Explain the trends in ionization energy across a period and down a group, and how these trends relate to the ionization energies of alkali metals and alkaline earth metals.

Trends in Ionization Energy:

• Across a period: Ionization energy generally increases. For example, Li has $I_1 = 520.2$ kJ/mol, while Be has $I_1 = 899.5$ kJ/mol. This is due to increasing nuclear charge and decreasing atomic radius, which holds valence electrons more tightly.
• Down a group: Ionization energy generally decreases. This occurs because atomic radius increases and inner electrons shield the valence electrons more effectively, making them easier to remove. (The provided table does not contain data to illustrate this trend).

Alkali Metals (e.g., Lithium): Lithium ($1s^2 2s^1$) readily loses its single valence electron ($I_1 = 520.2$ kJ/mol) to form a stable $\text{Li}^+$ ion with a $1s^2$ (noble gas) electron configuration. The second ionization energy ($I_2 = 7298.2$ kJ/mol) is significantly higher (approximately 14 times $I_1$) because it requires removing an electron from a stable, full inner shell. Therefore, forming +2 or +3 ions is energetically unfavorable.

Alkaline Earth Metals (e.g., Beryllium): Beryllium ($1s^2 2s^2$) readily loses both its valence electrons ($I_1 = 899.5$ kJ/mol, $I_2 = 1757.1$ kJ/mol) to form a stable $\text{Be}^{2+}$ ion with a $1s^2$ (noble gas) electron configuration. The third ionization energy ($I_3 = 14848.8$ kJ/mol) is significantly higher (approximately 8.5 times $I_2$) because it requires removing an electron from a stable, full inner shell. Therefore, forming +3 or +4 ions is energetically unfavorable.