Containing arsenic in the trivalent form (As³⁺), as opposed to the pentavalent form.
From arsenic + -ous (chemical suffix indicating lower valence). This systematic chemical nomenclature developed in the late 18th century to distinguish between different oxidation states of the same element.
Arsenous compounds are generally more toxic than arsenic pentoxide, which is why arsenic poisoning has a specific chemistry—the chemical form matters as much as the amount.
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