Question

Why is there a gradual decrease in the ionization energy from He to Li?

• A. Completely filled shell of Li
• B. In Li, electron is added to new shell
• C. Half filled shell of Li
• D. In Li, electron is added to p-orbital

Answer 1

The correct option is B In Li, electron is added to new shell

The electronic configuration of $He$ is $1s^2$. The electronic configuration of $Li$ is $1s^{2}2s^1$. As one more electron is added to a new shell in case of Li, it can be removed easily. Hence, there is a gradual decrease in the ionization energy from $He$ to $Li$.