Which of the following sets of elements represents the correct order of increasing first ionization enthalpy?

Solution

1. The general trend for first ionization enthalpy across a period is an increase from left to right.
2. However, $\text{Be}$ ($1s^{2}2s^2$) has a higher ionization enthalpy than $\text{B}$ ($1s^{2}2s^{2}2p^1$) due to the stability of the fully filled $2s$ subshell.
3. The correct order is $\text{Li}<\text{B}<\text{Be}<\text{C}$.
4. Hence the answer is (A).