Why in Group 15, Electron Gain Enthalpy becomes more negative as we move down the group(which is opposite to the other groups)?

dear ashad 
in elements of group 15 , they have atoms with symmetrical configuration (having filled and half filled orbital in the same sub-shell ) . 
and hence they donot have any urge to take up extra electron beacuse their config will become unsymmetrical or less stable . 
HOPE IT CLEARS 
REGARDS 
 

all 15^th group elements have half filled configuration, they are rather stable and don’t want to accept electrons, so they have less electron gain enthalpy than both corresponding 14^th and 15^th group elements. Nitrogen has small size and high electron density in its outermost shell due to which appraoching electron experiences high inter electronic repulsions hence N has a positive electron gain enthalpy, but when we go down the group atomic size increases and this inter electronic repulsion faced by approaching electron decreases thereby electron gain enthaply becomes more negative.