Question

Which of the following are isoelectronic species?
(i) NH\textsubscript{3 (ii) CH\textsubscript{3}\textsuperscript{+} (iii) NH\textsubscript{2}\textsuperscript{--} (iv) NH\textsubscript{4}\textsuperscript{+}
Choose the correct answer from the codes given below}

• A. (i), (ii), (iii)
• B. (ii), (iii), (iv)
• C. (i), (ii), (iv)
• D. (i), (iii), (iv)
• E. (ii), (iii)

Hint:

Most stable small molecules containing 2nd-row elements (like $N, O, F, Ne$) often aim for 10 total electrons to satisfy the octet rule ($2 + 8 = 10$).

Answer 1

The Correct Option is D

Concept: Isoelectronic species are atoms, molecules, or ions that have the same total number of electrons.
• Electron Counting: Total electrons = $\sum (\text{Atomic Number of atoms}) - (\text{Positive Charge}) + (\text{Negative Charge})$.

Step 1: Calculate total electrons for each species.
• NH\textsubscript{3: $N(7) + 3H(1) = 7 + 3 = \mathbf{10}$ electrons.
• CH\textsubscript{3\textsuperscript{+}}: $C(6) + 3H(1) - 1(\text{charge}) = 6 + 3 - 1 = \mathbf{8}$ electrons.
• NH\textsubscript{2\textsuperscript{--}}: $N(7) + 2H(1) + 1(\text{charge}) = 7 + 2 + 1 = \mathbf{10}$ electrons.
• NH\textsubscript{4\textsuperscript{+}}: $N(7) + 4H(1) - 1(\text{charge}) = 7 + 4 - 1 = \mathbf{10}$ electrons.

Step 2: Identify matches. Species (i), (iii), and (iv) all contain exactly 10 electrons. Therefore, they are isoelectronic with each other. This corresponds to code (D).