Final answer:
The characteristic property of gases in Group 18 (Noble gases) is that they are unlikely to react with other elements due to their stable electron configuration.
Step-by-step explanation:
The gases in Group 18 of the periodic table are known as the Noble gases. The most defining characteristic of Noble gases is that they are unlikely to react with other elements. This is due to their stable electron configuration, where each atom has a full set of electrons in its outermost shell. Therefore, they have little tendency to gain or lose electrons, making them sometimes called inert gases. Unlike metals, they do not display properties such as malleability or high electrical conductivity, and they are not brittle like some non-metals.
Learn more about Noble gases