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A unimolecular step is a reaction step in a chemical mechanism that involves only one reactant molecule. A single molecule undergoes a chemical change, such as the breaking of a bond or a change in its structure, which may either lead to the formation of products or proceed to further reaction steps.

A bimolecular step is a reaction step in a chemical mechanism where two reactant molecules collide and interact with each other, leading to a transfer of energy or electrons and the formation of products or the start of additional reaction steps.

Termolecular steps are reaction steps in a chemical mechanism that involve three reactant molecules colliding and reacting with each other simultaneously. Termolecular reactions would require a specific collision with the proper orientation and energy of all three participating molecules, making them relatively rare compared to unimolecular and bimolecular steps.

There are a few reasons why termolecular steps are infrequent in chemical reactions: 1. Low Probability of Simultaneous Collision: For a termolecular step to occur, three molecules must collide with each other simultaneously. The probability of this happening is significantly lower than that of simultaneous collisions between two molecules (bimolecular) or of a single molecule undergoing a structural change (unimolecular). 2. High Activation Energy Requirement: In a termolecular reaction, three molecules must have the proper orientation and energy upon collision for a reaction to occur. This often leads to a higher activation energy requirement compared to unimolecular or bimolecular reactions, making the reaction less favorable and less likely to occur. 3. Statistical Improbability: Given the lower probability of termolecular collisions and the higher activation energy barrier that must be overcome, termolecular reactions are statistically much less probable than unimolecular and bimolecular reactions. Overall, these factors contribute to the infrequency of termolecular steps in chemical reactions.