Question

why AsCl5 is highly reactive and unstable?

Answer 1

It is conditioned by very weak As-Cl bonds. The main factor which determines low stability is the huge size and the number of electrons for central atom. Therefore bonding electrons are attracted weaker that leads to weak As-Cl bonds. Furthermore, impact of completed low quantum level on valence electrons makes them more inert to bond formation. This causes a less availability of valence electrons and weak As-Cl interaction.