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Figure 13.1 Carbon oxidation state extremes.

Figure 13.1 Carbon oxidation state extremes.

Chemical Oxidation States

Chemical Oxidation States

outer shell (one from each of the four hydrogens), thereby changing its oxidation state to —4 (10 negatively charged electrons versus six positively charged protons). Conversely, the carbon found in carbon dioxide (CO2) essentially lends all four of its outer shell electrons to a pair of surrounding oxygen atoms, thereby shifting its oxidation state to +4.

Carbon, nitrogen, and sulfur are the three main elements of biological interest with a full eight-electron range in oxidation state. As shown in Figure 13.2, the ranges for these three elements (C, N, and S) are each, respectively, shifted one electron higher.

Table 13.1 shows the common oxidation states of some of the other elements of biological importance. Oxygen's (AN = 8) outer shell has six electrons, producing an aggressive tendency to add two additional electrons (thereby filling its outer shell) rather than releasing any to other adjacent atoms. This characteristic qualifies oxygen as having a high degree of electronegativity (Section 3.2). As a result, oxygen's metabolic range of oxidation states only includes —2 (e.g., H2O), besides 0 (diatomic oxygen, O2). Hydrogen (AN = 1) has possible states of —1, 0, and +1, since its single outer shell has only one electron of the two it would take to fill it. In living matter it typically donates its electron (thus, oxidation state = +1) to a more electronegative (e.g., oxygen, carbon, sulfur, nitrogen) receptor. Metabolic iron (AN = 26), occurs mainly in the +2 (ferrous) and +3 (ferric) oxidation states, while manganese (AN = 25) is found mainly as the +2 (manganous) and +4 (manganic) forms. Chlorine (AN = 17) as a disinfectant in water (HOCl, hypo-chlorous acid) has an oxidation state of +1, but as chloride or in organic compounds it is 1.

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