Why the noble gases become less ideal in their behavior down the group from helium to xenon?

Answer 1

As atoms get larger, they have a weaker grip on their electron shell. Remember that as indicated by Coulomb's Law, the attraction between opposite electrical charges is inversely proportional to the square of the distance between them. Whereas, the attraction increases in direct proportion to the amount of the electrical charge. So a larger atom has more protons in the nucleus, with which to attract electrons, but the increasing distance of the electrons from the nucleus will reduce the attraction to a greater extent than the increased number of protons is increasing it.

So, as noble gases get heavier, and their grip on their electrons weakens, they become more capable of losing an electron and therefore entering into chemical reactions of some kind.

Answer 2

Noble gases become less ideal as you move down the group because the size and polarizability of the atoms increase, leading to stronger van der Waals forces between the atoms. This causes deviations from ideal behavior as the intermolecular interactions become more significant at larger atomic sizes, affecting properties like compressibility and volume deviations.