Variation of electron affinity in a periodic table

• 
  It is the amount of energy, that is released when an electron is added in the valence shell of isolated gaseous atom. 
•  Exothermic process
• Units:--KCal/mole, KJoule/mole, eV/ atom
• A(g) + 1e^- :------ A^-(g) + EA¹(Exothermic) 
• A(g)  + 1e^-  - EA:----A^-(g) 

Successive EA or step wise EA

A(g) + 1e^-  :----A^-¹  + EA¹(Exothermic) 

A-(g)  + 1e^- :-----A^-²(g) - EA²(endothermic) 

Overall reaction 

A(g) + 2e^- :------A-² ( endothermic) 

We take another example:---

O + 1e^- :------O^-  +  EA¹(Exothermic) 

O^- + 1e^- :------O^-²   - EA¹(endothermic) 

Factors affecting :---

1. Size of atom :--   size of atom is indirectly proportional to the EA

When size of atom increases ,the attraction  of nucleus on electrons decreases and penetration power increases. So the attraction  of nucleus decreases  on outermost shell so we can easily add another electrons  to outermost shell. 

2.Zeffect:---Zeffect is directly proportional to the EA.

Attraction  of nucleus increases on outermost shell it increases electron affinity. 

3.screening constant or penetration power:---inner shell penetrate the attraction of nucleus on outermost shell.  If penetration power increases electron affinity decreases. 

4.half filled and fullfilled electronic configuration :---- such type of elements have zero electron affinity. 

 5.periodic Variation :--- moving acroos the period electron affinity  increases. 

Moving down the group electron affinity decreases. 

• Note:--- electron affinity of third period element is higher than second period element. 
• P>N
• S>P
• Cl>F
• Cl>F>S>O>P>N
• Second period elements have higher EA due to the availability of the vacant d orbital. Another factor is :-- group second elements already have a full valence s shell and the addition of a new electrons would move to the higher energy p shell that is already left empty in the neutral atom. That's why EA is so positive for the 2nd group elements. 
• Halogens generally have the highest EA

                      EA(I) + EA(II)  < 0

Trend of electron affinity across the period:--

Ne<N<Be<B<Li<C<O<F

Variation of EA moving down the group:---

Cl>F>Br>I = ( 17 group) 

Si>C>Ge>Sn>Pb = (14 group) 

P>N>As>Sb>Bi. = (15 group) 

S>Se>Te>Po>O. = (16 group)  

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