The correct order of first ionization enthalpy values of the following elements is: (A) O (B) N (C) Be (D) F (E) B

🧠 Period 2 IE has two famous exceptions: Be > B, and N > O The general trend is "IE rises across a period", but two anomalies break it. Be ($2s^2$ filled) is more stable than B ($2p^1$ alone). N ($2p^3$ half-filled) is more stable than O ($2p^4$ paired). You must apply both.

🗺️ Map letters to elements first A = O, B = N, C = Be, D = F, E = B (boron). Translation step is essential — students who skip it always pick a wrong order.

Now build the chemistry order: B (boron, $2p^1$) — easiest to ionize, smallest IE. Be ($2s^2$ filled) — beats B. So Be > B. O ($2p^4$, pairing repulsion) — easier than N. N ($2p^3$ half-filled) — hardest in this set except F. F ($2p^5$, highest Z) — largest IE.

Chemistry order (low → high): B < Be < O < N < F.

Translate back to letters: E < C < A < B < D.

⚠️ The trap Option (b) C < E < A < B < D puts Be (C) below B (E) — that ignores the famous $s^2$ vs $p^1$ anomaly. The "natural left-to-right rise" is wrong here. Always check for $s^2$ filled and $p^3$ half-filled stability before writing any IE order in Period 2 or 3.

$\boxed{\text{Answer: (a)}}$