How does electron configuration affect chemical behavior?
In simple terms - the more unstable an electron configuration may be, the more reactive an atom will become.
For example, if we look at the Group 1 elements in the periodic table, they are all highly reactive as they have 1 electron in their outermost shells - an unstable or 'unfavourable' configuration in terms of energy.
However, the noble gases in Group 8 in the periodic table are 'inert' that means they don't react (or more correctly, have an incredibly low reactivity). This is because they have 8 electrons in their outermost shell and thus have no need to acquire or lose electrons to possess a stable electron configuration.