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

Solution

1. The electronic configuration of $\text{Li}$ is $1s^{2}2s^1$, $\text{Be}$ is $1s^{2}2s^2$, $\text{B}$ is $1s^{2}2s^{2}2p^1$, and $\text{C}$ is $1s^{2}2s^{2}2p^2$.
2. Due to the fully filled $2s$ subshell, $\text{Be}$ has a higher first ionization energy than $\text{B}$.
3. The general trend of increasing ionization energy across a period is $\text{Li}<\text{B}<\text{Be}<\text{C}$.
4. Hence the answer is (A).