Question

What is responsible for the sharp decrease in ionization energy between the elements with 2 and 3 atomic number? 10 and 11?

Answer 1

An extra shell, which means further distance between those electrons in the valence shell and the nucleus. Ionisation energy refers to the energy required to remove valence electrons. There will be more energy required if the valence electrons are closer to the nucleus that have a positive charge, because of more attraction. If there is an extra shell, there is further distance, less attraction and hence less energy required.