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Unformatted text preview: HW KEY- 8 OXIDATION-REDUCTION CHM2045 Hw8K-G1-Redox.wpd Oxidation Numbers 1. C By definition, the oxidation number of an atom is equal to the charge that would be present on the atom if the compound was composed of ions. It does not matter whether the compound actually contains ions. If used consistently, the concept of oxidation number is a very useful bookkeeping system for keeping track of electrons in chemical reactions. C The calculation of formal charge is generally applied to covalent bonds within Lewis electron structures for molecules or polyatomic ions. All bonding electrons are assumed to be shared equally between the nonmetallic atoms. Formal charge rarely exceeds +2 or is less than -2. C Derivation of oxidation states operates on the assumption that the bonding electrons are not shared, but belong exclusively to the more electronegative atom of a bond. In some cases, oxidation states of +7 and - 4 are observed, which is hardly possible for ionic charge. 2. Recall the ranges of oxidation numbers for Main Group (Group A) elements: Theoretical maximum: + group number Theoretical minimum: group number - 8 (mostly nonmetals) Note: Most metals have only positive oxidation states a) N Group 5A nonmetal with 5 valence electrons. Oxidation range +5 to -3 b) Cl Group 7A nonmetal with 7 valence electrons. Oxidation range +7 to -1 c) F Group 7A nonmetal with 7 valence electrons. F is the most electronegative element and therefore must always have a negative oxidation state, specifically -1 in all the compounds it forms. d) Na Group IA metal with 1 valence electron. Na is a metal, and is not at all likely to gain electrons within a bond, and, therefore is always +1 in compounds. e) S Group VIA nonmetal with 6 valence electrons. Oxidation range +6 to -2 f) Ca Group IIA metal with 2 valence electrons. Ca is a metal, and is not at all likely to gain electrons within a bond, and, therefore is always +2 in compounds. g) C Group IVA nonmetal with 4 valence electrons. Oxidation range +4 to -4. h) O Group VIA with 6 valence electrons. O is the second most electronegative element and always carries a negative oxidation state in compounds (except when bonded to F in the compound F 2 O). Most of the time O is -2 but it may be -1 in peroxides (O 2 2- ) or some fractional negative in superoxides and ozonides. i) Al Group IIIA metal with 3 valence electrons. Al is a metal, and is not at all likely to gain electrons within a bond, and, therefore is always +3 in compounds. j) Mn Group VIIB metal with 7 valence electrons. Mn is a transition metal and is not likely to form a negative oxidation state. Its maximum oxidation state is +7, but it often forms intermediate oxidation states of +2 and +4....
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