate particles are unstable and thus exist for very short periods of time. It is during this time the atoms rearrange themselves by producing a grouping of molecules considered to have bonds that are simultaneously beings formed and being broken. Each intermediate particle is called an activated complex.
Figure 1 o illustrate the formation of an activated complex, consider the reaction between a molecule of hydrogen (H2) and a molecule of iodine (I2) to produce HI (Figure 1). For a reaction to occur, H2 and I2 must collide. If they approach from the wrong angle or have too little kinetic energy, they will rebound from each other without forming the product HI. If, however, the angle of approach is right and the particles have sufficient energy, an activated complex containing all for atoms (H2I2) is temporarily formed.
Shortly after the activated complex is formed, it may
break apart to re-form the original molecules of H2 and I2 or to form the product HI. The minimum amount of energy needed to form the activated complex is called the activation energy.