Questions

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.

Khan Abdul Ghaffar Khan

Second ozone hole was detected over Tibet in the year 2006. 

Ozone is a form of oxygen, acts as a high-altitude atmospheric screen against cancer-causing ultraviolet light.

The main source of these halogens in the stratosphere is photodissociation of man made halocarbon refrigerants, solvents, propellants and CFC's.