Q.With what neutral molecule is ClO- isoelectronic? Is that molecule a Lewis base?

Step 1: Count the electrons in $ClO^-$.

Cl (Z = 17) + O (Z = 8) + 1 extra electron (for the negative charge) = 26 total electrons, over 2 atoms.

Valence electrons: Cl (7) + O (6) + 1 (charge) = 14 valence electrons.

Step 2: Find a neutral diatomic molecule matching this electron count.

Chlorine monofluoride, ClF: Cl (17) + F (9) = 26 total electrons, over 2 atoms.

Valence electrons: Cl (7) + F (7) = 14 valence electrons.

Both the total-electron count (26) and the valence-electron count (14) over 2 atoms match exactly — so $ClO^-$ and $ClF$ are isoelectronic.

Step 3: Is ClF a Lewis base? …

Count total/valence electrons of ClO⁻ (26 total, 14 valence, over 2 atoms) and find a neutral diatomic molecule with the same count — ClF — confirming the isoelectronic relatio …

• 'Isoelectronic' requires matching BOTH the number of atoms and the number of electrons (total, or equivalently valence) — simply matching one criterion is not sufficient.
• Do not forget to add the extra electron for the negative charge on $ClO^-$ when counting electrons — omitting it gives the wrong total (25 instead of 26) and an incorrect isoelectronic match. …