O (Z=8 , A=16)
=> 8p+ , 8e- si 16-8 = 8 n0
1s2 2s2 2p4 => gr. 6 (VI-A) , per. 2
S (Z=16 , A= 32)
=> 16p+ , 16e- si 32-16 = 16n0
1s2 2s2 2p6 3s2 3p4 => gr. 16 (VI-A) , per. 3
Ar (Z=18 , A= 40)
18p+ , 18e- si 40-18 = 22n0
1s2 2s2 2p6 3s2 3p6 => gr. 18 (VIII-A) , per. 3