Why is fluorine so reactive?

The tendency of these species to be reduced is explained by the fact that both oxygen and fluorine occur to the RIGHT of the Periodic Table as we face it, and thus have HIGH effective nuclear charge. In fact, elemental fluorine is the most powerful oxidizing agent on the Periodic Table. Generally, non-metals are oxidizing agents which ACCEPT electrons and are thereby reduced.

Conversely, difluorine and dioxygen regularly undergo reduction to give anions; metals, on the other hand, are typically electron-rich materials that undergo oxidation and are therefore REDUCING AGENTS.

Please take note that I am assuming you are a first-year chemistry student, or an advanced A-level student.

Fluorine is highly reactive due to its electronic structure. It has seven valence electrons, requiring only one additional electron to achieve a stable octet configuration. This strong tendency to gain an electron makes fluorine highly reactive, as it readily forms bonds with other elements to acquire the missing electron and achieve a stable electron configuration. Additionally, fluorine's small atomic size and high electronegativity contribute to its reactivity, as it can attract electrons from other atoms with greater force than most other elements.