Two or more ions are said to be isoelectronic if they have the same electronic structure and the same number of valence electrons.
Na+^{+}+ = 10 electrons = 2, 8
Mg2+^{2+}2+ = 10 electrons = 2,8
O2−^{2-}2− = 10 electrons = 2,8
Si2+^{2+}2+ = 12 electrons = 2,8,2
⟹ \implies⟹ Si2+^{2+}2+ is not isoelectronic with the rest.
