Here's why:

* Electronegativity: Fluorine has the highest electronegativity of all elements, meaning it has a strong attraction for electrons. This makes it highly reactive, eager to gain an electron to complete its octet. Chlorine, while not as electronegative as fluorine, is still very reactive.

* Electron affinity: Fluorine also has the highest electron affinity, meaning it releases the most energy when it gains an electron. This further contributes to its high reactivity.

* Smaller size: Fluorine and chlorine are relatively small atoms, which means their outer electrons are closer to the nucleus and experience stronger electrostatic attraction. This makes them more likely to react.

Reactivity trend within halogens:

The reactivity of halogens decreases as you move down the group:

* F > Cl > Br > I > At

This is because the larger atoms have their outer electrons further from the nucleus, making them less attracted to additional electrons.

Let me know if you have any other questions!