Final Answer:
The first ionization energies of helium and beryllium are both higher than the first ionization energy of lithium because helium and beryllium have fully filled inner electron shells, resulting in greater stability and a stronger attraction between the nucleus and outer electrons.
Step-by-step explanation:
Helium, with an atomic number of 2, has a configuration of 1s². It possesses a fully filled 1s subshell, making it exceptionally stable. When an electron is removed from helium, it involves breaking into the stable inner shell, requiring a significant amount of energy. This results in a higher first ionization energy.
Beryllium, on the other hand, has an atomic number of 4, and its electron configuration is 1s² 2s². Like helium, beryllium also has a fully filled inner shell (1s²), leading to increased stability. When removing an electron, the outer electron has to overcome the strong attraction of the nucleus and the repulsion from the inner electrons, resulting in a higher first ionization energy compared to lithium.
In contrast, lithium has an atomic number of 3, and its electron configuration is 1s² 2s¹. The electron being removed is in the less stable outer shell, which experiences less electron-electron repulsion and is farther from the nucleus than the electrons being removed in helium and beryllium. Consequently, lithium exhibits a lower first ionization energy compared to helium and beryllium. Overall, the trend in first ionization energies reflects the increasing stability associated with fully filled inner electron shells.