All noble gas atoms have a stable duplet (two electrons in the 1st energy level) or octet(eight electrons in other outer energy level)in the outer energy level.

They therefore do not acquire/gain extra electron in the outer energy level or donate/lose.

They therefore are therefore zerovalent .

 The number of energy levels increases down the group from Helium   to Radon.

The more the number of energy levels the bigger/larger the atomic size/radius. e.g.

 The atomic size/radius of Argon is bigger/larger than that of Neon because Argon has more/3 energy levels than Neon (2 energy levels).

Atomic radius noble gases increase down the group as the number of energy levels increases.

The effective nuclear attraction on the outer electrons thus decrease down the group.

The noble gases are generally unreactive  because the  outer energy level has the stable octet/duplet.

 The stable octet/duplet in noble gas atoms lead to a comparatively very high 1st ionization energy.

This is because losing /donating an electron from the stable atom require a lot of energy to lose/donate and make it unstable.

As atomic radius increase down the group and the 1st ionization energy decrease, very electronegative elements like Oxygen and Fluorine are able to react and bond with      lower members of the noble gases.

Xenon reacts with Fluorine to form a covalent compound XeF6.

This is because the outer electrons in Xenon is far from the nucleus and thus experience less effective nuclear attraction.

Noble gases have low melting and boiling  points because they   exist as monatomic molecules joined by very weak inter molecular /van-der-waals forces that require very little energy to weaken and form liquid and break to form a gas.

The intermolecular/van-der-waals forces increase down the group as the atomic radius/size increase .

This also increases the melting and boiling points down the group.

Noble gases are insoluble in water and  poor electrical      conductors

1. Noble gases are generally unreactive. Explain