Which two elements have the most contrasting ionization energy?

The elements that exhibit the most contrasting ionization energy are helium and francium. Ionization energy refers to the energy required to remove an electron from an atom in its gaseous state.

Helium is located at the top of Group 18 in the periodic table, characterized by a complete valence shell and a small atomic radius. As a result, it has a very high ionization energy because its electrons are tightly held by the nucleus and removing one requires a significant amount of energy.

On the other hand, francium is positioned at the bottom of Group 1. It has a larger atomic radius and fewer protons in the nucleus relative to its electron shell compared to helium. This makes its outermost electron much easier to remove, resulting in a very low ionization energy.

The stark difference in ionization energy between helium and francium highlights the trends within the periodic table, where ionization energy generally increases across a period and decreases down a group. This understanding of their locations within the periodic table clarifies why these two elements display the most contrasting ionization energies.