Chapter 7 - Atomic Structure and Periodicity - Active Learning Questions - Page 341: 5

Lithium

Lithium should have a larger second ionization energy because lithium's oxidation number is 1. On the other hand, beryllium's oxidation number is 2. This matters because the amount of energy needed to remove an electron after the valence shell is full is significantly higher. Thus, when removing the second electron from Lithium we are removing an electron from an inner shell, while from beryllium, it is much lower as it is by itself in the valence shell.