Question

which halogens deviates it ideal behaviour

Answer 1

The ideal gas law is commonly used to model the behavior of gas-phase reactions. Ideal gases are assumed to be composed of point masses whose interactions are restricted to perfectly elastic collisions; in other words, a gas particles’ volume is considered negligible compared to the container’s total volume.
There are two notable situations in which the behavior of real gases deviates from that model:

1) At high pressures where the volume occupied by gas molecules does not approach zero
2) At low temperatures where the contribution of intermolecular forces becomes significant