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Valdayan/Zyryanka A Weichselian, loess-like silt from Siberia, in which there is a threefold division with the Zyryanka drift at the base, then Karginsky interstadial sediments, and finally the Sartan l of an atom, measured by the number of hydrogen ions (valency 1) that the atom could combine with or replace. In an ionic compound the valency (electrovalency) equals the ionic charge on each ion, e.g. in the compound MgO, Mg2+ shows a valency of +2, O2− a valency of −2. In a covalent compound the valency (covalency) of an atom is equal to the number of bonds it forms, e.g. in CH4 carbon has a valency of 4, hydrogen a valency of 1.