Question:  Why are the halogens so reactive?

Answer:

You can think of it like this:

The halogens have seven valence electrons, while the noble gases have eight valence electrons.  The octet
rule says basically that all elements want to have the same number of valence electrons as the nearest noble
gas.  They can do this either by gaining or losing electrons (or in the case of covalent compounds, sharing electrons).  Why do they want to be like the nearest noble gas?  Because the filled octet that makes the noble gas electron configuration is very stable.

Elements like sodium, which is on the far left of the periodic table are very reactive because they can lose only one electron to be like a noble gas or they can gain seven - since there's not much question about which it will do, it reacts vigorously to give electrons to anything that it can.  Carbon, on the other hand, can either gain or lose four electrons to be like the nearest noble gas - as a result, carbon is not as violently reactive as elements nearer the noble gases.  Fluorine can gain only one electron or lose seven - since there's not much choice about which it can do, it tends to react violently to gain electrons.   That's why the halogens are referred to as oxidizers - they take electrons away from just about anything else in their quest to be like the nearest noble gas.