Almost all known xenon compounds contain the electronegative atoms fluorine or oxygen. These covalent features are more substantial when acceptors bind hydrogens from more electronegative donors. Most known hypervalent molecules contain substituents more electronegative than their central atoms. During the time of Maxwell and Faraday, concerns came about for electropositive and electronegative activities. Therefore, complexes with more electronegative ligands are more oxidising than less electronegative ligand complexes. Fluorine is highly electronegative and lowers the polarizability of the material. Fluorine is too electronegative to be bonded to magnesium; it becomes an ionic salt instead. These hypercoordinated compounds usually have electronegative substituents. Most metalloids, like the less electronegative nonmetals, form allotropes. A hydrogen atom attached to a relatively electronegative atom is the hydrogen bond "donor". More electronegative metals and ligands can stabilize electrons better than their less electronegative counterparts. The carbonyl carbon is subsequently attacked by the electronegative oxygen. It is highly electronegative and non-metallic, usually diatomic, gas down to very low temperatures. However, this is not the case for certain substituents, typically those that are highly electronegative. Oxides of more electronegative elements tend to be acidic.