First slide
Periodicity in atomic properties

With in each pair of elements of F and Cl.S and Se, and Li  and Na. respectively. the elements that release more energy upon an electron gain are 


(i) Electron affinity of second period p-block elements is less than third period p-block element due to small size of second period p-block element. 

   E.A. order : F<Cl

(ii) Down the group electron affinity decreases due to size increases

    E.A. order : S>Se


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