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As we know that fully filled electronic configuration is more stable than half filled electronic configuration, so in my opinion beryllium's electron affinity should be less than nitrogen's.
Is my logic correct and if so why does beryllium demonstrate such an exception?
This is not true. This page reveals that both electron affinities are negative, meaning the anions are unstable and, if forced to form, they will spontaneously spring the extra electron off. Furthermore, the electron affinity is more negative for beryllium (-0.52 eV) which must accept the electron into a higher subshell, than for nitrogen (-0.0007 eV) which merely must disturb an already half-filled subshell.
