Isoelectronic Configurations
We already know that elements with similar electronic configurations tend to have similar chemical and physical properties. It is possible, in fact common, for elemental ions to have not only similar but exactly the same electronic configurations as other elements or ions. When two elements and/or ions have the same electronic configurations it is said that they are "isoelectronic" with one another. When two chemical species are isolectronic they again tend to have similar chemical properties.
Examples of Isoelectronic Elements and/or Ions
Element or ion pair Electronic congifuration
Li+, He  1s2 
Be+2, He 1s2 
F-, Ne  1s2 2s2 2p6 
S-2, Ar  1s2 2s2 2s6 3s2 3p6 
Mg+2, Na+   1s2 2s2 2p6
Ca+, K  [Ne]4s1 
P-3, S-2 1s2 2s2 2p6 3s2 3p6 

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C101 Class Notes
Prof. N. De Leon