For the second period elements the correct increasing order of first ionisation enthalpy is:

Solution & Explanation

\textbf{Answer:} (B) $\text{Li}<\text{B}<\text{Be}<\text{C}<\text{O}<\text{N}<\text{F}<\text{Ne}$. \textbf{Solution:} First ionisation enthalpy generally increases across a period, but there are two anomalies: $\text{Be}>\text{B}$ : the $2p^1$ electron of boron is easier to remove than a $2s$ electron from the filled $2s^2$ of beryllium. $\text{N}>\text{O}$ : nitrogen has a stable half-filled $2p^3$ configuration, so its first ionisation enthalpy is higher than that of oxygen. Including both anomalies, the increasing order is $\text{Li}<\text{B}<\text{Be}<\text{C}<\text{O}<\text{N}<\text{F}<\text{Ne}$.