When moving across the periodic table from Li to Be to B, the first ionization energy increases from Li to Be, then drops for B. The first ionization energy of B is greater than that of Li. Explain why.

• When moving across a period on the periodic table, the value of the effective nuclear charge increases with atomic number. This causes a general increase from Li to Be to B. 
• The even higher value of Be (greater than B) is due to the increased stability of the electron configuration of Be. 
• Beryllium has a filled s-subshell. Filled subshells have an increased stability, and additional energy is required to pull an electron away.