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Isoelectronic species are :
A
CO, $CN^-$, $NO^+$, $C_2^{2-}$
B
$CO^-$, CN, NO, $C_2^-$
C
$CO^+$, $CN^+$, $NO^-$, $C_2$
D
CO, CN, NO, $C_2$
Detailed Solution
Count the total electrons:
CO: 6 + 8 = 14; $CN^-$: 6 + 7 + 1 = 14; $NO^+$: 7 + 8 – 1 = 14; $C_2^{2-}$: 6 + 6 + 2 = 14.
All four have 14 electrons (like $N_2$), so they are isoelectronic.
In the other sets, the species have different numbers of electrons (e.g. CO 14, CN 13, NO 15, $C_2$ 12).
So CO, $CN^-$, $NO^+$ and $C_2^{2-}$ are isoelectronic.
CO: 6 + 8 = 14; $CN^-$: 6 + 7 + 1 = 14; $NO^+$: 7 + 8 – 1 = 14; $C_2^{2-}$: 6 + 6 + 2 = 14.
All four have 14 electrons (like $N_2$), so they are isoelectronic.
In the other sets, the species have different numbers of electrons (e.g. CO 14, CN 13, NO 15, $C_2$ 12).
So CO, $CN^-$, $NO^+$ and $C_2^{2-}$ are isoelectronic.