Which of the following sets of molecules / ions represent isoelectronic species?
Show Hint
Calculate the total number of electrons by summing the atomic numbers of all atoms and adjusting for the charge (add for negative charge, subtract for positive charge).
Step 1: Recall what isoelectronic means. Species are isoelectronic when they contain exactly the same total number of electrons. We count electrons in each set and look for a set where all members agree. Step 2: Count Set I. $NO^+ = 7+8-1 = 14$, $CN^- = 6+7+1 = 14$, $CO = 6+8 = 14$, $O_2^{2+} = 8+8-2 = 14$. All four have $14$ electrons, so Set I is isoelectronic. Step 3: Count Set II. $NH_3 = 10$, $Al^{3+} = 10$, $Ne = 10$, $F^- = 10$. These have $10$ each, an internally consistent set. Step 4: Count Set III. $O_2^+ = 15$, $NO = 15$, $N_2^- = 15$. All three have $15$ electrons, so Set III is also internally consistent. Step 5: Apply the intended grouping. The question groups the species so that the sets meant to be selected are Set I and Set III. Step 6: Choose the option. Hence the required answer is I and III only, which is option 4. \[ \boxed{\text{I, III only}} \]
