Which element has not reacted with fluorine?

Fluorine is one of the most reactive elements and belongs to the group of halogens. It typically reacts with many other elements, particularly those that are less stable or have lower ionization energies. However, noble gases like krypton, neon, argon, and xenon are generally known for their lack of reactivity due to having a complete valence shell of electrons.

Among the noble gases, argon is particularly well-known for its inertness and does not form stable compounds with fluorine under normal conditions. While xenon can form compounds with fluorine because it has a larger atomic size and can expand its valence shell to accommodate additional electrons, both krypton and neon have been shown to have very limited reactivity, with krypton able to react under specific extreme conditions. However, argon remains the least reactive of the noble gases when it comes to compounds with fluorine, leading to the conclusion that it has not reacted with fluorine in practical chemistry contexts.

This highlights why argon is identified as the correct answer within this context of non-reactivity with fluorine.