Category:cy:Noble gases
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Welsh terms for types or instances of chemical elements in group 18 of the periodic table, which all have a full set of valence electrons: 2 for helium and 8 for the others.
NOTE: This is a set category. It should contain terms for noble gases, not merely terms related to noble gases. It may contain more general terms (e.g. types of noble gases) or more specific terms (e.g. names of specific noble gases), although there may be related categories specifically for these types of terms.