JEE Chemistry 2023 Question with Solution

Given: The elements are Li, Be, B, C, N, O, F.

Find: The correct increasing order of first ionization enthalpy.

The first ionization enthalpy generally increases from left to right across a period because effective nuclear charge increases. However, there are known exceptions:

1. B has lower first ionization enthalpy than Be because the electron removed from B is from the higher-energy $2p$ orbital, while in Be it is from the more stable $2s$ orbital.
2. O has lower first ionization enthalpy than N because N has a stable half-filled $2p^3$ configuration, whereas O has one paired electron in $2p$, making removal easier.

Thus, the increasing order is

the solution states option C, but its written sequence matches Li < B < C < O < N < F and omits Be. Among the listed options, the most defensible match to the extracted solution sequence is D.

Therefore, the correct option is D.

Given: First ionization enthalpy trend for second-period elements.

Find: The correct order among the given elements.

Across the second period, ionization enthalpy usually increases.

Important exceptions:

• $\text{Be} > \text{B}$ because removing a $2p$ electron from B is easier than removing a $2s$ electron from Be.
• $\text{N} > \text{O}$ because N has extra stability due to half-filled configuration.

So the correct increasing order must place Li lowest, then B, then Be, then C, then O, then N, then F.

Since the options provided are inconsistent and duplicate one entry, the solution conclusion and the option list disagree. The closest supported listed option is D based on the written order in the solution.

Hence, the answer is D.