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                              Q:
                           
                           
                                                      | A) the existence of unpaired electrons in the valence shell | B) the energy differences between the electron shells | 
| C) the sum of the potential energies of all the electron shells | D) the average distance of the outermost electron shell from the nucleus | 
                              Answer:   A) the existence of unpaired electrons in the valence shell
                              
                                                            
                              
Explanation:
                               
                           
                           Explanation:
When atoms react they do so in order to reach the lowest energy, which is the most stable. The electrons orbitals will overlap in the way that reduces the energy the most. Orbitals of similar energy will produce the greatest stabilisation.The answer you gave isn't the whole story, it depends what species is reacting with what. The effective nuclear charge on the atom will affect the energy of the valance electrons.
 
Hence, the reactivity of an atom arises from the existence of unpaired electrons in the valence shell.
 
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