Br,grupa 17=> 7 electroni pe ultimul strat,perioada 4=> 4 straturi ocupate cu electroni
Br:1s22s2sp63s23p64s23d104p5 deci Z=35
-un atom de Br accepta un electron
1s22s22p63s23p64s23d104p5 + 1 electron ->1s22s22p63s23p64s23d104p6 Br+ + 1e
Z gaz nobil=36 (Kr)