Question

2 reason why noble gases become less ideal in their behaviour down the group

Answer 1

Well, to start with, since the atomic radius of the gas increases as it goes down the period, it causes there to be a decrease in the effect that the nucleus's positive charge has on the valence electrons ( more electrons in between, and more distance ). This causes the valence electrons to become less tightly bound to the atom, which translates to a deviation from the noble gases tendencies to remain nonreactive.