Final answer:
Elements in Group 18 are gaseous at room temperature and chemically stable, a description fitting the noble gases which are unreactive due to their full valence electron shells.
Step-by-step explanation:
The properties that best describe the elements in Group 18 of the periodic table are that they are gaseous at room temperature and chemically stable. This group is known as the noble gases, which include elements like helium, neon, argon, krypton, xenon, and radon. These elements are known for being extremely unreactive due to their full valence electron shells, which in most cases consists of eight valence electrons. They rarely form compounds because they do not easily gain or lose electrons.