Why is fluorine a very reactive halogen ?

The atomic number of sodium is 11, fluorine is 9 and neon is 10. Why are sodium and fluorine very reactive while neon shows almost no reactivity.

Why is Vinyl chloride less reactive than ethyl chloride ?

What are the interhalogen compounds ? Why are these more reactive than halogens ?

Why ICI is more reactive than I_2 ?

Why haloalkanes are more reactive than haloarenes towards nuclophilic subsition reaction ?