Question

Which of the following is not isoelectronic?

• A. CO
• B. CN$^-$
• C. NO$^+$
• D. O$_2^{2-}$

Hint:

To check for isoelectronic species, sum the atomic numbers and adjust for ionic charges to get the total number of electrons.

Answer 1

The Correct Option is D

Isoelectronic species have the same number of electrons. Let's count the total electrons in each:

• CO: C (6) + O (8) = 14 electrons
• CN$^-$: C (6) + N (7) + 1 (from charge) = 14 electrons
• NO$^+$: N (7) + O (8) − 1 (due to positive charge) = 14 electrons
• O$_2^{2-}$: O (8 × 2) + 2 (from charge) = 18 electrons

So, only O$_2^{2-}$ is not isoelectronic with the others.