Why Noble gases are mostly chemically inert?

The inertness to chemical reactivity of noble gases is attributed to the following reasons:

(1)The noble gases except helium (1s²) have completely filled ns²np⁶ electronic configuration in their valence shell.

(2) They have high ionisation enthalpy and more positive electron gain enthalpy.

Why Xenone (Xe) does not forms compounds such as xenone trifluoride (XeF3)and xenone pentafluoride (XeF5)?

By the promotion of one, two or three electrons from filled p-orbital to the vacant d-orbital in the valence shell, 2, 4 or 6 half filled orbitals are formed. Thus Xe can combine only with even number of fluorine and not odd.

Why Noble gases exhibit large positive values of electron gainenthalpy?

Since noble gases have stable electronic configurations, they have no tendency to accept the electron and therefore, have large positive values of electron gain enthalpy.

Why Noble gases have very low boiling points?

Noble gases being monoatomic have no interatomic forces except weak dispersion forces and therefore, they are liquefied at very low temperatures. Hence, they have low boiling points.

Why liquification of noble gases is quite difficult?

liquification of noble gases is quite difficult.It is due to the fact that there are only weak vander waal's forces which hold atoms together, but due to increase in these forces down the group the ease for liquification increases down the group.