The chemical property common to all elements in Group 18 (also known as Group VIIIA or the noble gases) is their inertness.

Here's why:

* Full outer electron shells: Noble gases have a complete outermost electron shell, meaning they have a stable electron configuration.

* Low reactivity: This stable configuration makes them extremely unreactive, as they don't readily gain or lose electrons to form chemical bonds.

While some noble gases can form compounds under extreme conditions, they are generally considered the least reactive elements on the periodic table.