Explain why fluorine is more reactive than chlorine.

There are three things to consider every single time relative reactivity is unknown: atomic radius, shielding, and number of electrons.

The reactivity is the halogens ability to gain an electron, so number of electrons already in the atom plays a vital role.

Chlorine has more electrons so repels a reacting electron with greater force than fluorine, making it less likely to react.

Fluorine also has fewer electron shells than chlorine, so there are fewer electrons between the positive nucleus and the reacting electron to essentially block, or weaken, the electromagnetic attraction. This is shielding.

Lastly, fluorine is a much smaller molecule than chlorine and the shorter distance, or radius, between the nucleus and the electron again makes it more likely to attract the electron and react to gain a noble gas configuration.