Electron affinity of sodium is negativ but magnesium has positive value-why?

Explanation

Electronic configuration of `""_(11)Na:1s^(2)2s^(2)2p^(6)3s^(1)`
Electronic configuration of `""_(12)Mg:1s^(2)2s^(2)2p^(6)3s^(2)`
Addition of one electron to the 3s-orbital of Na leads to a comparatively stable electronic configuration with fulfilled orbital. So, this process of addition of electron to Na is an exothermic process. so, the electron affinity of Na is negative stable electronic configuration so addition of an electron to the 3p-orbital destablises thhe electronic configuration of Mg. additional energy is required for addition of electron to outermost shell of Mg i.e., this process is endothermic and thus the valueof electron affinity of Mg is positive.