Isoelectronic species:- Atoms and ions which contains the same number of electrons are called isoelectronic species.

(i) F^- has the electronic configuration,

F        +       e^-       →        F^-

2,7                                  2,8

             (Total of 10 electrons)

So any species having 10 electrons would be isoelectronic with F^-. From the periodic table, the possible ions/atoms are,

Ne(10) 2,8    [∵ Na → Na⁺ + e^-]

Na⁺(10) 2,8    2,8,1      2,8

Therefore, Ne and Na⁺ are isoelectronic with F^-

(ii) Ar (atomic no. = 18) has an electronic configuration (1s² 2s² 2p⁶ 3s² 3p⁶ (18 electrons). So, any atom or ion having 18 electrons with the same distribution as shown by Ar would be isoelectronic with it. from the periodic table, the elements on either side of the Ar are Cl (17) and K (19). These atoms can acquire Ar structure by the reaction

Cl           +        e^-      →         Cl^-

(2,8,7)                                 (2,8,8)

                            (Total of 8 electrons)

and K     →       K⁺    +     e^-

  (2,8,8,1)       (2,8,8)

Therefore, Cl^- and K⁺ are isoelectronic with Ar

(iii) Mg²⁺ has the electronic configuration

Mg        →         Mg²⁺      +      2e^-

(2,8,2)                (2,8)

Thus all the atoms and ions having 2,8 configuration are isoelectronic with Mg²⁺. From the periodic table, the elements on either side of magnesium are Ne(10) and Na(11). Na can acquire Mg²⁺ structure by losing electronic as:

Na              →             Na⁺     +      e^-

(2,8,1)                       (2,8)

Al             →          Al³⁺      +       3e^-

(2,8,3)                  (2,8)

Therefore, Ne and Na⁺ and Al³⁺ are isoelectronic with Mg²⁺

(iv) Rb⁺ is obtained from Rb (atomic no. = 37) by losing one electron

Rb           →           Rb⁺      +      e^-

(2,8,18,8,1)       (2,8,18,8)

Thus, the atoms or ions having 2,8,18,8 configurations are isoelectronic with Rb⁺. From the periodic table, the following species have (2,8,18,8) configuration:

(a) Kr      →      (2,8,18,8)

(b) Br         +       e^-       →       Br^-

(2,8,18,1)                             (2,8,18,8)

(c) Sr        →           Sr²⁺       +       2e^-

(2,8,18,8,2)                    (2,8,18,8)

Thus, Kr, Br^- and Sr²⁺ are isoelectronic with Rb⁺