Arrange the following elements in increasing order of first ionization enthalpy: $\mathrm{Li}$, $\mathrm{Be}$, $\mathrm{B}$, $\mathrm{C}$, $\mathrm{N}$.

Solution & Explanation

\textbf{Answer:} (A) $\text{Li}<\text{B}<\text{Be}<\text{C}<\text{N}$ \textbf{Solution:} First ionization enthalpy generally increases from left to right across period 2 as $Z_{\text{eff}}$ increases, but there is an anomaly at boron. $\mathrm{Be}$ has a fully filled, stable $2s^2$ configuration, so it is harder to ionize than $\mathrm{B}$, whose single $2p^1$ electron is removed more easily. Thus $\text{B}<\text{Be}$. The correct increasing order is $\text{Li}<\text{B}<\text{Be}<\text{C}<\text{N}$.