Praxis Middle School Science (5442) Practice Test

Group 18 on the periodic table is the set of noble gases. These elements have full outer electron shells, which makes them very stable and unlikely to react with other elements. Because their outer shell is complete, they don’t need to gain or lose electrons, so they stay as individual, mostly unreactive atoms. That’s why they’re called noble gases—their chemistry is very “upper crust,” in a sense: they rarely form compounds. They sit in the far-right column of the table and are typically colorless, odorless gases at room temperature (with radon being radioactive). For comparison, halogens in the neighboring column are highly reactive nonmetals that readily gain one electron, alkaline earth metals to the left have two valence electrons and form basic compounds, and transition metals in the center have varied properties and multiple oxidation states.