National Council of Examiners for Engineering and Surveying (NCEES) Fundamentals of Engineering (FE) Chemical Practice Exam

The oxidation number of chlorine in most compounds is commonly -1. This is particularly true in binary compounds where chlorine is combined with metals or with nonmetals that are less electronegative than chlorine. For instance, in compounds like sodium chloride (NaCl) and potassium chloride (KCl), chlorine exhibits an oxidation state of -1, reflecting its tendency to gain one electron and achieve a stable electron configuration, akin to that of a noble gas.

In addition to its common -1 oxidation state, chlorine can exhibit other oxidation states in different compounds, but -1 is the most prevalent due to the reactivity and electronegativity of chlorine. In certain compounds, like chlorine dioxide (ClO2), it takes on a +4 oxidation state, but these situations are exceptions rather than the rule. Therefore, when considering the typical behavior of chlorine in most chemical environments, -1 is the standard oxidation number.