3. Electron concentration (e/a Electron concentration(e/a)is the number of valence electrons per atom on the average i.g. for Cuzn: e/a=3/2=1.53. Electron concentration ( e/a ) Electron concentration ( e/a ) is the number of valence electrons per atom on the average. i.g. for CuZn : e/a = 3/2 =1.5