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For a chemical reaction to occur, the reactant molecules must collide with enough energy. The minimum kinetic energy required for a reaction to occur is called the activation energy (E_A).

This example shows the stages of reaction between hydrogen and bromine.

Reactant molecules collide

As the reactant molecules collide they must have enough energy to overcome the repulsive forces (caused by outer electrons) and start to break the bonds between the atoms.

Activated complex

An intermediate stage is reached in which a high energy, unstable arrangement of atoms is formed called the activated complex.

Product molecule forms

Energy is given out as new bonds are formed and the atoms are rearranged into the product molecule(s).

Collision geometry

For a successful collision to take place, the collision geometry must be right (the reactant molecules have to be facing the right way!) so that the activated complex can be formed. Looking at the reaction between hydrogen and bromine: