Types of Elementary Reaction Studied

Typical reactions that have been studied include 

These are classified as a heavy atom approaching a molecule, or as a light atom approaching a molecule. Light and heavy are relative terms, and depend on the reaction in which the atoms are taking part, e.g. in the following two reactions Na can be light or heavy. 

In the first reaction Na is light with respect to I, but in the second is heavy with respect to H.  

Predictions depend on whether a heavy or a light atom is attacking a molecule, and on whether the overall reaction is exo- or endothermic, or thermoneutral. 

Question. In the above examples which of the attacking atoms are heavy, and  

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