Which of the following is isoelectronic pair?

1. ${\mathrm{ICl}}_{2}$ , ${\mathrm{ClO}}_{2}$

2. ${\mathrm{BrO}}_{2}^{-}$ , ${\mathrm{BrF}}_{2}^{+}$

3. ${\mathrm{ClO}}_{2}$ , $\mathrm{BrF}$

4. ${\mathrm{CN}}^{-}$ , ${\mathrm{O}}_{3}$

Hint: Isoelectronic species have the same number of electrons.

Isoelectronic pairs have the same number of electrons
 ${\mathbf{BrO}}_{\mathbf{2}}^{\mathbf{-}}$ ${\mathbf{BrF}}_{\mathbf{2}}^{\mathbf{+}}$ Total number of electrons 35 + 2 $×$ 8 + 1 = 52 35 + 9 $×$ 2 – 1 = 52

Hence,(b) is the correct choice, while in another case this value is not equal.

 ICl2 ClO2 53 + 2 x 17 = 87 17 + 16 = 33

 ClO2 BrF 17 + 16 = 33 35+ 9=44

 ${\mathbf{CN}}^{\mathbf{-}}$ O3 6 + 7 + 1 = 14 8 x 3 = 24

Hence, only (b) is the correct choice