Chemical reactions breaking and making covalent bonds

Chemical reactions breaking and making covalent bonds 

More commonly, the excited electrons may adopt a different stable configuration, by interacting with electrons associated with different atoms and molecules. The result is the formation of new covalent bonds, and hence the formation of new compounds. In this case, there are three possibilities (as shown in Eigure 2.1)  

There may be an output of energy equal to the activation energy of the reaction, so that the energy level of the products is the same as that of the starting materials. Such a reaction is energetically neutral (thermoneutral). 

In general, reactions in which relatively large complex molecules are broken down to smaller molecules are exothermic, whereas reactions that involve the synthesis of larger molecules from smaller ones are endothermic. 

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