All the noble gases are colourless gases at room temperature, and each exists as single atoms. Their physical properties change in a regular way down Group 0 from helium to neon, argon, krypton, xenon and radon.
The boiling points increase down the group. Helium has the lowest boiling point, at about -269 °C, and radon, at the bottom, has the highest. The melting points increase in the same way, and the density of the gases will also increase.
A pattern like this can be used to make predictions. Argon boils at about -186 °C and xenon at about -108 °C. Krypton comes after argon and before xenon in the group, so its boiling point should fall in the range set by these two values. A sensible prediction is about -150 °C. Radon is below xenon, so it is predicted to have a higher boiling point than xenon and a higher density, but its boiling point is still well below room temperature so it is a gas.