Consider the following pairs of ions

A) ${\mathrm{SC}}^{+3} \mathrm{and} {\mathrm{Ti}}^{+4}$

B)${\mathrm{Mn}}^{+3} \mathrm{and} {\mathrm{Fe}}^{+4}$

C)$F{e}^{+3} \mathrm{and} {\mathrm{Co}}^{+4}$

D)${\mathrm{Cu}}^{+} \mathrm{and} {\mathrm{Zn}}^{+2}$

Among these pairs of ions, isoelectronic pairs would include

The species or ions which have same no. of electrons are called “isoelectronic”. Let us see below the pair of ions.

Case 1:${\mathrm{SC}}^{+3} \mathrm{and} {\mathrm{Ti}}^{+4}$

Total no. of electrons in ${\mathrm{SC}}^{+3}$ion, 21- 3 =18

Total no. of electrons in ${\mathrm{Ti}}^{+4}$ on, 22 -4=18

Case 2:${\mathrm{Mn}}^{+3} \mathrm{and} {\mathrm{Fe}}^{+4}$

Total no. of electrons in ${\mathrm{Mn}}^{+3}$ion, 25 -2 =23

Total no. of electrons in ${\mathrm{Fe}}^{+4}$ion, 26- 2= 24

Case 3:$F{e}^{+3} \mathrm{and} {\mathrm{Co}}^{+4}$

Total no. of electrons in $F{e}^{+3}$ion, 26 -2 =24

Total no. of electrons in${\mathrm{Co}}^{+4}$ ion, 27 -3 =24

Case 4:${\mathrm{Cu}}^{+} \mathrm{and} {\mathrm{Zn}}^{+2}$

Total no. of electrons in ${\mathrm{Cu}}^{+}$ion, 29 -1 =28

Total no. of electrons in ${\mathrm{Zn}}^{+2}$ion, 30- 2= 28

Hence, the pair of ions in A, C and D are isoelectronic