Final answer:
Beryllium has a higher first ionization energy than lithium because it has a fully filled s sub-shell, which is more stable and requires more energy to remove an electron from. Furthermore, beryllium's nucleus has a +2 charge while lithium's has a +1 charge, making beryllium's hold on its electrons stronger.
Step-by-step explanation:
The first ionization energy (IE₁) of beryllium is higher than that of lithium, despite lithium having fewer protons and therefore a smaller nuclear charge. This can be explained by examining the electronic configurations of both elements. Beryllium has an electronic configuration of [He]2s², meaning its outermost s sub-shell contains two electrons, which completes the sub-shell. In contrast, lithium has an electronic configuration of [He]2s¹, with only one electron in its outermost s sub-shell.
The higher first ionization energy of beryllium is attributed to the fully filled s sub-shell. A filled sub-shell is more stable and requires more energy to remove an electron from it compared to a sub-shell that is not full. Additionally, the electron in lithium is more easily removed due to the +1 charge of lithium's nucleus being less effective at holding onto the single outer electron than beryllium's +2 charge holding onto two electrons in its full s sub-shell. Consequently, the first ionization energy of beryllium is higher.