"Electron affinity of lithium is negative but the electron affinity of beryllium is positive"-why?

Explanation

Electronic configuration of `""_(3)Li:1s^(2)2s^(1)`
Electronic coniguration of `""_(4)Be:1s^(2)2s^(2)`
On addition of an electron to Li-atom, 2s-orbital of Li becomes fully filled with electrons and consequently that electronic configuration attains stability. this process is acompanied by liberation of energy on the other hand, Be has a stable electronic configuration with fully filled 2s subshell. when an electron is added to Be-atom it occupies 2p-subshell causing destabilisation of the stable electronic configuration. this process is accompanied by the absorption of heat. naturally electron affinity of Li is negative but the electron affinity of Be is positive.